The laboratory diagnosis of malaria

On a world-wide basis, malaria is a prevalent infection with 300-500 million clinical cases every year. The majority of cases occur in less developed countries where access to expensive medical technology is limited. In Australia malaria is not endemic and is only seen following travel. The diagnosis of malaria continues to be based on the examination of stained blood smears (the 'gold standard'). Properly stained and expertly examined thick and thin blood films enables species diagnosis and estimation of parasitaemia. In a country where malaria is uncommon (300 reported cases per annum in Australia) this requires ongoing maintenance of microscopy skills. Commercial diagnostic tests have been introduced in an attempt to simplify the diagnosis. These include Quantitative Buffy Coat (QBC) and the use of monoclonal antibodies to parasite-specific molecules. QBC is based on differential centrifugation of blood with parasite visualisation by acridine orange using fluorescence microscopy. The limitations of QBC are low sensitivity, the requirements for fluorescence microscopy, it is non-quantitative and there is no permanent record. The development of antigen capture assays for P. falciparum based on the detection of histidine rich protein II (HRPII) has simplified the diagnosis of P. falciparum malaria. These dipstick methods are quick, simple to perform and interpret, do not require specialised skills and have high sensitivity and specificity. These tests are proving to be of value in possible mixed infections, low levels of parasitaemia, for staff who do not have specialised microscopy skills and for remote sites. Nucleic acid detection methods (especially PCR) have also been developed but are generally not species specific and are not suitable for routine diagnosis. These methods are most applicable to large scale batch analysis, reference studies and epidemiology. Examination of stained blood films remains the 'gold standard' for the diagnosis of malaria. As microscopy requires specialised skills ongoing education and QA are critical. Detection of HRPII is a useful quick adjunct for the detection of P. falciparum.