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I've done a search and can't find anything as to what specifically makes a Covid-19 positive that identifies it as unique. I would expect to see something like this:
The SARS-CoV-2 diagnostic tests that are currently in use, both serological and nasal swab, are binary tests - they either are positive or negative, like a pregnancy test. They do not (yet) report viral titre, like the first image you linked, or look at restriction enzyme "gene fingerprinting" patterns, like the second. They simply tell you if either a part of the virus is in the sample (RNA or capsid protein in the two nasal swab tests) or if your immune system has seen enough of the virus to mount an antibody-mediated immune response (the IgM/IgG blood tests).
It's worth pointing out that all of the current tests in the US are available under emergency use authorizations from the FDA, meaning they haven't gone through the usual (and time-consuming) process of full qualification and validation. That type of work needs to be done in order to develop quantitative testing ("You have a viral titre of X, compared to an average range of Y through Z."), which will likely appear in the coming months and years as the proper development work is done to support their validity.
To my knowledge, all of the PCR-based tests that have been authorized for diagnostics use fluorescently labeled reporter probes specific to the the SARS-COV-2 viral genome. In real-time PCR, the intensity of this fluorescent signal is measured (spoiler alert) in real time, after each amplification cycle. The fluorescent signal increases with the amount of the target sequence present in a reaction. If the signal exceeds a certain baseline threshold before a certain number of PCR cycles, the test is considered positive for that target. It really is as simple as that, but a lot of work goes in on the development and validation of these threshold cutoffs to ensure they are sufficient for sensitive detection.
What identifies the readout as unique to this virus is the design of the probes and the primer sets that go with them. These are designed to be highly specific to a given target, and are validated in silico against existing sequence databases, and also empirically against closely related organisms, or just against other things that might be present in a specimen. They should also target conserved regions of the genome that are unlikely to mutate. Most of the tests I have seen use at least two different SARS-CoV-2-specific probe targets, and at least one amplification control (often targeting a conserved human gene) to ensure that the reaction enzymes are functioning properly. But the specific regions of the genome that are targeted can vary between different tests.
Using real-time PCR saves clinical lab techs the time and effort of making gels and running the amplified PCR products out on the gel, like those in the second example you posted. This not only improves the turnaround time for testing results, but saves countless work hours for the clinical labs processing these tests, especially when you consider the scale of how many tests are being processed right now.
As with any nucleic acid test, a positive result can only indicate if the target is (or was) present. It does not provide information to clinicians about whether it is actively replicating or even infectious. But in general, this kind of testing is highly sensitive and specific when properly designed and validated.
A Covid-19 diagnostic test won’t pick up other coronaviruses
Covid-19 tests detect any coronavirus, not just the one that causes Covid-19.
Incorrect. Covid-19 diagnostic tests are not perfect, but they won’t misdiagnose other coronaviruses as the virus which causes Covid-19.
A post on Facebook makes a number of claims about the Covid-19 pandemic, centring on the assertion that Covid-19 tests do not test for Covid-19 specifically, but any coronavirus, including ones which cause &ldquonothing more than cold/flu like symptoms&rdquo. (Some coronaviruses cause what we know of as the common cold.)
As we have written about extensively here, the diagnostic test used in the UK is called a PCR (polymerase chain reaction) test, which looks for the virus&rsquos genetic material to see if someone currently has Covid-19.
The Facebook post questions the reliability of PCR tests and there is something in this.
PCR tests can sometimes indicate that someone does not have the virus when they do (false negative). They can also indicate that someone has the virus when they don&rsquot (false positive).
It&rsquos hard to say how many false negatives and positives PCR tests produce, but in general, these tests are &ldquohighly accurate&rdquo.
Most importantly, what a PCR test won&rsquot do is misinterpret the presence of other coronaviruses as the presence of the virus which causes Covid-19, as this post claims.
The possibility that a test might pick up related viruses that have genetic similarities to the virus you&rsquore looking for (technically known as &ldquocross-reactivity&rdquo) is something that is looked at when designing PCR tests.
For example, one of the earliest PCR testing protocols, which was published on 13 January, specifically checked that the test did not pick up the four human coronaviruses that cause infections including the common cold.
