The limitations of testing

I READ with interest about calls to the Malaysian government for more testing by some quarters. While this may be well-intentioned, we should be aware of the limitations of tests, particularly the specificity and sensitivity of a test. The specificity of a test is the ability of a test to label a patient without disease, negative, while sensitivity is the ability of a test to label a patient with disease, positive.

It is well established that the real time polymerase chain reaction (qPCR) test has a higher sensitivity and specificity than rapid test kits (RTK). However, even qPCR does not have a sensitivity or specificity of 100%. Factors affecting sensitivity or specificity include the skill of the technician, quality of sample collected and conditions of the machine. A test with a sensitivity or specificity above 95% is widely considered acceptable.

Here comes the problem. Assuming a million tests were conducted, the prevalence is 1% and the sensitivity of the test is 95%. This means there will be 500 positive cases (1,000,000 x 0.01 x 0.05) which we fail to detect. This will give us a false sense of security and these 500 patients will go about their business as usual and start spreading the virus to those around them. We all know how much havoc one infected individual can cause let alone 500.

Again, assuming a million tests were conducted, the prevalence is 1% and the specificity of the test is 95%. There will be 49,500 patients (1,000,000 x 0.99 x 0.05) who are actually negative but due to statistical error, we consider them positive. This will certainly skew national infection counts. From an individual’s point of view, imagine the anguish and suffering he or she experiences, and it will be worse if he or she is isolated alongside patients who are actually positive.

One way of mitigating this potential pitfall is to perform the tests multiple times. However, from sample collection to RNA extraction to qPCR takes half a working day. Imagine doing this multiple times on a test subject, how many man-hours will be used up? And how do you deal with conflicting results? What if two tests are negative and one test positive? This presents more problems which are even harder to solve. Most importantly, precious Health Ministry resources will be diverted into testing while very little is achieved in terms of patient management.

While testing may have guided our government’s policy in the early stages, in our current situation, I am of the opinion that our government is on the right track in emphasising vaccination. All other intervention methods that might divert resources away from vaccination will be counter productive to our fight against Covid-19.


PhD candidate


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Covid-19 , testing , vaccination , false negatives


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