The estimation of platelet count from a blood smear on the basis of the red cell: platelet ratio.

OBJECTIVE The estimation of platelet count from blood smears is a daily routine laboratory test, which should be systematic each time the automated count is erroneous. In our laboratory, we estimate the platelet count indirectly by using the automated red blood cell (RBC) and calculating the platelet count on the basis of the red cell: platelet ratio in a stained blood film. In this study, we attempted to verify the reliability of this technique. MATERIAL AND METHODS One hundred ninety-one platelet counts were executed by two laboratory methods: an automated count using an impedance cell counter and then a manual method by reviewing microscopic blood smears. The number of platelets per 1000 erythrocytes was multiplied by the automated RBC (x106 cells/µl) to give an approximate manual count (x103 cells/µl). Two paired t-test was used for comparison of the two methods. RESULTS The regression analyses for the entire data set collected in our study with the two laboratory methods gave the following least squares equation by comparing the automated (y) to the manual method (x): y=0.8548x + 12.013 (r=0.908). The paired t-test showed no significant difference between the two methods (p>0.05) and the Intra-class Correlation Coefficient (ICC) was equal to 0.905. The plot of the differences between the automated and manual values against their means according to Band and Altman design showed that the difference mean was 3.209 with a standard deviation SD=46.331. We noticed that 93% of the differences were within the agreement limits (mean±2SD), and that 77% of the differences were less than 20,000 platelets/µl. CONCLUSION Estimating platelet count on the basis of the red cell: platelet ratio is a reliable technique and it should be proposed as a method of reference.