Results for a range of available PCR tests show that they do not cross-react with any viruses analysed, including other coronaviruses.
Where some of the confusion might have come about is because of the accuracy of antibody tests. The US Centers for Disease Control and Prevention says: &ldquoThere is a chance that a positive result means you have antibodies from an infection with a different virus from the same family of viruses.&rdquo
This article is part of our work fact checking potentially false pictures, videos and stories on Facebook. You can read more about this—and find out how to report Facebook content—here. For the purposes of that scheme, we’ve rated this claim as false because PCR tests are specific to the virus that causes Covid-19, not just any coronavirus.
Who Can Get Tested?
The Centers for Disease Control and Prevention (CDC) continues to issue evolving recommendations for getting tested.
Situations in which they recommend you seek a COVID-19 test include:
- If you have symptoms of COVID-19
- If you have been in close contact (less than 6 feet for a total of 15 minutes or more) with documented SARS-CoV-2 infection and do not have symptoms
- If you are in a high SARS-CoV-2 transmission zone and attended a public or private gathering of more than 10 people (without universal mask-wearing and/or physical distancing)
- If you work in a nursing home
- If you live in or receive care in a nursing home
- If you are a critical infrastructure worker, healthcare worker, or first responder
The CDC adds that there may be other situations in which public health officials or healthcare providers may advise specific people to get tested. If specifically recommended by an official or provider you should get tested. Situations like this can include:
- If you're about to be admitted to the hospital or preparing for a procedure
- If there is significant spread of the virus in your community, your public health department may request significant numbers of asymptomatic “healthy people” to be tested in order to help stop the spread of the virus
The indications for testing for COVID-19 continue evolving as more information is being gathered about this infection.
COVID-19 Antibody Test
Currently, Rush is offering COVID-19 antibody testing in limited situations. Antibody testing is not used to diagnose whether a person currently has COVID-19, the disease caused by the novel 2019 coronavirus. The test results may show whether a person has been infected with the virus, depending on the results. Since no standard exists yet for determining accuracy, these results are not definitive. Please see additional information if you are a Rush employee or Rush University student.
You can learn more at the CDC website.
COVID-19 (SARS-CoV-2) IgG Antibody Negative Test Result
If your antibody test result was negative, this means that the test did not detect any COVID-19 antibodies in your blood. This result suggests that you have not been infected with the COVID-19 virus.
However, all tests, including the COVID-19 antibody test, can produce negative results that are incorrect (i.e., false negative results). A negative result also may occur if you have an antibody test too soon after an active COVID-19 virus infection.
In most people who recover from COVID-19, antibodies appear in their blood about 14 days after the start of the illness. We don’t know if people who have had COVID-19 and who do not develop antibodies are at risk of infection with COVID-19 in the future. Researchers at Rush and elsewhere are working hard to answer this question. In the meantime, we recommend that you wear a face mask in public, practice frequent hand hygiene and follow social distancing recommendations, just as you were doing before antibody testing. If you have questions, please consult with your health care provider.
COVID-19 (SARS-CoV-2) IgG Antibody Positive Test Result
If your antibody test result was positive, this means that the test shows that you have COVID-19 antibodies in your blood. This result means that you were likely infected with COVID-19 in the past.
However, all tests, including the COVID-19 antibody test, can give positive results that are incorrect (i.e., false positive results). Furthermore, we do not know whether the antibodies that were detected by this test will protect you from COVID-19 infection in the future. Researchers at Rush and elsewhere are working hard to answer this question.
In the meantime, we recommend that you continue to wear a face mask in public, practice frequent hand hygiene and follow social distancing recommendations, just as you were doing before antibody testing. If you have questions, please consult with your health care provider.
SARS-CoV-2 is detected by using one of the following assays:
The UW SARS-CoV-2 Real-time RT-PCR assay targets two distinct regions within the N gene of SARS-CoV-2 (the causative agent for COVID-19). Amplification of both targets results in a presumptive positive (detectable) test result, while amplification of one of two targets results in an inconclusive result, and amplification of neither target results a negative (non-detectable) test result. Testing is limited to the high complexity CLIA clinical laboratory at UW Virology in Seattle, WA.
