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Parasitemia determined from conventional light microscopy:
clinical correlation AND INTERPRETATION

Parasitemia (%)	No. of Parasites/µl	Clinical Correlationa
0.0001-0.0004	5-20	Number of organisms required for positive thick film (sensitivity) Examination of 100 thick-blood-film (TBF) fields (0.25 µl) may miss up to 20% of infections (sensitivity 80-90%); at least 300 fields should be examined before reporting a negative result Examination of 100 thin-blood-film fields (THBF) (0.005 µl); at least 300 fields should be examined before reporting a negative result; both thick and thin blood films should be examined for every specimen submitted for a suspect malaria case (report final results using 100 x oil immersion objective) BinaxNOW® rapid lateral flow method (dipstick) (0-100 = 53.9% sensitivity for P. falciparum) BinaxNOW® rapid lateral flow method (dipstick) (0-100 = 6.2% sensitivity for P. vivax)) One set (TBF + THBF) of negative blood films does not rule out a malaria infection
0.002	100	Patients may be symptomatic below this level, particularly if they are immunologically naïve (no prior exposure to malaria) (Example: travelers) BinaxNOW® rapid lateral flow method (dipstick) (100-500 = 89.2% sensitivity for P. falciparum) BinaxNOW® rapid lateral flow method (dipstick) (100-500 = 23.6% sensitivity for P. vivax)
0.02	1,000	Level often seen in travelers (immunologically naïve) – results may also be lower than this BinaxNOW® rapid lateral flow method (dipstick) (1000-5000 = 99.2% sensitivity for P. falciparum) (500-1000 = 92.6% sensitivity for P. falciparum) BinaxNOW® rapid lateral flow method (dipstick) (1000-5000 = 81.0% sensitivity for P. vivax) (500-1000 = 47.4% sensitivity for P. vivax)
0.1	5,000	BinaxNOW® rapid lateral flow method (dipstick) (>5000 = 99.7% sensitivity for P. falciparum) BinaxNOW® rapid lateral flow method (dipstick) (>5000 = 93.5% sensitivity for P. vivax)
0.2	10,000	Level above which immune patients will exhibit symptoms
2	100,000	Maximum parasitemia of P. vivax and P. ovale (which infect young RBCs only)
2-5	100,000 – 250,000	Hyperparasitemia, severe malariab; increased mortality
10	500,000	Exchange transfusion may be considered; high mortality

The BinaxNOW® malaria test (Inverness Medical, Scarborough, ME) is FDA approved. The BinaxNOW® Malaria Test detects antigen from both viable and non-viable malaria organisms, including gametocytes and sequestered P. falciparum parasites. Test performance depends on antigen load in the specimen and may not directly correlate with microscopy performed on the same specimen. Samples with positive rheumatoid factor titers may produce false positive results. Analytical reactivity testing demonstrates that the pan malarial test line on the BinaxNOW® test is capable of detecting all five malaria species. However, during clinical trials, insufficient data was generated to support clinical performance claims for the detection of P. malariae, P. ovale, or P. knowlesi. Clinical performance claims for this test are made for P. falciparum and P. vivax detection only. The test is not intended for use in screening asymptomatic populations. (BinaxNOW® malaria test package insert (Inverness Medical, Scarborough, ME). The BinaxNOW Positive Malaria Control, as well as the BinaxNOW® Malaria test, is available commercially (Inverness Medical, Scarborough, ME).

World Health Organization criteria for severe malaria are parasitemia of >10,000/µl and severe anemia (hemoglobin, <5 g/liter). Prognosis is poor if >20% of parasites are pigment-containing trophozoites and schizonts and/or if >5% of neutrophils contain visible pigment.