Microscopy and RDT test sensitivity

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The value of RDT compared with microscopy: thick blood films In ideal conditions a well-trained microscopist with experience in malaria, viewing a well-prepared thick blood film using a good quality microscope may detect better than 1 infected cell per 1000 total red cells. This is equivalent to around 50 parasites/ul. However, in practice it is estimated that a less experienced microscopist in equivalent conditions may acheive less sensitivity - perhaps 1 parasite in 100 red cells - equivalent to 500 parasites/μl. Both these outcomes are as good or better than an RDT test for detecting P.falciparum, and exceed what might be expected for other malaria species. In less ideal conditions where equipment, staining quality, or microscopic experience is less good, then RDTs may have equivalent or superior sensitivity to microscopy. This includes many malarial endemic regions where this sensitivity will not be achieved. It is important in these conditions however to understand the limitations of the selected test in terms of species identiication and sensitivity. The value of RDT tests compared with microscopy: thin blood films Thin films may provide similar sensitivity to a routine laboratory for an experienced morphologist with sufficient time and following recommended practices. Many people are more familiar with thin films and feel more comfortable with this approach, although it is not the currently recommended practice. Thin films however offer some advantages: Species may be identified much more confidently using thin films (including mixed infectionz) The % of red cells infected by parasites is much more accurately assessed by thin film. Summing up RDT tests are less affected by operator skill, facilities or training and may be as good or better than morphology where expertise is limited or conditions do not allow microscopy. RDTs also provide a more rapid turnaround time and when used together with morphology may help increase diagnostic confidence. However, operators must be aware of test limitations. Morphology has a superior achievable sensitivity for all malaria species, and particularly for lower levels of parasite infection and for some species. Outcomes are however highly dependent on the quality of preparation, the facilities available to examine the film, and the skill of the operator. Morphology provides additional advantages of: allowing the % parasite infection to be assessed and are unaffected by problems of gene deletion.