In addition to evaluating white blood cells during a manual differential, providing information about the platelet population on the peripheral blood smear is essential. The findings in the image to the right would elicit a report comment of "increased platelets" of a high magnitude, such as "marked" or "4+." Estimates of platelet counts from a review of peripheral blood should be made on each smear examined. This provides a simple estimate of "increased," "decreased," or "normal," which usually corroborates the value generated from an automated cell counter.
A formula for estimating platelet counts must be established for each laboratory. One guideline for the estimation of platelets is as follows:
- Count platelets on 5 fields using 1000X magnification (care should be taken to ensure the fields used for counting are not too thick or too thin)
- Average the platelet counts obtained
- Multiply by 15 X 109/L to obtain an estimated platelet count (some laboratories prefer a 20 X 109 multiplier in this step if capillary blood is used)
A platelet counting scheme, when clustered, as in the image, is probably not needed, as there are more than 100 platelets in the field. This translates into a platelet count of 1500 X 109/L or more.
