Posts or Comments 23 May 2024

Diagnosis Bill Brieger | 26 Jul 2010 06:00 am

When negative is positive

rdt-algorithm-color2.jpgThe need for rapid diagnostic tests (RDTs) for malaria case management has never been more clear since of the publication by Gething et al. that only 43% of febrile episodes in malaria endemic countries are actually malaria. They did find that fever is a better indicator of malaria in areas of higher transmission, but in no situation can a clinician be confident that fever equates automatically with malaria.

Unfortunately, that confidence is more the rule than the exception.  In Burkina Faso, for example, health workers are taught to use the algorithm to the right in diagnosing malaria in patients greater that 5 years of age (at present there are not enough RDTs to use on children below 5 years).

In reality, health workers still provide artemisinin-based combination therapy (ACT) antimalarial drugs to most people who present with the classical symptoms of fever and headache.  Even when RDTs are used, negative tests are frequently treated with ACTs.

Health workers explain that during their training they were told that a negative test does not mean the person does not have malaria. While this is true to a very small extent, RDTs in current use are more that 90% accurate if stored and used properly. Gething’s results should cause health workers to think harder.

Earlier this month Bisoffi and colleagues reported that …

According to microscopy, in the rainy season versus the dry season, the prevalence of malaria infection in patients presenting at primary health care centres was three times higher in febrile patients and twice in non-febrile patients. In the dry season, only a small proportion of fevers were attributable to malaria.

dscn8925a.JPGThe Burkina Faso research found seasonal variations in the sensitivity and specificity of the RDTs, but concluded for adult patients that, “RDTs appear to be most useful during the low transmission season: a negative test safely excludes malaria and would avoid most unnecessary treatments, if prescribers are convinced to rely on the negative result.” Health workers need to hear this information.

Other concerns about positive predictive value of RDT use in Burkina Faso, and people should read the article by Bisoffi, but the take home lesson is that a negative result can be trusted and is an important guide to using expensive stocks of ACTs more judiciously.

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