- Parasite Classification |
- Parasite, Body Site |
- Stool Testing Order Recommendations | - STAT Testing |
- Fecal Fixatives | - Stool Collection Options | - Report Comments |
- Tips for Fecal ImmunoAssay | - Malaria (5 Species) |
- Malaria (5 Species) Images | - Rapid Malaria Testing |
- Malaria Parasitemia Method | - Malaria Parasitemia Interpretation |
Determination of Parasitemia
It is important to report the level of parasitemia when blood films are reviewed and found to be positive for malaria parasites. Because of the potential for drug resistance in some of the Plasmodium species, particularly P. falciparum and P. vivax, it is important that every positive smear be assessed and the parasitemia reported exactly the same way on follow‑up specimens as on the initial specimen. This allows the parasitemia to be followed after therapy has been initiated. In cases where the patient is hospitalized, monitoring should be performed at 24, 48, and 72 h after initiating therapy. Generally, the parasitemia will drop very quickly within the first 2 h; 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.
Another approach is to count the number of parasites per 100 white blood cells (WBCs) on the smear. Either the thick or thin film can be used for this purpose. This figure can be converted to the number of parasites per μl of blood; divide the number of parasites per 100 WBCs by 100, and multiply that figure by the number of WBCs per μl of blood. Depending on the parasitemia, 200 or more WBCs may have to be counted, so the denominator may vary (it may be 200 or even more). In this case, blood for both the peripheral smears and cell counts must be collected at the same time.
It is critical that the same reporting method be used consistently for every subsequent set of blood films so that 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. Drug resistance has also been reported in P. vivax cases; mixed infections are also much more common than suspected.