Tag Archives: prevalence

“Think twice”

What have the following got in common?

  • E.coli resistant to nitrofurantoin
  • E.coli resistant to fosfomycin
  • Haemophilus influenzae resistant to ciprofloxacin
  • Group B streptococcus resistant to penicillin.
  • Coagulase negative staphylococci resistant to vancomycin
  • Candida albicans resistant to fluconazole

In my area of the world anyway (New Zealand), the percentage resistance rates of the above micro-organism/antimicrobial combinations is less than 1%. i.e. the prevalence is very low.

And because the prevalence is very low, unless your susceptibility testing methods are very specific, the positive predictive value of the result will also be very low. Thus , there will generally be a large number of false positives amongst such results.

Such a result should therefore automatically trigger a double check of everything, with a close look at the audit trail leading to the result. In some circumstances, repeating the test or sending the isolate to a reference laboratory may be the best option even if the result looks genuine.

We always need to be very careful when reporting low prevalence results, because even though we would like them to be, our tests are generally not perfect…




“The Shopping List”

shopping list

Take the following hypothetical list of laboratory tests:

  • FBC
  • U&Es
  • LFTs
  • Thyroid Function tests
  • Hepatitis A, B & C screen
  • HIV
  • Treponemal screen
  • ANA
  • Coagulation screen
  • Complement
  • Autoantibody screen
  • EBV serology
  • CMV serology
  • Urine culture
  • MRSA screen
  • Serum lipids

In laboratories across the world, this kind of “shopping list” request form coming into the laboratory is seen all too frequently, and often without any clinical justification or rationale included.

Add up the costs of these tests and you have a total of several hundred dollars, maybe even over a thousand. A CT scan would be roughly an equivalent cost, and you would be hard pushed to get one of those without justifying exactly why you wanted it….

There may of course be a very valid reason for ordering all the tests above, although it is difficult to think of a clinical syndrome which would justify everything in the list above.

Such a list may be done as a “wellness screen”, but whether you believe in wellness screening or not, there are tests in here (EBV, CMV, urine culture for starters) which should never be part of any wellness screen laboratory testing.

Such a list may be done as a “fishing expedition” when the patient presents to the clinician with vague or ill defined symptoms. The problem is that the prevalence rates for most of these tests in such patient populations are going to be very low with the consequent problems of low positive predictive value and false positive results.

Personally I don’t believe that shopping list requests such as the above that are sent into the lab by clinicians should be permitted unless there is a clear rationale on the request form detailing why so many tests have been requested in the first place…..

Courtesy, respect, and common sense.


Of course if your laboratory is on a fee for service funding arrangement you might welcome “shopping lists” or at least turn a blind eye to them…. Personally however I have never been a great fan of shopping!


“Over the top testing….Hepatitis A”

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.