25 October 2011 (Tuesday) - Rapid Malaria Testing


Malaria antigen detection tests are a group of commercially available tests that allow the rapid diagnosis of malaria

There are currently over 20 such tests commercially available, but none of the rapid tests are currently as sensitive as a thick blood film, nor as cheap. A major drawback in the use of all current dipstick methods is that the result is essentially qualitative. In many endemic areas of tropical Africa, however, the quantitative assessment of parasitaemia is important, as a large percentage of the population will test positive in any qualitative assay.

Antigen tests look for one of four main markers:




pGluDH



The enzyme pGluDH does not occur in the host red cell and is therefore specific to plasmodium sp.

GluDH activity in P.vivax, P.ovale and P. malariae has never been tested, but given the importance of GluDH as a branch point enzyme, it is assumed that every cell must have a high concentration of GluDH.



Histidine Rich Protein II



The histidine-rich protein II (HRP II) is a histidine - and alanine -rich, water-soluble protein, which is localized in several cell compartments including the parasite cytoplasm.

The antigen is expressed only by P. falciparum trophozoites.



pLDH



P.falciparum lactate dehydrogenase (pLDH) is a 33 kDa oxidoreductase It is the last enzyme of the glycolytic pathway, essential for ATP generation and one of the most abundant enzymes expressed by P.falciparum.

pLDH does not persist in the blood but clears about the same time as the parasites following successful treatment. The lack of antigen persistence after treatment makes the pLDH test useful in predicting treatment failure.

In this respect, pLDH is similar to pGluDH. LDH from P. vivax, P.malariae, and P.ovale exhibit 90-92% identity to pLDH from P.falciparum.



pAldo



Fructose-bisphosphate aldolase catalyzes a key reaction in glycolysis and energy production and is produced by all four species. The P.falciparum aldolase is a 41 kDa protein and has 61-68% sequence similarity to known eukaryotic aldolases.

The presence of antibodies against p41 in the sera of human adults partially immune to malaria suggest that p41 is implicated in protective immune response against the parasite.




However these techniques are not without drawbacks.



Cross-reactions with autoantibodies:



Studies have reported cross reactivity of the various RDTs with autoantibodies such as rheumatoid factor, resulting in false positive tests for malaria. Studies in patients with positive rheumatoid factor have shown that the false positive reactions.




Sensitivity:



RDTs for the diagnosis of P. falciparum malaria generally achieve a sensitivity of >90% at densities above 100 parasites per µL blood and the sensitivity decreases markedly below that level of parasite density.

Many studies have achieved >95% sensitivity at parasitemia of ~500 parasites/µL, but this high parasitemia is seen in only a minority of patients. For the diagnosis of P. vivax malaria, many tests have a lower sensitivity compared to that for P. falciparum malaria; however, the pLDH test has an equal or better sensitivity for P. vivax malaria compared to P. falciparum malaria.

For the diagnosis of P. malariae and P. ovale infections, the sensitivity is lower than that of P. falciparum malaria at all levels of parasitemia on pLDH tests.





The sensitivity of the RDTs at low levels of parasitemia and for non-immune populations remains a problem. Compared to microscopy,many tests were found to be less sensitive in detecting asymptomatic patients, particularly at low parasitemias.

Also, the RDTs have been reported to give false negative results even at higher levels of parasitemia. Therefore, in cases of suspected severe malaria or complex health emergencies, a positive result may be confirmatory but a negative result may not rule out malaria. A negative RDT result should always be confirmed by microscopy.


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