The Hologic Emergency Use Authorization (EUA) SARS-CoV-2 Real-time RT-PCR assay targets two conserved regions of the SARS-CoV-2 (the causative agent for COVID-19) ORF1ab gene. The two regions are not differentiated amplification of either or both regions is a presumptive positive (detectable) test result and amplification of neither target results a negative (non-detectable) test result.
The Roche cobas Emergency Use Authorization (EUA) SARS-CoV-2 Real-time RT-PCR assay (Fact Sheet) targets two regions of the SARS-CoV-2 (the causative agent for COVID-19) genome, the E gene and ORF1ab gene. Amplification of both targets results in a presumptive positive (detectable) test result, while amplification of one of two targets results in an inconclusive result, and amplification of neither target results a negative (non-detectable) test result.
The Abbott Alinity m Emergency Use Authorization (EUA) SARS-CoV-2 Real-time RT-PCR assay targets two regions of the SARS-CoV-2 (the causative agent for COVID-19) genome, the RdRp gene and N gene. The two regions are not differentiated amplification of either or both regions is a presumptive positive (detectable) test result and amplification of neither target results a negative (non-detectable) test result.
The DiaSorin Molecular Simplexa™ COVID-19 Direct Emergency Use Authorization (EUA) SARS-CoV-2 Real-time RT-PCR assay targets two regions of the SARS-CoV-2 (the causative agent for COVID-19) genome, the OEF1ab gene and S gene. Amplification of both targets results in a presumptive positive (detectable) test result, while amplification of one of two targets results in an inconclusive result, and amplification of neither target results a negative (non-detectable) test result.
The Hologic Emergency Use Authorization (EUA) SARS-CoV-2 Transcripton Mediated Amplification (TMA) assay targets two conserved regions of the SARS-CoV-2 (the causative agent for COVID-19) ORF1ab gene. The two regions are not differentiated amplification of either or both regions is a presumptive positive (detectable) test result and amplification of neither target results a negative (non-detectable) test result.
Due to the shortage of kits and false negative rate of RT-PCR, the Hubei Province, China temporarily used CT scans as a clinical diagnosis for COVID-19. 47 Chest CT scans are non-invasive and involve taking many X-ray measurements at different angles across a patient’s chest to produce cross-sectional images. 48,49 The images are analyzed by radiologists to look for abnormal features that can lead to a diagnosis. 48 The imaging features of COVID-19 are diverse and depend on the stage of infection after the onset of symptoms. For example, Bernheim et al. saw more frequent normal CT findings (56%) in the early stages of the disease (0𠄲 days) 50 with a maximum lung involvement peaking at around 10 days after the onset of symptoms. 51 The most common hallmark features of COVID-19 include bilateral and peripheral ground-glass opacities (areas of hazy opacity) 52 and consolidations of the lungs (fluid or solid material in compressible lung tissue). 50,51 De Wever et al. found that ground-glass opacities are most prominent 0𠄴 days after symptom onset. As a COVID-19 infection progresses, in addition to ground-glass opacities, crazy-paving patterns (i.e., irregular-shaped paved stone pattern) develop, 51 followed by increasing consolidation of the lungs. 50,51 Based on these imaging features, several retrospective studies have shown that CT scans have a higher sensitivity (86%) and improved false negative rates compared to RT-PCR. 3,25,53,54 The main caveat of using CT for COVID-19 is that the specificity is low (25%) because the imaging features overlap with other viral pneumonia. 3
COVID-19 is currently diagnosed with RT-PCR and has been screened for with CT scans, but each technique has its own drawbacks. There are three issues that have arisen with RT-PCR. First, the availability of PCR reagent kits has not kept up with demand. Second, community hospitals outside of urban cities lack the PCR infrastructure to accommodate high sample throughput. Lastly, RT-PCR relies on the presence of detectable SARS-CoV-2 in the sample collected. If an asymptomatic patient was infected with SARS-CoV-2 but has since recovered, PCR would not identify this prior infection, and control measures would not be enforced. Meanwhile, CT systems are expensive, require technical expertise, and cannot specifically diagnose COVID-19. Other technologies need to be adapted to SARS-CoV-2 to address these deficiencies.
