Tag Archives: sensitivity

“First do no Harm”

The Science of Microbiology, , ,

We need to remember that there is always a patient at the receiving end of a test result, and that, wherever possible, the result should have a positive bearing on the management of the patient.

However some results can cause potential harm to the patient. How can this be?

A classic example of this is the urinary antigen test for Legionella pneumophilia. At first glance, this test, easily obtained and giving a result within 20 minutes, seems to do the job just fine.

But all is not quite as it seems….

The urinary antigen test for L pneumophilia only picks up serogroup 1. What about all those other serogroups of L. pneumophilia? It also has a sensitivity of about 70-75%. So 25-30% of true L pneumophilias (serogroup 1) are being missed. And finally, in NZ at least, the vast majority of Legionella disease is caused by Legionella longbeachae, for which the urinary antigen test is no good whatsoever.

The big worry is that a negative urinary antigen test in a sick patient will lead to antibiotic cover for Legionella being discontinued, and I have seen this happen quite a few times over the years, even in the ICU setting. We see this happen because the recipient of the result often takes it for gospel, without considering the whole picture of both the result and the patient, the sensitivity, specificity, PPV, clinical picture etc.

With the increasing availability of PCR testing for Legionella, maybe the days of urinary antigen tests for Legionella pneumophilia serogroup 1 are numbered, in much the same way as rapid turnaround PCR for RSV and Influenza are gradually pushing out their rapid antigen equivalents for these infections.

It is always a concern when a laboratory test can potentially do more harm than good, and I think this sometimes happens with urinary antigen for Legionella.

We have to be brave enough to both embrace and introduce new tests, and at the same time get rid of the old…

Michael

“Don’t believe everything you read…”

Confessions of a Microbiologist, virology, , , , ,

I know it is difficult to believe but I was doing a little bit of background reading recently on Monospot tests (looking for heterophile antibodies to Epstein Barr Virus). On reading a guideline I came across this statement…

“The presence of heterophile antibodies in a symptomatic adolescent or young adult has a sensitivity of approximately 90%, and specificity of almost 100% for glandular fever.”

Now the question I have is what is wrong with this statement?

The answer is that sensitivity and specificity are functions of the test itself. Different tests for the same disease from different manufacturers may have different sensitivity and specificity.

However once you start applying the test to population cohorts such as symptomatic adolescents, then you need to start talking in terms of positive and negative predictive value.

….and the paradox is when you use a test such a Monospot, with a sensitivity of approximately 80%, in a high prevalence population such as symptomatic adolescents, then your negative predictive value will be relatively lower than in a low prevalence cohort, as there will be a significant amount of people who will test negative who actually have the disease (false negatives).

Sometimes we need to think about the science behind the statements in the guideline and make sure it makes sense in our heads.

Don’t believe everything you read, (especially when it is written by me!)

Michael

For a really nice presentation on sensitivity, specificity, PPV etc click here (5-10 minute read)

“An imperfect test”

The Science of Microbiology, , ,

Influenza_virusAs the Influenza season gets underway here in the Southern Hemisphere, I am thinking here about the Rapid Antigen assays that are available for Influenza diagnosis. These are generally very specific, ie a positive result really means a positive. However their sensitivity is not great (70% at best, meaning that a negative result does not always mean that the patient does not have influenza).

This would be ok if it could somehow be ensured that the result was interpreted correctly by the requestor every time.

However even when the results go out with the standard comments expressing the limitations of the test as described above, there is still a tendency for a negative result to be interpreted as the patient not having influenza and the patient to be taken out of isolation etc etc.

In our laboratories we were fortunate enough to get funding for Influenza PCR, so even though the assay takes a bit longer, we know that the sensitivity is much higher than the rapid Influenza tests so a negative is more likely to actually mean a flu free patient.

The rapid test for influenza is only one example amongst many laboratory tests that are “imperfect”.

Given that the result of the rapid Influenza antigen tests are available in 15-20 minutes, it can be a valuable test when in the right hands. Unfortunately, in the wrong hands, it becomes a liability…

Michael

On a completely different matter altogether, I have added some MCQs on Pasteurella multocida to the website.