SOP – Malaria Microscopy - NVBDCP

Quality Assurance of Malaria Diagnostic tests8.3.8.2.1 Method 1: Parasites per microlitre of bloodThis is a practical method of reasonable and acceptable accuracy. The number of parasitesper microlitre of blood in a thick film is counted in relation to a standard number of leukocytes(8000). Although there are variations in the number of leukocytes between healthyindividuals and even greater variations between individuals in ill health, this standard allowsfor reasonable comparisons. For this two tally counters are required, one to count parasitesand the other to count leukocytes. Step 1 Step 2(a) If 10 or more parasites have been identified and counted, against 200leukocytes, record the results on the record form in terms of the number ofparasites per 200 leukocytes.(b) In case, if only 9 or fewer parasites have been counted against 200leukocytes counted, then continue counting until 500 leukocytes are counted;then record the number of parasites per 500 leukocytes.In each case, the number of parasites relative to the leukocyte count can beconverted to parasites per microlitre of blood by the simple mathematicalformula:Number of parasites X 8000---------------------------------- = parasites per microlitreNumber of leukocytesIn effect, this means that if 200 leukocytes are counted, the number ofparasites is multiplied by 40 and if 500 leukocytes are counted the number ofparasites is multiplied by 10.8.3.8.2.2 Method 2: the plus systemA simpler method of counting parasites in thick blood film is to use the plus system. Thissystem is less satisfactory and should not be used when parasite count is done forpreparation of QC/EQA samples for use with malaria RDTs. However, in the peripherallaboratory under QA/EQA of malaria microscopy for competency/ proficiency testing of thelaboratory technicians, this method would be suitable. The system entails using a codebetween one and four plus signs, as follows:+ = 1-10 parasites per 100 (thick film) fields+ + = 11-100 parasites per 100 (thick film) fields+ + + = 1-10 parasites per single (thick film) fields+ + + + = more than 10 parasites per single (thick film) fieldSOP – Malaria Microscopy© Copyright to Dte. NVBDCP Only. Any modification are prohibited