Are RT-PCR tests failing to detect some coronavirus variants? What doctors say
Doctors TNM spoke to said that they are using a combination of methods to detect if a person has COVID-19 and then prescribing the necessary treatment.Image for representation/PTI
In the recent past there have been several anecdotes as well as reports of the RT-PCR test giving a false negative even for people who may be infected with the novel coronavirus. This has raised several concerns. Apart from the possible delay in getting treatment itself, there are also questions about whether the new variants of the SARS-CoV-2 virus found in India could be evading detection by RT-PCR tests.
While doctors TNM spoke to said that they believe that the RT-PCR test should still be able to detect coronavirus if an individual is infected with a variant, they also say that more research is needed to ascertain the variants’ detectability with the RT-PCR tests. They add that they are not relying on RT-PCR tests alone, and have been using a combination of methods to detect if a person has COVID-19 and then prescribing the necessary treatment.
Detectability of variants through RT-PCR tests
Dr A Velumani, the CEO of Thyrocare Technologies, a chain of diagnostics labs, explains that the RT-PCR test they do does not look for a single virus gene of the virus, but three. “The virus has a certain gene that we look for specific sequences are the signature of the virus. There are particular locations on the gene that are used to detect if it’s SARS-CoV-2,” he says. He adds that since the RT-PCR test looks for the gene in three locations of the virus, even if it is a variant, at least two out of three locations should show up the identifying gene. “However, a three-gene test cannot identify if the virus detected is a variant. For that you need at least a five-gene indicator test,” Dr Velumani says.
Dr Siri Kamath, consultant physician at BGS Gleneagles Global Hospital in Kengeri, Bengaluru, points out that the variants have affected the spike protein of the virus, and the RT-PCR test looks for genetic material of the virus to identify it. “RT-PCR test detects the most conserved part of the genome of the virus. It is yet to be proven if variations in spike protein of the virus will affect its detection. We don’t have the correlation between false negatives being because of infection by variants,” he says.
Dr Srivatsa Lokeshwaran, a consultant, interventional pulmonology with Aster CMI Hospital in Bengaluru, agreed, saying, “RNA viruses like the coronavirus are bound to mutate over a period of time. Despite minor mutations, RT-PCR would still detect the virus.”
Further, Dr Velumani points out that false negatives are not new, and happened earlier also during the first wave of coronavirus infections. “It is because of variation in human response to the virus and how the virus or its variant would react with the body that causes variations in result. A laboratory can’t figure that out.”
What the research says so far
The doctors TNM spoke to agreed that there needs to be more study into whether variants of the virus have varied detectability by RT-PCR tests as well. So far, there is limited study and reporting on the same.
In March, France had reported a new variant of the novel coronavirus from its Brittany region. Eight of the 79 COVID-19 patients had then tested negative for the disease on RT-PCR test, which is considered the gold standard. The illness was confirmed for the eight persons who subsequently died. They showed typical COVID-19 symptoms later, after blood samples and tissue in their respiratory system were analysed. In February, Finnish researchers had also found a variant of the virus that was evading detection by at least one standard RT-PCR test, though not all. A researcher was quoted as saying that this new variant did not “genetically resemble any other known variant.”
This research paper further states that mutations “may impact the diagnostic sensitivity and specialty, and therefore, they should be considered in designing new testing kits as part of the current efforts in COVID-19 testing, prevention, and control.” The study genotyped 31,421 SARS-CoV-2 genome samples collected up to July 23, 2020 and found that “essentially all of the current COVID-19 diagnostic targets have undergone mutations.”
However, American regulatory body Food and Drug Administration (FDA) in January said that genetic variants of SARS-CoV-2 may lead to false negatives in molecular tests. “[…] false negative results may occur with any molecular test for the detection of SARS-CoV-2 if a mutation occurs in the part of the virus’ genome assessed by that test,” it said, adding, “Molecular tests designed to detect multiple SARS-CoV-2 genetic targets are less susceptible to the effects of genetic variation than tests designed to detect a single genetic target.” RT-PCR is a molecular test. The FDA did not mention if variants that affect the spike protein can evade detection.
