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Why do better diagnostic tests give false impressions of malaria control?

January 2, 2013 - 13:40 -- William Jobin

Epidemiological Nugget Number Two........Many malaria programs are slowly converting to better diagnostic methods for reporting the prevalence of malaria. However the numbers have to be carefully analyzed, because of the artifacts caused by changes in False Positivity of the better diagnostic tests. The most common diagnostic method used in Africa - and reported through WHO - is called Clinical Diagnosis. This method, relying on the clinical judgement of the health care worker, has a False Positivity Rate of about 80%. Yes, even 90% sometimes!.........Thus if the MOH annual statistics for 2011 reported 100,000 cases of malaria in the district, in fact there were only 20,000 infected people because the Clinical Diagnoses confuse all sorts of fevers with malaria.......... So what happens when a better diagnostic test is introduced, such as the Gold Standard - a microscopic exam of a stained blood slide? The blood slides give a False Positive Rate of only about 10%. So if blood slides were examined the next year, in 2012 - instead of the traditional Clinical Diagnosis - only 21,000 cases would have been reported from the district for 2012, giving a huge drop in reported cases from 100,000 cases in 2011. At first you would say - "What a Fantastic improvement! It must have been due to the wonderful efforts of our malaria program"..........However, if you correct for the lower rate of False Positives by the new test, you would find that there was no change in malaria prevalence ! It is simply a more accurate assessment of the true prevalence of malaria in the district.........So we have to learn to understand the reported numbers before we make any conclusions about progress in malaria control. We might have a very false impression, just because the diagnostic tests were improved. I call it the Paradox of Improved Diagnoses. The Paradox also applies to the Rapid Diagnostic Tests now being used in some places instead of the Blood Slides.

Comments

Submitted by Pierre Lutgen on

Does anybody have a paper describing clinical trials with "old" ACTs stored for one year in tropical climate?