4

A recent RT-PCR test for SARS-CoV-2 from a private laboratory lists the test's sensitivity and specificity as 100%. The test certificate lists this next to the result:

Ref. value/Норма Negative (sensitivity 100%, specificity 100%) Отрицательный (чувствительность 100%, специфичность 100%)

When I called to ask about false negative and positive rate, they confirmed that the test is 100% accurate, referring to the high number of cycles they run (42 I believe).

This sounds fishy to me, as surely one can at best claim something like ">99%" or similar, but not 100%. Other PCR tests I have done don't list the sensitivity and specificity, so unfortunately I can't compare.

Can a PCR test really claim 100% sensitivity and specificity, or has this been dishonestly rounded up?

Antigen tests seem to have a sensitivity of about 65.3% and a specificity of 99.9%, for example.enter image description here

2
  • 2
    The problem isn't lack of prior research. You're asking us to address a claim we can't see for ourselves. If the only way you have of showing us the claim is doing a screen shot and redacting your personal information then that's what you need to do. We need to see the company making the claim and how they make it.
    – Carey Gregory
    Jan 11 at 5:09
  • 1
    The claim is "100% sensitivity, 100% specificity" for a RT-PCR test for SARS-CoV-2. I've added a screenshot of the PDF without personal details. I'm really not sure what more info you get from the screenshot other than what I have already described in the original post.
    – dk2ax
    Jan 11 at 5:44

1 Answer 1

7

A diagnostic test should never claim 100% sensitivity or specificity.

So we are all on the same page, let's review what sensitivity and specificity actually mean. According to the US Food and Drug Administration, the organization which regulates medical diagnostic testing in the US:

In studies of diagnostic accuracy, the sensitivity of the new test is estimated as the proportion of subjects with the target condition in whom the test is positive. Similarly, the specificity of the test is estimated as the proportion of subjects without the target condition in whom the test is negative.

As an FDA guidance document notes:

These are only estimates for sensitivity and specificity because they are based on only a subset of subjects from the intended use population; if another subset of subjects were tested (or even the same subjects tested at a different time), then the estimates of sensitivity and specificity would probably be numerically different. ... This type of uncertainty decreases as the number of subjects in the study increases.

Sensitivity and specificity estimates (and other estimates of diagnostic performance) can be subject to bias. Biased estimates are systematically too high or too low. Biased sensitivity and specificity estimates will not equal the true sensitivity and specificity, on average. Often the existence, size (magnitude), and direction of the bias cannot be determined. Bias creates inaccurate estimates.

One of the key issues is that of the "reference standard", that is what actually defines that a tested condition is actually present or absent. The US Centers for Disease Control and Prevention notes:

The “gold standard” for clinical diagnostic detection of SARS-CoV-2 remains laboratory-based [nucleic acid amplification tests].

Thus, the definition of sensitivity and specificity for a new nucleic acid amplification test (NAAT) is circular, with the reference standard being another NAAT.

As FDA guidance continues:

Two sources of bias that originally motivated the development of this guidance include error in the reference standard and incorporation of results from the test under evaluation to establish the target condition.

These sources of bias and uncertainty are why reputable testing companies don't claim 100% sensitivity or specificity. The reality is that ">99% sensitivity" is also almost certainly overly optimistic and does not incorporate all of the possible sources of bias described in depth by the FDA.

One final note is that a diagnostic test can have 100% sensitivity if it reports all tests positive or 100% specificity if it reports all tests negative. Obviously, such a test has no clinical utility.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.