In India, variants from the United Kingdom (B.1.1.7), South Africa (B.1.351) and Brazil (P.1) have been detected, as per the Consortium on Genomics (INSACOG), which is a group of 10 national laboratories established by the Union Ministry for Health. Further, there is also a double variant which has been found in some samples from Maharashtra, which has ‘immune escape’ qualities i.e. it may evade detection by the human immune system, hence affecting the infected individual’s capacity to generate an immunogenic response and eliminate it.
However, it is not clear if these variants are less likely to be detected by the RT-PCR test, and medical professionals as well as scientists have called for more study into this aspect.
How doctors are tackling COVID-19 if RT-PCR test is negative
Doctors are using a combination of methods to check if a person has COVID-19 even if the individual tests negative on the RT-PCR test. Dr Anoop Amarnath, member of the Critical Care Support Unit of the Karnataka government, who works at Manipal Hospital, explains that they are looking at diagnosis from four aspects – the RT-PCR test, certain biochemical parameters, radiological parameters (such as a high-resolution CT scan), and clinical symptoms (presenting with cough, fever, breathlessness, oxygen saturation levels).
Acknowledging that they are seeing negative test results even where a person is symptomatic, Dr Anoop says, “As per the guidelines, we look at the other markers and if those suggest they have COVID-19, then they are given the relevant treatment.”
Dr Siri says that their first hint is how a person’s symptoms. “If the clinical symptoms are there, we first assume that it’s COVID-19 and advise them to isolate etc. Then even if the RT-PCR test result is negative, we see the age and vulnerability of the person. Say, if the patient is 40+, we do a CT scan of the lungs. If the person is younger and does not have serious symptoms, we may put them on appropriate medication, tell them to self-isolate at home, and keep checking their oxygen saturation levels. If it has not dropped below a certain level in the first 10 days or so, it is likely their condition will not deteriorate.”
Fact check: Inventor of method used to test for COVID-19 didn’t say it can’t be used in virus detection
Correction Nov. 13, 2020: The verdict of this fact check has been changed from false to misleading, to reflect that the quote examined may have been a fair reflection of Mullis’s views, even if not a direct quote. The body of the text is updated in places to further clarify this.
Social media users have been sharing a quote attributed to the inventor of the Polymerase Chain Reaction (PCR) test, currently being used to detect COVID-19, which says “PCR tests cannot detect free infectious viruses at all”. This quote appears not to be a direct quote from the inventor, Kary Mullis, has lost some context and does not mean COVID-19 testing is fraudulent, as suggested by some social media posts.
The posts have been shared over 1,000 times on Facebook ( here , here , here).
The post begins with the words “COVID-19 TEST a FRAUD?”, then introduces the alleged quote from Mullis, who invented the PCR method in 1985 and was recognized for this achievement by being awarded the Nobel Prize in Chemistry in 1993 ( here).
However, the quote is actually from an article written by John Lauritsen in December 1996 about HIV and AIDS, not COVID-19 ( here).
The context around the quote shows Lauritsen is not saying PCR tests do not work. Instead, he is clarifying that PCR identifies substances qualitatively not quantitatively, detecting the genetic sequences of viruses, but not the viruses themselves: “PCR is intended to identify substances qualitatively, but by its very nature is unsuited for estimating numbers. Although there is a common misimpression that the viral load tests actually count the number of viruses in the blood, these tests cannot detect free, infectious viruses at all they can only detect proteins that are believed, in some cases wrongly, to be unique to HIV. The tests can detect genetic sequences of viruses, but not viruses themselves.”
Even if Mullis had voiced a similar statement before his death in 2019, this quote does not mean the PCR test is unable to detect the presence of SARS-CoV-2 - the virus that causes COVID-19 - rather that it cannot determine whether the individual tested is infectious.
The PCR test is the preferred COVID-19 testing method in England ( tinyurl.com/u9xxxup). It detects the presence of the virus by amplifying the virus’genetic material to a point where it can be detected by scientists ( tinyurl.com/y7rno7pf).
