I'm a simple man and I'm getting lost in all the specifics of the antibody tests so could you guys please answer a question for me: Which antibody tests are reliable? I'm in Houston and would like to give it a go. If the board docs have suggestions, I'm all ears and greatly appreciative.

The Roche test has a specificity of 99.8% and sensitivity of 100%. The Abbott test has a specificity of 99.5%

What does sensitivity mean? That it's 100% sensitive to sars-cov-2?

Crocs said:

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What does sensitivity mean? That it's 100% sensitive to sars-cov-2?

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Sensitivity means if it is negative it is absolutely negative!

Do you know if the test that Quest Diagnostics is offering is either of these?

HERE is a good concise article that discusses Sensitivity, Specificity, PPV and NPV.

From Quest website:

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Quest Diagnostics is committed to providing testing that is high quality and reliable. Most of our antibody testing is performed on a test platform from Abbott that received FDA emergency use authorization (EUA) on April 26. We also use a platform from EUROIMMUN, a PerkinElmer company, that received FDA EUA today. We have also performed verification studies on these platforms as a further check on quality.

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I would tap the brakes on getting antibody tested just yet. The science is catching up, but the FDA has yet to evaluate the tests on the market. Our ability to make a test at the moment is much greater than our understanding of what those antibodies being tested for mean. Additionally, there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection. We have yet to determine whether or not the antibodies detected are protective.

Douph said:

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I would tap the brakes on getting antibody tested just yet. The science is catching up, but the FDA has yet to evaluate the tests on the market. Our ability to make a test at the moment is much greater than our understanding of what those antibodies being tested for mean. Additionally, there is currently no evidence that people who have recovered from COVID-19 and have antibodies are protected from a second infection. We have yet to determine whether or not the antibodies detected are protective.

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I think Reveille did post a study that's shows you do get immunity.

Thank you, Dr. Rev. Now I just need to find a place in Houston that offers that test.

RandyAg98 said:

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HERE is a good concise article that discusses Sensitivity, Specificity, PPV and NPV.

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I think it is important for people to read this before going out and spending money on antibody testing (ignoring the question of whether immunity to COVID is certain or whether these tests have cross-reactivity with non-novel CV strains). If you are in an area of low disease prevalence, as most of Texas is, the positive predictive value plummets making the test nearly worthless, especially when coupled with questions that are being raised regarding the test, along with with delays from disease onset to testing positive for antibodies on these tests.

To give an example. If the COVID prevalence of Houston was 1% (which is a reasonable estimation), and we trusted the sn,sp of 100%, 99.5% (which I have doubts about the specificity claim), that would mean that the positive predictive value of a positive test would be 0.67 or a 33% false positive rate, which is high enough to render the test useless in that population, especially given that our main concern is to limit the number of false positives with this test due to concern for an increase in riskier behavior in those that believe they are now immune with a positive result.

Unfortunately we just don't have enough evidence yet to recommend this to people. However, if you are dead-set on getting an antibody test, or it will give you some peace of mind, just make sure you at least understand that a positive result does not necessarily mean you ever had COVID (unless your pre-test probability was high due to known exposure history or had travel to a high prevalence area), and you should not presume to have immunity.

Dr. Not Yet Dr. Ag said:

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RandyAg98 said:

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HERE is a good concise article that discusses Sensitivity, Specificity, PPV and NPV.

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I think it is important for people to read this before going out and spending money on antibody testing (ignoring the question of whether immunity to COVID is certain or whether these tests have cross-reactivity with non-novel CV strains). If you are in an area of low disease prevalence, as most of Texas is, the positive predictive value plummets making the test nearly worthless, especially when coupled with questions that are being raised regarding the test, along with with delays from disease onset to testing positive for antibodies on these tests.

To give an example. If the COVID prevalence of Houston was 1% (which is a reasonable estimation), and we trusted the sn,sp of 100%, 99.5% (which I have doubts about the specificity claim), that would mean that the positive predictive value of a positive test would be 0.67 or a 33% false positive rate, which is high enough to render the test useless in that population, especially given that our main concern is to limit the number of false positives with this test due to concern for an increase in riskier behavior in those that believe they are now immune with a positive result.

Unfortunately we just don't have enough evidence yet to recommend this to people. However, if you are dead-set on getting an antibody test, or it will give you some peace of mind, just make sure you at least understand that a positive result does not necessarily mean you ever had COVID (unless your pre-test probability was high due to known exposure history or had travel to a high prevalence area), and you should not presume to have immunity.

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WH has a document that was recommending retests of any positive results. The PPV goes up dramatically if two positives. Basically, the probability of both tests showing up false positive for the same individual is really small.

I had a sputum test for the virus and a blood test for the antibodies.

What I'm most curious about is the chance for false positives with the sputum test.

