We’ve been treated to a series of accounts concerning Ebola tests of subjects that produced negative results.  Ebola test subjects with purportedly negative results have been drawn from places such as London’s Gatwick airport, New York City, and Ohio   With respect to the New York subjects, six in total have been tested, and Breitbart reports that this was withheld from the public.

It would be surprising if additional subjects are not tested in the United States in the future.

Unfortunately, whether by dint of stupidity or design, MSM sources have not conveyed certain very important information about the nature of scientific assays for disease.

This has left many in the general public with the misguided impression that when Ebola tests come back negative, we can be certain that the subject does not have the malady.

Scientific assays vary in terms of their sensitivity and specificity.

The sensitivity of an assay measures the likelihood that the assay produces a positive result when the subject is in fact afflicted (true positive).  The specificity of an assay measures the likelihood that the assay produces a negative result when the subject is in fact not afflicted (true negative).

Each of these in turn has a complement.  The complement of true positives is false positives, while the complement of true negatives is false negatives.  False negatives are results where the subject is afflicted but the test says they’re not. 

Since false negatives are the complement of specificity, if the specificity of a test is 95%, the false negative rate is 5%.

Clearly, the prospect of false negatives in an Ebola setting is one of grave concern.

One test for Ebola, the indirect fluorescence assay, is known to have a rather low specificity, and therefore a rather high false negative rate.  PCR testing has also been known to miss cases of affliction. 

While there are other tests for Ebola, we know that there is at least one July case in which an initial negative test proved wrong when the subject later succumbed to Ebola. 

Therefore, meaningful questions to ask are:

1. How many testing cases will there be?
2. What will the responses to negative results be?
3. What test or tests is/are subjects receiving?
4. Crucially, what is the false negative rate of the test/tests?

Statistically, the more false negative test results there are, the more likely it becomes that at least one of them is wrong.  How likely it is that at least one is wrong is of course a function of the false negative rate as well as the prior probability of the subject’s being afflicted, which of  course turns on such factors as travel history.

Dr. Jason Kissner is associate professor of criminology at California State University, Fresno.  You can reach him at crimprof2010@hotmail.com.