Q&A column

Drummer OH. Drugs in bone and bone marrow. In Jenkins AJ, ed. Drug Testing in Alternate Biological Specimens. Humana Press; 2008.

Graham R. Jones, PhD
Forensic Toxicologist
Office of the Chief Medical Examiner
Alberta, Canada
Member, CAP Toxicology Committee

Michael A. Graham, MD
Department of Pathology
St. Louis University School of Medicine
St. Louis, Mo.
Member, CAP Toxicology Committee

Q. Is there a standardized procedure for performing platelet estimates that incorporates the dilution effect for low hemoglobin in anemic patients? I am having a hard time proving what I see in practice. The formula I found for platelet estimation works well with low hemoglobin levels but not with levels greater than 13 g/dL.

A.A number of manual platelet count estimators have been proposed to account for variation in hemoglobin levels.^1,2 However, none of them appear to be as accurate as traditional estimation of the platelet count by magnification area, regardless of hemoglobin: Multiply the number of platelets in an average of 10 microscopy fields with non-overlapping red blood cells using a 100× (oil immersion) objective lens by 20,000 to get an estimate of the platelet count per microliter (µL).^3,4

Malok and colleagues compared the aforementioned formula for the traditional manual method, in which the average platelet count per high-power field is multiplied by 20,000, to an alternative manual method in which the average platelet count per high-power field is multiplied by the hemoglobin in g/dL and 1,000.⁵ Both manual methods were compared to an automated platelet count across 184 samples and two slides per sample. The study found strong concordance between the traditional manual method and the automated method. In contrast, platelet counts by the alternative manual method differed significantly from those generated by the automated method. With regard to this query, it is noteworthy that these findings held true across hemoglobin values, including normal hemo­globin, which was defined as ≥13 g/dL, as well as low hemoglobin.

In a separate study, Anchinmane and Sankhe used 100 blood samples randomly collected from inpatients at a tertiary care hospital in India with EDTA vacutainer tubes to compare the traditional manual platelet count estimate calculator by Brecher and Cronkite to an automated analyzer method.⁶ Their study also demonstrated excellent concordance between the traditional manual and automated methods. While the authors do not mention other complete blood count parameters for these samples, including hemoglobin, it is likely that hospitalized patients in a random sampling demonstrated a wide range of hemoglobin values.

Based on these data, accounting for hemoglobin in manual platelet count estimation is not recommended, regardless of hemoglobin range. Malok and colleagues conclude their article with this encouraging statement: “Clinical laboratory professionals should feel confident in using the traditional multiplication factor of 20,000 for their platelet estimates for comparison to automated platelet counts as one measure of quality assurance.”⁵

1. Torres SL, Velez EL. Platelet verification under microscope calculated by the patient’s hemoglobin factor. Lab Med. 2004;35(7):430–433.
2. Bajpai R, Rajak C, Poonia M. Platelet estimation by peripheral smear: reliable, rapid, cost-effective method to assess degree of thrombocytopenia. Inter J Medical Sci Prac. 2015;2(2):90–93.
3. Brecher G, Cronkite EP. Morphology and enumeration of human blood platelets. J Appl Physiol. 1950;3(6):365–377.
4.  Gao Y, Mansoor A, Wood B, Nelson H, Higa D, Naugler C. Platelet count estimation using the CellaVision DM96 system. J Pathol Inform. 2013;4:16.
5. Malok M, Titchener EH, Bridgers C, Lee BY, Bamberg R. Comparison of two platelet count estimation methodologies for peripheral blood smears. Clin Lab Sci. 2007;20(3):154–160.
6. Anchinmane VT, Sankhe SV. Utility of peripheral blood smear in platelet count estimation. Int J Res Med Sci. 2019;7(2):434–437.

Alexandra E. Kovach, MD
Associate Professor of Clinical Pathology
Keck School of Medicine, University of Southern California
Director of Hematology and Bone Marrow Laboratories
Staff Hematopathologist and Pediatric Pathologist
Children’s Hospital of Los Angeles
Member, CAP Hematology/Clinical Microscopy Committee

Timothy Skelton, MD, PhD
Medical Director, Core Laboratory and Laboratory Informatics
Lahey Hospital and Medical Center
Burlington, Mass.
Member, CAP Hematology/Clinical Microscopy Committee