An Explanation of the Discrepancy between Direct and Indirect Platelet Counts

The indirect platelet count is higher than the direct count because the red cells, which are used as a point of reference in the indirect method, are not randomly distributed beneath the coverslip. The red cells are concentrated at the edge of the coverslip so that the true ratio of red cells to platelets cannot be accurately established. Indirect platelet counts based on the ratio in the central areas of the coverslip are too high.