We like to mention malaria in this book due to the frequency of specimens tested in Nordic laboratories. Accurate diagnosis of Plasmodium spp. is essential for successful treatment. Generally microscopic examination of smears is adequate for diagnosis of the majority of cases. The species do differ morphologically; which can be detected by microscopy, but it suffers from poor sensitivity in cases with low-level parasitemiae. PCR has been developed both for diagnostic purposes as well as for the possibility to detect drug resistant P. falciparum (Weiss, 1995).
Various assays based on the PCR have been described in the diagnosis of malaria, which were specific either for P. falciparum (Zalis et al. 1996) or Plasmodium vivax (Kain et al. 1993). A PCR based on the small subunit rRNA has been described, which enables detection of all four human Plasmodium species (Waters 1988; Snounou et al. 1993). In field studies, sampling of blood direct to membranes followed by PCR detection has been shown to be the best and easiest way of testing large number of samples (McLaughlin et al. 1991). In most of the reports, PCR detection was reported to be more sensitive than microscopic evaluation of blood smears, but did, due to PCR inhibitory components, sometimes fail to detect Plasmodium in specimens known to be positive (Walliker 1994; Oliveira et al. 1996). However, generally microscopy produced adequate sensitivity and specificity (Payne 1988; Weiss 1995).
The previous demonstrated sensitivity and specificity of PCR in the diagnosis of malaria may be preferable to that of microscopy in evaluating new diagnostic tests. Recently the diagnosis of P. falciparum was evaluated in 151 travellers comparing the dipstick test 'Parasight F', PCR, and direct microscopy (Humar et al. 1997). Using PCR as a gold standard, both the dipstick test and microscopy, had a sensitivity of 88%. The PCR was superior in cases of low parasitemiae.
Antifolate drugs such as pyrmethamine and proguanile are widely used for treatment and prophylaxis of malaria. Antifolate resistance is increasingly common in areas where malaria is endemic. Point mutations in the dihydrofolate gene are responsible for developing resistance. Two reports employed mutation specific primers and PCR followed by electrophoretic analysis to detect the presence or absence of specific products to distinguish pyrmethamine-susceptible from drug resistant as well as proguanile resistant isolates (Peterson et al. 1991; Plowe et al. 1995).
PCR diagnosis or detection of resistance will probably be a test that need to be centralized in reference laboratories, due to the low number of samples.
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