A spokesperson for Public Health England told Reuters why PCR tests are being used widely in England: “Molecular diagnostic tests, such as real-time PCR, are the gold standard methods for identifying individuals with an active viral infection, such as SARS-CoV-2 (the cause of COVID-19 disease), in their respiratory tract. These tests are rapid and produce results in real-time.
“It is important to note that detecting viral material by PCR does not indicate that the virus is fully intact and infectious, i.e. able to cause infection in other people. The isolation of infectious virus from positive individuals requires virus culture methods. These methods can only be conducted in laboratories with specialist containment facilities and are time consuming and complex.”
Understanding Your PCR (Nasal Swab) Test Results
Update: The average turnaround time for PCR (nasal swab) lab results is currently 2 to 3 days, but can take longer depending on lab partner and other factors.
All PCR testing is performed by one of our commercial lab partners. The testing platforms used are Roche Cobus or Hologic Panther, both with Emergency Use Authorization by the FDA. Both platforms search for 2 targets target 1 that is specific to SARS-CoV-2 (the coronavirus causing COVID-19) and target 2 for general Corona Viruses (including but not limited to SARS-CoV-2).
Positive Results: You have tested positive for Sars-CoV-2, the virus causing COVID-19. Either target 1 alone or both targets 1 and 2 were detected (our lab partners do not specifically call out if you tested positive for target 1 alone or target 1 and 2 as it is not relevant, either scenario is positive). You are likely actively contagious and should home quarantine (sleep alone in bed, if possible use your own bathroom, wipe down surfaces, and wear a mask when in the same room as others). Please note, a small percentage of patients with active infection may be completely asymptomatic. CityMD recommends the CDC's most up to date return to work recommendation of the rule of 10/3. If you did not have symptoms at the time of your PCR nasal swab, you may return to work in 10 days (provided you do not have a fever 3 days prior to return to work). If you are asymptomatic and test positive, you are still likely contagious to others. A positive PCR test does not yield any information about potential immunity. Please be re-evaluated immediately for worsening symptoms such as shortness of breath or lightheadedness.
Negative Results: With a high likelihood, the results state you were not infected with Sars-CoV-2 at the time of testing. Neither target 1 or target 2 were detected. We recommend following quarantine recommendations and universal precautions (hand washing, social distancing, and when appropriate PPE such as masks and gloves). This test does not give information about past infections or future immunity.
Inconclusive Results (Presumptive Positive) or Presumed Positive: Inconclusive/Presumptive Positive or Presumed Positive means target 1 was not detected but target 2 was detected. It is presumed if you had symptoms consistent with COVID-19 and test positive for target 2, you have COVID-19. You are likely actively contagious and should home quarantine (sleep alone in bed, if possible use your own bathroom, wipe down surfaces, and wear a mask when in the same room as others). Please note, a small percentage of patients with active infection may be completely asymptomatic. CityMD recommends the CDC's most up to date return to work recommendation of the rule of 10/3. If you did not have symptoms at the time of your PCR nasal swab, you may return to work in 10 days (provided you do not have a fever 3 days prior to return to work). A positive PCR test does not yield any information about potential immunity. Please be re-evaluated immediately for worsening symptoms such as shortness of breath or lightheadedness.
Sunrise Labs will report your test as:
Quest Labs will report your test as:
If you have any questions or need to discuss your results further, please call Aftercare at 844-824-8963 Extension 8120. We apologize as we are currently experiencing high call volumes to our Aftercare Department. We appreciate your patience and understanding during this time.
If you test positive
- A positive test result shows you may have antibodies from an infection with the virus that causes COVID-19. However, there is a chance that a positive result means you have antibodies from an infection with a different virus from the same family of viruses (called coronaviruses). Note: Other coronaviruses cannot produce a positive result on a viral test for SARS-CoV-2.
- Having antibodies to the virus that causes COVID-19 may provide protection from getting infected with the virus again. But even if it does, we do not know how much protection the antibodies may provide or how long this protection may last. Confirmed and suspected cases of reinfection have been reported, but remain rare.
