It is really useful to know your “hit rate” for any particular laboratory test. I.e. the average number of tests required to produce one positive result in your tested population.

So what is an acceptable hit rate?

Well, this depends on the severity of the disease you are looking for, how easy it is to treat, and also how easy it is to clinically suspect based on the symptomatology.

Low hit rates are generally more acceptable for serious diseases. e.g. our hit rate for gonorrhoea based on our NAAT is roughly one in a hundred. For HIV it is approximately one in several hundred. But the consequences of missing these diseases are very serious, so the low hit rate can be rationalised.

Other low hit rates are not so easy to explain away. Our hit rate for Hepatitis A is approximately 1 in a 1000. This is a disease which is rarely serious and also has some distinctive epidemiological and clinical characteristics.

After rotavirus vaccination commenced in NZ last year the rotavirus hit rate has dropped to less than 1 in a 100. Rotavirus generally does not cause particularly serious illness in the relatively affluent NZ population.

Detection of Trichomonas in genital specimens also has a local hit rate of approximately one in a hundred. Move into the older age groups and this hit rate drops to 1 in a 1000. Trichomonas infection is not a particularly severe disease in the bigger scheme of things.

Hit rates are not just important from an economic/efficiency point of view. There is an important quality issue here as well. Unless you are using a highly specific test, low hit rates often lead to poor positive predictive value, and the dreaded false positives….

So what can we do about it?

If the hit rate is unjustifiably low, one can try and focus the testing on patients more likely to have the disease. For example, Hepatitis A testing could be restricted to patients with a significant increase in liver function tests. Along the same lines routine Trichomonas testing could be restricted to certain age categories most likely to have the disease.

With more sophisticated IT capability these days, one can often determine hit rates for different tests within a few minutes with a simple computer search. Thus looking at hit rates for different tests is becoming an increasingly important aspect of laboratory management.

 Have a think about a few of the tests that you perform in your local laboratory. What are your hit rates like? And is there anything that you can do about them?

Michael