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Enzyme-linked immunosorbent assays (ELISAs) allow for the testing of large numbers of samples within a short time frame. We tested the sensitivity and specificity of a histidine-rich protein 2 (HRP2)-based, commercially available ELISA antigen detection assay for Plasmodium falciparum (Malaria Antigen CELISA; Cellabs, Sydney, Australia). A total of 700 whole blood samples obtained from symptomatic outpatients of malaria clinics along the Thai-Myanmar border were tested relative to blinded duplicate expert microscopy adjusted with species-specific polymerase chain reaction (PCR). PCR-adjusted microscopy showed that 79 (11.3%) were infected with P. falciparum, 118 (16.9%) with P. vivax, 1 (0.1%) with P. malariae, 7 (1.0%) with mixed infections (P. falciparum and P. vivax), and 495 (70.7%) were negative. The geometric mean parasite density for P. falciparum was 7547/muL (range: 12-363,810/muL). The overall sensitivity of the HRP2 ELISA for P. falciparum malaria was 98.8% (95% CI, 93.6-100%) and the specificity was 100% (95% CI, 99.5-100%). The positive and negative predictive values for the ELISA were 100% (95% CI, 96.5-100%) and 99.8% (95% CI, 99.1-100%), respectively. The results for P. falciparum were clearly superior to expert microscopy alone, particularly in mixed infections. Microscopy combined with ELISA reaches a sensitivity and specificity similar to PCR-adjusted microscopy for the diagnosis of P. falciparum while being considerably less expensive and faster. We conclude that ELISA serves as an excellent tool to augment microscopy as the gold standard for P. falciparum diagnosis in research settings and should be further evaluated for screening in blood banks.

A hospital-based study was conducted along the Thai-Myanmar border to provide greater knowledge of the causes of febrile illness and to determine what zoonotic and vector-borne emerging infectious diseases might be present. A total of 613 adults were enrolled from June 1999 to March 2002. Cases were classified based on clinical findings and laboratory results. An etiologic diagnosis was made for 48% of subjects. Malaria was the most common diagnosis, accounting for 25% of subjects, with two-thirds Plasmodium falciparum. Serologic evidence for leptospirosis was found in 17% of subjects. Other etiologic diagnoses included rickettsial infections, dengue fever, and typhoid. The most frequent clinical diagnoses were nonspecific febrile illness, respiratory infections, and gastroenteritis. Clinical associations were generally not predictive of etiologic diagnosis. Apparent dual diagnoses were common, particularly for malaria and leptospirosis. Findings have been used to modify treatment of unspecified febrile illness in the area.

This study examines hematologic profiles of persons with acute Plasmodium falciparum or P. vivax infection in Maesod on Thailand's western border with Myanmar compared with febrile, non-parasitemic persons also reporting to malaria clinics. Nine hundred seventy-nine subjects were malaria-negative, 414 were infected with P. falciparum, and 646 were infected with P. vivax. Persons with patent parasitemia tended to have significantly lower white blood cell, red blood cell, platelet, and hemoglobin levels than those who were malaria-negative. For the first time, a parallel trend in thrombocytopenia with parasitemia was found to be associated with both P. falciparum, and P. vivax infection. Using logistic regression, persons with platelet counts < 150,000/microL were 12-15 times more likely to have malaria than persons with platelet counts > or = 150,000/microL. This study supplements previous literature on the hematologic effects of malaria and helps define those alterations for a semi-immune population. Thrombocytopenia is identified as a key indicator of malaria in these febrile patients.

The NOW ICT Malaria P.f./P.v. for Whole Blood (Binax, Inc., Portland, ME) is a new malaria rapid diagnostic device that represents a technical advance over previous assays, such as ICT Malaria P.f./P.v. and ICT Malaria P.f.. We evaluated this device in March 2001 in symptomatic patients at malaria clinics in Maesod, Thailand. Microscopic examination of Giemsa-stained blood smears was the reference standard. In 246 patients, microscopy showed 32 (13.0%) infected with Plasmodium falciparum, 63 (25.6%) with P. vivax, 6 (2.4%) with mixed infections of P. falciparum and P. vivax, 5 (2.0%) with P. malariae, and 140 (56.9%) negative. Sensitivity for P. falciparum was 100% and specificity was 96.2% (200 of 208; 95% confidence interval [CI] = 92-98). For P. vivax, sensitivity was 87.3% (55 of 63; 95% CI = 77-93) and specificity was 97.7% (173 of 177; 95% CI = 95-99), but all the four false-positive results were microscopically positive for P. malariae; thus, specificity for non-falciparum Plasmodium was 100%. These results suggest improved performance over NOW ICT predecessors.