MFF0066: Tweaking diagnostic tests


MFF Tweaking diagnostic tests Gary Lum

It’s Thursday 18 May 2017. It’s pay day for me so I’m a happy camper.

I was recently asked how can diagnostic tests be made more sensitive.

Not long ago in episode 52 I discussed the limitations of diagnostic tests. It’s probably worth going back to that episode and listening again or reading the show notes.

Sensitivity is simply defined as the ability of a test to detect a true positive. We want our diagnostic tests to be highly sensitive so that when we receive a positive result we can be confident the result reflects the presence of disease. Likewise, we want our tests to be specific too. Specificity is the ability of a test to exclude a true negative. That means we want a negative result to really mean an absence of the disease. What we don’t want though are tests that are so sensitive that we cannot trust the result, or more accurately, we cannot interpret the result adequately in the context of the patient in front of us. For example, PCR for the diagnosis of clinical chlamydia as well as screening sexually active asymptomatic people as part of a sexual health screen is regarded as the gold standard. The problem is that PCR cannot be used for test of cure because it is too sensitive. It will detect the nucleic acid from dead bacteria that are yet to be removed by neutrophils and other cellular inflammatory mechanisms.

Unfortunately, sensitivity and specificity exist in a state of tension. When we do things to crudely increase sensitivity we may end up diminishing specificity so the extra positive test results we see may also mean we lose specificity and increase the number of false positive results. It’s a tension that must be handled carefully and it is why modifications to tests should not occur in isolation. Any test that is modified, especially commercial tests, become in-house modifications and require validation and verification that the change in method reflects reality.

In the last episode of Medical Fun Facts, I hinted on changes occurring in the world of medical microbiology with advances in technology. For a couple of centuries, the gold standard has been culture. If we can grow the pathogen, identify it and characterise it we have certainty. Certainty can never be placed in serology and uncertainty remains a factor in molecular microbiology only because now we cannot get a handle on viability, quantity and context like we can when we culture pathogens.

It’s easy to cheat an assay to make it more sensitive. For serology, you can lengthen incubation periods, reduce the stringency of interpretation and add cross reacting antigens to the test substrate. In PCR, the easy cheat is to increase the number of reactions or cycles. If you increase the incubation period of an assay long enough or increase the cycle numbers enough, you can make any test positive. These are of course extreme examples, but it demonstrates why test validation and verification are vital components to high quality medical testing. It’s why in Australia and like-minded nations, the standard for medical testing focuses on quality and competency.

If you disagree with anything in these podcasts or if you would like to voice a different view, please feel free to write a comment. If I have said something incorrect I welcome correction. Please also feel free to share your comments on social media.

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