BackgroundFor the calculation of parasite index (PI) by microscopy method, an assumed total leucocyte count (TLC) of 8,000/μL is used conventionally. However, due to obvious variation in the population and individual TLCs, use of 8,000/μL may result in either over/underestimation of the PI.MethodsThis study was aimed at ascertaining the utility of 8,000/μL TLC, as well as other assumed TLCs, with respect to measured TLC for the calculation of PI. A tertiary care hospital and five primary health centres were the base for the prospective study conducted among microscopically proven, symptomatic Plasmodium vivax mono-infection patients aged ≥18 years. PIs calculated by assumed TLCs ranging from 4,000-11,000/μL were compared with those calculated by measured TLCs. Geometric mean with 95% confidence interval, Bland-Altman plot and Wilcoxon signed rank test were used for statistical analysis.ResultsA total of 284 P. vivax mono-infection patients, including 156 from a tertiary care hospital and 128 from five primary health centres, were recruited in the study. Assumed TLCs below 5,000 cell/μL and above 5,500 cell/μL in tertiary care setting resulted in significant (p <0.05) underestimation and overestimation, respectively. However, in primary health centres, it was an assumed TLC of 5,000 cell/μL, below and above which there was significant (p <0.05) underestimation and overestimation observed, respectively.ConclusionsAssumed TLC of 8,000/μL is not suitable for the calculation of PI. Either actual TLC of the patient should be measured or a representative TLC should be derived for the population under investigation for any study requiring calculated PI by microscopy.