WBC estimate from smear.

Q I am looking for guidelines or a formula to help determine an estimated of the white blood cell (WBC) count in whole blood from the peripheral smear.

A When properly performed, the estimated white blood cell count from a stained peripheral blood smear is a useful tool to verify flagged results from automated hematology analyzers. Automated WBC counts are prone to interferences from conditions such platelet clumps, large platelets, lysis-resistant red blood cells, or nucleated red blood cells. (1) Flagged results require a blood-smear review by a technologist prior to reporting.

Various formulas exist for estimating the WBC count from a peripheral blood smear. Given the various instrumentation and slide-preparation techniques found in different laboratories, each lab should develop its own formula correlating the automated WBC count with the blood-smear findings. This can be done by following these steps:

1) Run a number of patient blood samples, (we recommend 30 samples), through the automated hematology analyzer;

2) Examine the same group of patients' stained blood smears and count the number of WBCs seen per field (20x, 40x, or oil, whichever you like to choose.) Count 10 fields and average them; and

3) Determine the WBC estimation factor (conversion factor) of each sample. (The WBC estimation factor equals the automated WBC count divided by manual WBC counts per field. (4)) Calculate the average estimation factor from the 30 samples.

Once the WBC estimation factor has been determined, the estimated WBC count equals the average manual WBC count per field multiplied by the estimation factor. For example, if the WBC estimation factor equals two, and there are four WBC per 40x field, the WBC estimate is 8x[10.sup.9]/L.

The December 2001 "Q & A" published on the College of American Pathologists' website offers a procedure for developing the WBC estimation factor. (2)

--Eric Nutt, MT(ASCP)

--Guang Fan, MD, PhD

Hematology Service

Department of Pathology

Oregon Health and Science University

Portland, OR

References

1. Stiene-Martin EA, Lotspeich-Steininger CA, Koepke JA. Clinical Hematology: Principles, Procedures, Correlations. Philadelphia, PA: J.B. Lippincott Co; 1992.

2. Sandhaus L. Q & A. CAP Today. December 2001. Available at: http://www.cap.org/apps/cap.portal?_nfpb=true&cntvwrPtlt_actionOverride=%2Fportlets%2FcontentViewer%2Fshow&_windowLabel=cntvwrPtlt&cntvwrPtlt%7BactionForm.contentReference%7D=cap_today%2Fq_and_a%2Fq_1201.html&_state=maximized&_pageLabel=cntvwr.

Edited by Daniel M. Baer, MD

MLO's "Tips from the Clinical Experts" provides practical, up-to-date solutions to readers' technical and clinical issues from a panel of experts in various fields. Readers may send questions to Dan Baer by e-mail at [e-mail].