Tag Archives: hepatitis A

“The Hit Parade”

The Art of Microbiology, , , ,

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

 

“Over the top testing….Hepatitis A”

The Science of Microbiology, virology, , ,

Over the next few weeks I will do a few articles on micro-organisms where I believe “overtesting” to occur and where there is ample scope to reduce the amount of tests without missing diagnoses.

The first of these tests I would like to look at is Hepatitis A.

There are a few reasons why Hepatitis A is over-tested which I will describe as follows:

  • Hepatitis A is essentially a non-endemic disease in NZ, and is becoming so in many other developed countries. Occasionally there are clusters associated with a known imported case, but rarely in a non-traveller without a contact history. Requestors are not always aware of this.
  • Hepatitis A almost always causes an acute illness with significant derangement of Liver Function Tests (LFTs) with the liver enzymes ALT and AST>100). Yet many times Hepatitis A is requested for the investigation of chronic mild derangement of LFTs.
  • Hepatitis A is often requested as part of a “Hepatitis Screen” which may comprise a panel of Hepatitis A, B & C, even though they usually have completely different clinical presentations.

So what is the solution:

Firstly I am not a big fan of “Hepatitis Screens” and I think requestors should be made to request these tests individually.

Secondly, I would support rejection of the request for Hepatitis A in countries of low endemicity unless one of the following apply:

  • The patient has a travel history.
  • The patient has significant derangement of LFTs. (ALT or AST >100)

I think such measures (if not already used) applied in countries with low endemicity for Hepatitis A would see their testing volume for this test decrease by at least 2/3, and probably more, without adversely affecting the sensitivity of the test. On the contrary, the specificity of the test should increase due to the relatively higher prevalence of disease in the tested cohort.

In the era where health budgets are getting tighter, we need to start thinking seriously about how to test smarter…..Hepatitis A is a great place to start.

Michael