The average number of platelets per oil immersion field, in a count of ten oil fields, multiplied by 20,000 has proved to be a reliable estimation of platelets in normal and thrombocytopenic states. In the presence of thrombocytosis, ie, 25 or more platelets per oil immersion field, the smear count is less reliable. The multiplication factor of 20,000 was determined after statistical analysis of 288 consecutive unselected patients. Certain precautions should be observed, such as use of a fresh EDTA sample or freely flowing finger blood, a well made and well stained smear without platelet aggregates or precipitated stain particles, and an acceptable counting area on the film.
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