- Talk with your healthcare provider about your test result and the type of test you took to understand what your result means. Your provider may suggest you take a second type of antibody test to see if the first test was accurate.
- You should continue to protect yourself and others since you could get infected with the virus again.
- If you work in a job where you wear personal protective equipment (PPE), continue wearing PPE.
If you test negative
- You may not have ever had COVID-19. Talk with your healthcare provider about your test result and the type of test you took to understand what your result means.
- You could have a current infection or been recently infected.
- The test may be negative because it typically takes 1&ndash3 weeks after infection for your body to make antibodies. It&rsquos possible you could still get sick if you have been exposed to the virus recently. This means you could still spread the virus.
- Some people may take even longer to develop antibodies, and some people who are infected may not ever develop antibodies.
If you get symptoms after the antibody test, you might need another test called a viral test. Viral tests identify the virus in samples from your respiratory system, such as a swab from the inside of your nose.
Regardless of whether you test positive or negative, the results do not confirm whether you are able to spread the virus that causes COVID-19. Until we know more, continue to take steps to protect yourself and others.
Antibody (Serology) Testing for COVID-19: Information for Patients and Consumers
SARS-CoV-2 antibody (often referred to as serology) tests look for antibodies in a sample to determine if an individual has had a past infection with the virus that causes COVID-19. COVID-19 antibody tests can help identify people who may have been infected with the SARS-CoV-2 virus or have recovered from a COVID-19 infection.
At this time, researchers do not know whether the presence of antibodies means that you are immune to COVID-19 or if you are immune, how long it will last.
In people who have received a COVID-19 vaccination, antibody testing is not recommended to determine whether you are immune or protected from COVID-19.
Many antibody tests are currently in development or available for use to detect antibodies to SARS-CoV-2. However, not all antibody tests that are being marketed to the public have been evaluated and authorized by the FDA. For details on specific tests authorized by the FDA, see In Vitro Diagnostics EUAs.
On this page:
Antibodies and Antibody Tests: The Basics
Q: What are antibodies?
A: Antibodies are proteins made by the immune system to fight infections like viruses and may help to ward off future occurrences by those same infections. Antibodies can take days or weeks to develop in the body following exposure to a SARS-CoV-2 (COVID-19) infection and it is unknown how long they stay in the blood.
Q: Are antibody tests used to diagnose COVID-19?
A: No. An antibody test does not detect the presence of the SARS-CoV-2 virus to diagnose COVID-19. These tests can return a negative test result even in infected patients (for example, if antibodies have not yet developed in response to the virus) or may generate false positive results (for example, if antibodies to another coronavirus type are detected), so they should not be used to evaluate if you are currently infected or contagious (ability to infect other people).
Q: If antibody tests cannot be used to diagnose COVID-19, what tests are available for that?
A: Currently, there are two types of diagnostic tests for COVID-19:
- Molecular (RT-PCR) tests, which detect the virus' genetic material
- Antigen tests that detect specific proteins on the surface of the virus
Molecular and antigen tests can detect if you have an active coronavirus infection. If you test positive on either type of test, you should follow the CDC's guidelines to protect yourself and others.
Molecular and antigen tests are performed using samples taken mostly from the nose and throat using a long swab, or other respiratory specimens.
For more information on the different types of tests, see:
Understanding Antibody Test Results
Q: What does a positive antibody test mean?
A: If you have a positive test result on a SARS-CoV-2 antibody test, it is possible that you have recently or previously had COVID-19. There is also a chance that the positive result is wrong, known as a false positive. False positive tests may occur:
- Because antibody tests may detect coronaviruses other than SARS-CoV-2, such as those that cause the common cold.
- When testing is done in a population without many cases of COVID-19 infections. These types of tests work best in populations with higher rates of infection.
Q: Does a positive antibody test mean that I am immune to COVID-19?
A: A positive antibody test does not necessarily mean you are immune from SARS-CoV-2 infection, as it is not known whether having antibodies to SARS-CoV-2 will protect you from getting infected again. It also does not indicate whether you can infect other people with SARS-CoV-2.
Q: What does a negative antibody test mean?