The reason for that is that when I am tested in a week or so to see if I am still infected, I wonder whether whether I will continue to be in isolation for a false positive. I don't mind staying isolated for a true positive, but I would hate to stay isolated another couple of weeks or more for a false positive.

eric76 said:

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I had a sputum test for the virus and a blood test for the antibodies.

What I'm most curious about is the chance for false positives with the sputum test.

The reason for that is that when I am tested in a week or so to see if I am still infected, I wonder whether whether I will continue to be in isolation for a false positive. I don't mind staying isolated for a true positive, but I would hate to stay isolated another couple of weeks or more for a false positive.

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I wouldn't think false positive is any higher for sputum than nasal pharyngeal swab. The concern would be more of a false negative, but since there is a move to at home tests, the false negatives seem to not be a huge problem. In other words, there seems to be enough virus in a easy to self administer (cheek swab, sputum, etc) to detect without having to stick a swab all the way up your nose.

If you have a false positive, it is unlikely the second test would also be a false positive.

That said, there are plenty of instances where the rtPCR tests are turning up positive while a person is no longer infectious. This isn't technically a false positive in that the test is actually detecting viral RNA, it just can't distinguish between live virus and virus fragments, nor does a qualitative (Yes/No) answer provide an indication of viral load and amount of shedding.

BiochemAg97 said:

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That said, there are plenty of instances where the rtPCR tests are turning up positive while a person is no longer infectious. This isn't technically a false positive in that the test is actually detecting viral RNA, it just can't distinguish between live virus and virus fragments, nor does a qualitative (Yes/No) answer provide an indication of viral load and amount of shedding.

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That's an excellent point. I've been considering that to be false positives as well.

Dr. Not Yet Dr. Ag said:

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RandyAg98 said:

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HERE is a good concise article that discusses Sensitivity, Specificity, PPV and NPV.

---

I think it is important for people to read this before going out and spending money on antibody testing (ignoring the question of whether immunity to COVID is certain or whether these tests have cross-reactivity with non-novel CV strains). If you are in an area of low disease prevalence, as most of Texas is, the positive predictive value plummets making the test nearly worthless, especially when coupled with questions that are being raised regarding the test, along with with delays from disease onset to testing positive for antibodies on these tests.

To give an example. If the COVID prevalence of Houston was 1% (which is a reasonable estimation), and we trusted the sn,sp of 100%, 99.5% (which I have doubts about the specificity claim), that would mean that the positive predictive value of a positive test would be 0.67 or a 33% false positive rate, which is high enough to render the test useless in that population, especially given that our main concern is to limit the number of false positives with this test due to concern for an increase in riskier behavior in those that believe they are now immune with a positive result.

Unfortunately we just don't have enough evidence yet to recommend this to people. However, if you are dead-set on getting an antibody test, or it will give you some peace of mind, just make sure you at least understand that a positive result does not necessarily mean you ever had COVID (unless your pre-test probability was high due to known exposure history or had travel to a high prevalence area), and you should not presume to have immunity.

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So while I don't dispute the point you are making about being cautious of how a person uses the results, unless I misunderstand how to use specificity, your math assumes that every person in Houston gets the antibody test. The numbers get better if you assume only people who think they had it are getting antibody tested. For instance, in this self selective scenario, let's say for every person that had it and gets tested, 9 falsely assume they had it and also get tested. Now if you have a positive result there is only a 4% chance of that being a false positive. Which is much more useful to the individual.

BlackGoldAg2011 said:

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Dr. Not Yet Dr. Ag said:

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RandyAg98 said:

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HERE is a good concise article that discusses Sensitivity, Specificity, PPV and NPV.

---

I think it is important for people to read this before going out and spending money on antibody testing (ignoring the question of whether immunity to COVID is certain or whether these tests have cross-reactivity with non-novel CV strains). If you are in an area of low disease prevalence, as most of Texas is, the positive predictive value plummets making the test nearly worthless, especially when coupled with questions that are being raised regarding the test, along with with delays from disease onset to testing positive for antibodies on these tests.

To give an example. If the COVID prevalence of Houston was 1% (which is a reasonable estimation), and we trusted the sn,sp of 100%, 99.5% (which I have doubts about the specificity claim), that would mean that the positive predictive value of a positive test would be 0.67 or a 33% false positive rate, which is high enough to render the test useless in that population, especially given that our main concern is to limit the number of false positives with this test due to concern for an increase in riskier behavior in those that believe they are now immune with a positive result.

Unfortunately we just don't have enough evidence yet to recommend this to people. However, if you are dead-set on getting an antibody test, or it will give you some peace of mind, just make sure you at least understand that a positive result does not necessarily mean you ever had COVID (unless your pre-test probability was high due to known exposure history or had travel to a high prevalence area), and you should not presume to have immunity.

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So while I don't dispute the point you are making about being cautious of how a person uses the results, unless I misunderstand how to use specificity, your math assumes that every person in Houston gets the antibody test. The numbers get better if you assume only people who think they had it are getting antibody tested. For instance, in this self selective scenario, let's say for every person that had it and gets tested, 9 falsely assume they had it and also get tested. Now if you have a positive result there is only a 4% chance of that being a false positive. Which is much more useful to the individual.

