REPORTING: Quantitation

Tips and Pifalls!

1. Because Plasmodium falciparum and, to a lesser extent, P. vivax have been reported to be resistant to chloroquine, it is particularly important for these two species and recommended for all four species that the organisms be quantitated on the blood smear.
   
   Generally, the parasitemia will drop very quickly within the first 24 hours; however, in cases of drug resistance, the level may not decrease, but actually increase over time. Malarial infections should be reported as the percentage of infected RBCs per 100 RBCs counted (0.5%, 1.0%, etc.). Considering the low parasitemia frequently seen in patients within the United States, several hundred RBCs may need to be counted to arrive at an accurate count and determination of the percentage. The thin blood film must be used for this approach. Although there are other methods for determining parasitemia, it is critical that the same reporting method be used consistently for every subsequent set of blood films, so the parasitemia can be tracked for decrease or possible increase, indicating resistance. Also, remember, that drug resistance may not become evident for a few days. The parasitemia may actually appear to decline, but may begin to increase after several days. Therefore, it is very important that patient parasitemia be monitored, particularly if an infection with P. falciparum has been diagnosed.
   
   
2. Because there may be an association between Blastocystis hominis and numbers in the stool, these organisms should be quantitated (rare, few, moderate, many, packed). The same quantitation approach as that used for proficiency testing (permanent stained smear) can be used.
   
   
3. Human cells in the stool should also be reported [WBCs, RBCs, Charcot-Leyden crystals, and yeast (providing the fresh specimen was immediately examined or placed in stool preservative). Quantitation can be as indicated above for Blastocystis hominis on the permanent stained smear.