Medical Fun Facts Episode 5: Positive and negative pathology tests

Apologies for the washing machine and dishwashing machine noise in the background.

One of the common misconceptions about pathology test results is that a positive result means you have a disease and a negative result means you don’t have a disease.

I’ve written elsewhere about this and in the blog post you’ll find links to a couple of posts I’ve written before.

This notion that we shouldn’t expect a positive result to mean a disease process is present is especially true for serology. It can also apply to nucleic acid amplification assays. Despite the claims of manufacturers and the theory of high specificity of PCR, it is not a panacea.

I’d like to cite a few examples:

Polymerase chain reaction

A common situation I come across is when a certain respiratory panel is used by one manufacturer and it cannot differentiate between rhinovirus and enterovirus. When the PCR result signals positive for this reaction, we still require a decent clinical and epidemiological history to make an accurate diagnosis.


In a similar vein, this chemistry instrument which is used to identify bacteria and yeasts in microbiology medical testing laboratories, cannot differentiate between Escherichia coli and Shigella species.


Cross reactions are common in serology. If we take for example, Rickettsial infections, we usually see many cross reactions. The patient is not going to be ‘positive’ for all these pathogens despite all the reactive serology results. We need to rely on the clinical features of the illness as well as pertinent epidemiological features.

While the result may be positive, it doesn’t mean the patient positively has the disease in question.

So just because your doctor says you have a positive test for something, it doesn’t mean you have the associated disease. This is true for many microbiology results and especially when the method of testing is an indirect form of diagnosis.

My final word is that tests used should be fit for purpose. A test should only be used for patients with a typical clinical presentation. Using a test for patients with nonspecific symptoms will likely give a higher proportion of false reactive rather than true reactive results. In this situation, it is important the referring medical practitioner and the specialist pathologist agree that testing in such situations is inappropriate and can do more harm than good.

Recommended reading

Diagnostic test limitations

Reactive versus positive serology

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