Diagnostic Medical Parasitology. Lynne Shore Garcia

      Figure 7.13 Diagram of the NOW malaria test format. (Adapted from package insert, Binax, Portland, ME.) doi:10.1128/9781555819002.ch7.f13

      Prior to the development of the improved product (Binax NOW malaria test), in a number of studies testing over 1,300 assays, the overall sensitivity ranged from 80 to 100% (32). Although some of the same issues apply to this test as to the ParaSight F test, the false-positive rate from the presence of rheumatoid factor appears to be less of an issue (33).

      In many areas of the world where both P. falciparum and P. vivax occur, rapid diagnostic tests for malaria must be able to differentiate the two species. It is also interesting that when a mixed-species infection with both P. falciparum and P. vivax is misdiagnosed as a single-species P. vivax infection, treatment for P. vivax can lead to a surge in P. falciparum parasitemia. The combined P. falciparum-P. vivax immunochromatographic test (NOW ICT Malaria Pf/Pv) was used in Indonesia, where both species occur (34). Blinded microscopy was used as the gold standard, with all discordant and 20% of concordant results cross-checked blindly. Of those with a presumptive clinical diagnosis of malaria, only 50% were parasitemic. The NOW ICT Malaria Pf/Pv test was sensitive (95.5%) and specific (89.8%) for the diagnosis of P. falciparum malaria, with a positive predictive value of 88.1% and a negative predictive value of 96.2%. Although the specificity and negative predictive value for the diagnosis of P. vivax malaria were 94.8 and 98.2%, respectively, the overall sensitivity of 75% and positive predictive value of 50% were lower than desired. With parasitemias of >500/µl, the sensitivity for the diagnosis of P. vivax malaria was 96%; with parasitemias of <500/µl, it was only 29%. However, by using this test, undertreatment rates would be reduced from 14.7 to 3.6% with a modest increase in the rate of overtreatment of microscopy-negative patients from 7.1 to 15.4%. Unfortunately, cost remains an obstacle to widespread implementation.

      In a study using the improved product (NOW malaria test), the sensitivity for P. falciparum was 100% and the specificity was 96%; the sensitivity for P. vivax was 89%, and the specificity was 98%. Testing was performed using P. falciparum- and P. vivax-positive specimens. These results suggest improved performance over NOW ICT predecessors (Fig. 7.13) (35). In another study, the NOW ICT test showed a sensitivity of 94% for the detection of P. falciparum malaria (96% for pure P. falciparum infection) and 84% for non-P. falciparum infections (87% for pure P. vivax infections and 62% for pure P. ovale and P. malariae infections) compared with PCR, with an overall specificity of 99% (36). The Binax NOW ICT may represent a useful adjunct for the diagnosis of P. falciparum and P. vivax malaria in febrile returned travelers. However, particularly in this patient group and in tests with a negative result, the rapid test should be performed in association with more traditional methods such as examination of thick and thin blood films (37, 38).

Flow Anti-pLDH Plasmodium Monoclonal Antibodies

      pLDH is a soluble glycolytic enzyme produced by the asexual and sexual stages of the live parasites and is present in and released from the parasite-infected erythrocytes. It has been found in all five human malaria species, and different isomers of pLDH for each of the five species exist. With pLDH as the target, a quantitative immunocapture assay, a qualitative immunochromatographic dipstick assay using monoclonal antibodies, an immunodot assay, and a dipstick assay using polyclonal antibodies have been developed.

      Flow anti-pLDH Plasmodium monoclonal antibodies detect a malaria pLDH antigen by using an antibody incorporated into the dipstick format (39) (Fig. 7.14). The test is positive only when viable parasites are present. Although the test has both a P. falciparum-specific and a panspecific antibody against all four species, the panspecific antibody detects only P. vivax with any degree of consistency (40, 41). The sensitivity and specificity of the Flow monoclonal antibodies are comparable to those of microscopy in detecting malaria infections at a parasitemia of >100/µl; however, the test failed to identify more than half of the patients with a parasitemia of <50/µl. In one study, the sensitivity of the Flow monoclonal antibodies was 97% at a high parasitemia (>100/µl) but fell to 39% at <50/µl (42). Therefore, the test is similar to the HRP2 assays and should not replace conventional microscopy in the diagnosis of malaria infection (43). However, a definite advantage is the ability to confirm a cure, since the test detects only viable organisms. Also, the number of false-positive results due to rheumatoid factor is much smaller than for some of the other tests. In a comparison of results with the ICT Malaria Pf and ParaSight F tests, low monoclonal antibodies correctly identified P. falciparum malaria in patient blood samples more often than the other two procedures did. In this study, the Flow monoclonal antibodies exhibited 94% sensitivity and 100% specificity for P. vivax and 88% sensitivity and 99% specificity for P. falciparum (41). In another study, compared with PCR, the sensitivity was 93% and the specificity was 99.5%. The authors felt that the test had sufficient sensitivity and specificity to detect P. vivax under laboratory conditions and could also be useful for malaria diagnosis in the field in Mexico (44). As with the other rapid malaria tests, cost is always an issue; however, Flow monoclonal antibodies provide a simple, rapid, and effective test in the diagnosis of malaria, especially where well-trained microscopists are not available or the work load is too high (45).