Identifying Plasmodium vivax under a microscope
Malaria parasites can be identified by examining a drop of the patient’s blood under a microscope, spread out as a “blood smear” on a microscope slide. To give the parasites a distinctive appearance, the specimen is stained prior to examination.
Microscopy is a low-cost, effective method that allows for the detection of the species, stages and densities of the parasite, and the therapeutic efficacy of antimalarial drugs. It requires at least a minimally equipped laboratory to perform blood smear staining and reading. It can take up to one hour or more to rule out an infection with a high degree of confidence.
Training of microscopists is absolutely essential to ensure good sensitivity and specificity. In addition, quality control procedures should be in place. Thus, microscopy can be impractical or inaccurate in remote areas, as facilities may be lacking. The threshold of detection for an expert microscopist is considered to be about 50 parasites/µL.1 This limit of detection can, however, be significantly higher in many endemic areas.1
Microscopy is not sensitive enough to detect a large proportion of all infections, as in some regions, up to 70% of P. vivax infections can be below the limit of detection.2 Where there is a mixed P. falciparum / vivax infection, vivax may be missed owing to the greater density of falciparum parasites.2