Context: Platelet counts from automated haematologyanalysers are crosschecked in peripheral smears by counting the number of platelets in ten oil immersion fields and multiplying their average by 15000. Since different microscopes have different field diameters, the area viewed and the number of platelets counted in ten fields differs between microscopes. Hence it is inappropriate to use the same multiplication factor of 15000 in all microscopes. Aims: To determine whether the multiplication factor of 15000 should be modified in a microscope with field number 20. Settings and Design: Platelet counts were estimated by two different methods from peripheral blood smears by using a microscope with field number 20. Multiplication factor used was 12000 in method A and 15000 in method B. Results of the two methods were compared with automated platelet counts. Methods and Material: Automated platelet counts were obtained from Sysmex XT1800i for 200 blood samples and compared with the manual counts estimated from Leica microscope with field number 20 by the two methods mentioned above. Statistical analysis used: ANOVA, student’s t test, correlation coefficient. Results: Results from method A correlated strongly with automated platelet counts (correlation coefficient of 0.978) and did not differ significantly from them (p value of 0.28). Method B results differed significantly from automated counts (p < 0.001). Conclusions: Modifications to multiplication factor are essential when microscopes of different field diameters are used for platelet count estimation. We have suggested those modifications needed for various types of microscopes in this article.

Keywords: Automated Analysers; Haematology; Microscopes; Platelet; Platelet Count.