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How Do We Best Diagnose Malaria in Rural India?

03.03.2018 Posted By : Dr. Ashok Rattan Share :

Traditional teaching mandates that diagnosis of malaria cannot be excluded by a single blood film. If the first blood film is negative, it is recommended that serial blood films be obtained 6 to 12 hours apart for 48 hours until three films have been found to be negative. (1) The rationale is that parasitemia may be cyclic, at low levels in the early stages of illness or sequestered in the case of Plasmodium falciparum and therefore missed if only one film is prepared. In addition, detection by light microscopy is operator and laboratory dependent, which may be overcome by serial examination.

The advent of rapid diagnostic tests (RDTs) for malaria is an important advance. Multiple immunochematographic tests, incorporating a number of different parasite antigens and produced by many different manufacturers are now available. RDTs for malaria are based principally on the detection of one of three antigens, a histidine rich protein -2 (HRP-2), lactate dehydrogenase (LDH) and aldolase. HRP-2 is only expressed by P.falciparum. LDH tests are generally less sensitive than HRP-2 tests. However, LDH tests may be more specific, because the antigen circulates only briefly after eradication of infection. Genus specific aldolase based RDTs offer pan species diagnosis of malaria and when combined with HRP-2 detection, offer detection of P.falciparum as well as all other species in one test. (2)

Consequent upon change in the recommended therapy for uncomplicated malaria to highly effective artemisinin based combination therapies, it is now recommended that the treatment of malaria be confined to parasitologically confirmed cases. (3)

Though over 100 comparisons between slide microscopy and RDTs have been published, most have used smear microscopy as gold standard, but since smear microscopy could have performance issues, it is best to use PCR as gold standard. A study performed is USA comparing traditional smear to rapid antigen capture test demonstrated that the RDT, as performed in a routine clinical setting, is superior to a single set of Giemsa stained blood smears for quickly evaluating a patient for malaria. Importantly, the rapid antigen capture assay had a 100% NPV for P.falciparum malaria. This FDA approved test appears to be an ideal tool for timely decision making when P.falciparum must be considered for patient disposition. This is especially true when reliable, experienced microscopy is not immediately available, such as after hours or in small or rural settings with physicians or technologists without expertise in reading malaria blood smear.(4) The choice of RDTs for malaria diagnosis must be based on its stability, low cost,  high sensitivity (97%) and specificity (88%) and ease of use. (5)

References:

1.       The Malaria Working Party for Standards Haematology.  The laboratory diagnosis of malaria. Clin Lab Haematol 1997; 19; 165 – 170

2.       Murray CK et. Update on rapid diagnostic testing for malaria. Clin Micro Review 2008; 21; 97- 110

3.       World Health Organization. Guidelines for the treatment of malaria WHO 2 ed., Geneva, 2010

4.       Stauffer WM et al. Diagnostic performance of rapid diagnostic tests versus blood smears for malaria in US clinical practice. Clin Infect Dis 2009; 49; 908 – 913

5.       Batwala V et al. Are rapid diagnostic tests more accurate in diagnosis of Plasmodium falciparum malaria compared to microscopy at rural health centres? Malaria Journal 2010; 9: 349

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