The page below is a sample from the LabCE course White Cell and Platelet Disorders: Peripheral Blood Clues to Nonneoplastic Conditions. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Platelet Estimates

In addition to evaluating white blood cells during a manual differential, it is important to provide information about the platelet population on the peripheral blood smear. 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 review of a 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:
  1. 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)
  2. Average the platelet counts obtained
  3. Multiply by 15 X 109/L to obtain estimated platelet count (some laboratories prefer a 20 X 109 multiplier in this step if capillary blood is used)
Such a counting scheme for platelets 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.