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Individually, your chance of being false positive is the false positive rate of the test, so if the test has a 5% false positive rate and you come back positive, then individually you have 95% chance of being a true positive and a 5% chance of being a false positive. We are now getting some tests that are 99+% accurate. And the other way to improve the positive predictive value is to run a 2nd test when the first comes back positive.

As a population, if the negative people heavily dominate, then you can get a situation where a big chunk of the positive results are false. If we are self selecting for those most likely to have antibodies, then we enrich for true positives. However, much of the antibody testing is being done as a population survey to test a broad population, so selecting for those most likely to have it is not desirable. When you see headlines that say X% are antibody positive, you need to consider if it was a selective sample (healthcare workers or meat packing plant) vs a population survey which give very different percentages.

Also recall that the tests of virus resulted in ~10% positives (which is now around 5-6% positives in Texas) so we generally suck at guessing who has/had it.

I agree that I don't think the antibody testing for general population is quite ready for prime time yet due to the low infection rate In the population.

I think it could be very beneficial to people with known repeat exposure such as healthcare workers. I had mild symptoms a few weeks ago (low grade fever, headache for a few days, chills) but no significant cough or shortness of breath. I happened to be off work that week and when I came back, employee health didn't think I was worthy of a nasal swab. Two of my coworkers had very similar symptoms on the same days, one was turned away at the door due to low grade fever, was tested with a swab and was positive. All of us had worked at one nurses station the week before.

I'm getting the Abbott antibody test today out of curiosity. Not going to change much in terms of me staying away from my older relatives and carefully using PPE, but may give me a little bit of peace of mind.

We also really suck in how we do the nasal swabs. Patients scream/yell, and pull away pretty often. I'm sure a lot of collectors are sending swabs that aren't getting to the nasopharynx. I really wish there was a blood test for active infection that we could draw along with other routine labs.

Moxley said:

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We also really suck in how we do the nasal swabs. Patients scream/yell, and pull away pretty often. I'm sure a lot of collectors are sending swabs that aren't getting to the nasopharynx. I really wish there was a blood test for active infection that we could draw along with other routine labs.

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There have been some efforts towards a home collection. I think generally a cheek swab or nasal swab without going all the way back. Seems to suggest there is enough virus in easy to reach spaces.

Obviously, blood could be less risk for the healthcare worker since you can keep a mask on the patient, but I think virus would present in blood later than nose/mouth.

Moxley said:

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We also really suck in how we do the nasal swabs. Patients scream/yell, and pull away pretty often. I'm sure a lot of collectors are sending swabs that aren't getting to the nasopharynx. I really wish there was a blood test for active infection that we could draw along with other routine labs.

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This is something I've been concerned about from the very beginning when seeing how low the positive test %age has been even when tests were strictly rationed to people most likely to to actually have the disease.

Mordred said:

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Moxley said:

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We also really suck in how we do the nasal swabs. Patients scream/yell, and pull away pretty often. I'm sure a lot of collectors are sending swabs that aren't getting to the nasopharynx. I really wish there was a blood test for active infection that we could draw along with other routine labs.

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This is something I've been concerned about from the very beginning when seeing how low the positive test %age has been even when tests were strictly rationed to people most likely to to actually have the disease.

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The reality is there are a lot of diseases that cause the same respiratory symptoms. If you look at flu tests, early in the season, there is also a low % of positive tests.

In NYC the virus actually had a relatively normal cycle (spiked before social distancing and lockdown went into effect) and the % positive rate went much higher, closer to 50% or more.

I attribute this pattern to something like a basic level of respiratory symptoms from all sources. Then in flu season, we see a spike in flu which then dominates the cause. With COVID, we had a similar baseline and then COVID spies and dominates the cause. Only in much of the country, COVID didn't really spike that far above respiratory disease baseline, so we don't see the positive rate increase.

Nm.

My antibody test was negative on the Abbott test. A little bit surprised but not really.

I'm glad I went though. Went ahead and got some other labs done since I haven't seen a physician in a few years after my insurance changed. AST/ALT, bilirubin all high and absolute monocyte count is also high. Looks like I'll be finding me a doctor pretty quick.

If my wife and I both get tested and both are positive for antibodies, wouldn't that increase the confidence that neither of us got a false positive?

Quest antibody test negative for me, but not surprising. GP is offering with annual bloodwork.

Moxley said:

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We also really suck in how we do the nasal swabs. Patients scream/yell, and pull away pretty often. I'm sure a lot of collectors are sending swabs that aren't getting to the nasopharynx. I really wish there was a blood test for active infection that we could draw along with other routine labs.

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A study last week came out showing saliva collection just as good as nasal swabbing.

And another some weeks back now that showed swabbing just the interior of the nose 95% as effective as the deep swabbing.

Alternatives are out there.