Plasmodium lactate dehydrogenase assay to detect malarial parasites

Microscopic examination of blood smears remains the gold standard for the diagnosis of malaria. However, it is labour-intensive and requires skilled operators. Immunochromatographic dipstick assays provide a potential alternative. One such dipstick, the Plasmodium lactate dehydrogenase assay (pLDH), is based on detection of the Plasmodium intracellular metabolic enzyme, LDH. The differentiation of malarial parasites is based on the antigenic differences between the pLDH isoforms. This study was designed to assess the sensitivity and specificity of pLDH assays in detecting and differentiating between various malarial species compared with microscopy.

Blood samples (n = 124) submitted to our laboratory for routine diagnosis of malaria were included in this study. From each blood sample, two thin films and a quantitative buffy coat (QBC) were made for microscopy. Thin films were stained with Giemsa and acridine orange. The pLDH assay was performed on all the samples according to the manufacturer's instructions.

Of the 124 blood samples, 84 were negative by all methods (Giemsa, acridine orange, QBC and pLDH assay). Of the 38 samples positive for Plasmodium falciparum on microscopy, pLDH assay correctly identified 36 at parasite counts as low as < 40 parasites/microl and had a sensitivity and specificity of 94.3% and 97.6%, respectively. Of the 21 samples positive for Plasmodium vivax, pLDH assay correctly identified 19 at parasite counts as low as < 80/microl, and had a sensitivity and specificity of 90.4% and 100%, respectively. However, it failed to identify two Plasmodium vivax infections at parasite counts of 5000/microl and > 200/microl, suggesting that plasmodial gene deletions could be responsible for non-expression of pLDH.

Our data demonstrate that pLDH assay, given its accuracy, rapidity (10-15 minutes), ease of performance and interpretation, can be a useful tool for the detection of malaria in countries where both plasmodial species are co-endemic and where laboratory support is limited.