Review of platelet estimates

May 1, 2011
Answering Your Questions

Question

What factor do you use when doing a platelet slide estimate?

Answer

Platelet (PLT) estimates are an integral part of peripheral blood-smear evaluation. Platelet estimates help to ensure accuracy of the automated platelet count and/or provide a blood-smear quantitative platelet analysis. To perform a reliable platelet estimate of a peripheral blood smear, it is essential that the following specimen criteria are met:

The following factors are responsible for the difficulty in the D antigen typing:

  • Macroscopically, the smear should exhibit a gradual transition from thick to thin, ending with an acceptable feather edge free of streaks, waves, or holes. If the smear is prepared from an EDTA sample, the specimen should be free of clots.
  • Microscopically, there should be a uniform distribution of platelets without clumping.
  • The smear should exhibit proper stain quality. Proper stain quality should be assessed according to the Romanowsky-type stain utilized in the staining process, and the stained smears should have minimal to absent precipitated stain.

The procedure for performing a platelet estimate varies, depending on how the smear is prepared (wedge method, semi-automated, or automated), the type of specimen collected (EDTA-anticoagulated blood vs. capillary), and the magnification of the microscopes ocular and objective pieces. PLT estimates are routinely performed using a 100X oil objective in an area of the smear where the red blood cells (RBCs) barely touch. A rough estimate can be determined by the number of platelets per 100X oil-immersion field (OIF). References state that anywhere from eight to 25 platelets per OIF correlates with a normal platelet count.

To provide a more accurate PLT estimate, one can calculate the ratio of an automated platelet count to the number of platelets per oil immersion field. This calculation should be performed whenever a different brand of microscope is used due to variations in the viewing field. Here is an overview of how to calculate the ratio:1

  1. Perform 30 consecutive automated patient samples. Ensure the PLT QC is within two Standard Deviations.
  2. Prepare and stain a smear on each specimen.
  3. For each smear, count the number of PLTs in 10 consecutive OIFs and divide by 10 to get the average number per field.
  4. Divide the automated PLT count by the average number of PLTs per field for each specimen.
  5. Add the numbers obtained in step four and divide by number of specimens (30) to get the average ratio of automated PLTs to PLTs per OIF.
  6. Round ratio value to the nearest whole number. For example, if the ratio value is 15.0, one platelet per oil field 15.0 x 109/L.
  7. Calculate the average number of platelets per OIF and multiply by the platelet factor. Therefore, for an average of 15 PLTs per OIF, the calculation would be: 15 x 15 = 225 x 109/L

Our laboratory prefers a slightly more tedious procedure, which we refer to as the indirect platelet count. When a platelet count has an instrument flag or is outside our checking criteria, we utilize the Miller disc to determine the number of platelets per 1,000 RBCs. The total platelet count per 1,000 RBCs is then multiplied by the RBC count and divided by 1,000 to convert units from 1012/L to109/L.

Therefore, with a total of 70 PLTs per 1,000 RBCs and a RBC count of 4.10 x 1012/L, the calculations would be:

RBC count/1,000 = 4.10 x 109/L
PLTs/1,000 RBCs = 70
Indirect PLT: 70 x 4.10 = 287 x 109/L

This type of platelet calculation helps to minimize the variation one would see with an anemic patient or with an elevated RBC count.2

—Debbie Bennes, BS, MLT Education Specialist,
Hematopathology Morphology
Instructor in Laboratory Medicine and Pathology
Mayo Clinic Rochester, MN

References
  1. Stiene-Martin EA, Lotspeich-Steininger CA, Koepke JA. Clinical Hematology, 2nd ed. Philadelphia, PA: J.B. Lippincott-Raven; 1992:337.
  2. Mogadam L. Application of the Miller's Disc for the Estimation and Quality Control of the Platelet Count. Lab Med. 1980;11:131.

Brad S. Karon, MD, PhD, is associate professor of laboratory medicine and pathology, and director of the Hospital Clinical Laboratories, point-of-care testing, and phlebotomy services at Mayo Clinic in Rochester, MN.

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