A: A negative result on a SARS-CoV-2 antibody test means antibodies to the virus were not detected in your sample. It could mean:
- You have not been infected with COVID-19 previously.
- You had COVID-19 in the past but you did not develop or have not yet developed detectable antibodies. It is unknown if all infected individuals will develop a detectable antibody response.
- The result may be wrong, known as a false negative. This occurs when the test does not detect antibodies even though you may have specific antibodies for SARS-CoV-2.
There are several reasons why negative antibody test results do not indicate with certainty that you do not have or have not had an infection with SARS-CoV-2. For example, if you are tested soon after being infected with SARS-CoV-2, the test may be negative, because it takes time for the body to develop an antibody response. It is also unknown if antibody levels decline over time to undetectable levels.
Q: What do sensitivity and specificity mean in antibody testing?
- Sensitivity is the ability of the test to identify people with antibodies to SARS-CoV-2. This is known as the true positive rate. A highly sensitive test will identify most people who truly have antibodies with few people with antibodies being missed by the test (false negatives).
- Specificity is the ability of the test to correctly identify people without antibodies to SARS-CoV-2. This is known as the true negative rate. A highly specific test will identify people who truly do not have antibodies, with few people without antibodies being identified as having antibodies by the test (false positives).
FDA has included information regarding sensitivity and specificity expectations for SARS-CoV-2 serology tests in the EUA serology templates for commercial manufacturers and laboratories. For information on authorized serology test performance, see EUA Authorized Serology Test Performance.
Q: What does positive predictive value mean in antibody testing?
Positive predictive value is the probability that people who have a positive test result truly have antibodies. Positive predictive values for SARS-CoV-2 antibody tests are impacted by how common SARS-CoV-2 antibodies are in the population being tested at a certain time.
Predictive values are probabilities calculated using a test's sensitivity and specificity, and an assumption about the percentage of individuals in the population who have antibodies at a given time (which is called "prevalence" in these calculations).
The lower the prevalence, the lower the predictive value. This means that COVID-19 antibody tests with high specificity used in areas with low prevalence (small number of people that have SARS-CoV-2 antibodies) will have a positive predictive value lower than in an area with higher prevalence.
Low positive predictive value may lead to more individuals with a false positive result. This could mean that individuals may not have developed antibodies to the virus even though the test indicated that they had. If a high positive predictive value cannot be achieved with a single test result, two tests may be used together to help identify individuals who may truly be SARS-CoV-2 antibody positive.
Q: What if I get different results on two tests from two different laboratories? Which one should I believe?
A: The test results from different laboratories may vary depending on several factors such as the accuracy of the test itself and also how long it may take for your body to develop antibodies after you had the coronavirus infection, if you were in fact infected. For this and other reasons, you should always review your test results with your health care provider.
Practical Information on Antibody Tests: Who Needs Them, Where to Get Them
Q: Who can get an antibody test?
A: If you have questions about whether an antibody test is right for you, talk with your health care provider or your state and local health departments.
Q: There are many antibody tests available. How do I know if I need one?
A: Talk to your health care provider or your state or local health department to discuss whether antibody testing is right for you. Antibody testing requires a prescription from a health care provider.
Q: Where can I get an antibody test or a diagnostic test?
A: Antibody tests and diagnostic tests are available by prescription from a health care provider and may be available at local health care facilities and testing centers. Contact your health care provider or your local or state health department for more information.
Q. I received a COVID-19 vaccination. Do I need an antibody test to know if I am immune to COVID-19?
A: Antibody tests are not recommended to determine your level of immunity or protection from COVID-19.
If you have questions about whether an antibody test is right for you, talk with your health care provider or your state and local health departments.
Q: Will I be able to return to work without having an antibody test done?
A: The requirements for returning to work may be determined by your employer or your state and local governments. Ask your employer about your workplace's criteria for returning to work and any actions your employer will be taking to prevent or reduce the spread of COVID-19 among employees and customers. For additional information, see Interim Guidelines for COVID-19 Antibody Testing.
Watch the video: PCR Test For International Traveling. covid-19 test for international traveling. #shorts (August 2022).