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Stepwise Success

Blood Smear Estimates

Published November 7, 2011 6:23 AM by Scott Warner

I shudder when I remember that, not terribly long ago, patients had appendectomies based on a manual differential. I remember a surgeon thinking aloud, “Only nine bands? We’ll hold off.” Hopefully, other variables were considered.

A 100-cell differential is notoriously inaccurate, never mind what a “band” is. A range can be estimated:

2 SD = 2 x SQRT((# cells x (100 - # cells)) / total cells counted)

Thus, a nine-cell band count in a 100-cell diff has a range of 3 to 15. Any copy of Clinical Diagnosis and Management by Laboratory Methods will have a table from 1960 based on a study of true variation for counts up to fifty percent. In this table, our band count has a range of 4 - 17. A savvy surgeon waiting for the band count to go over eleven percent might have taken the patient to the OR anyway.

It’s one reason some labs don’t report bands. But it also raises questions of accuracy. Other variables (hematocrit of the sample, wedge slide feathered edge, staining technique, humidity, human factors, etc.) suggest that these manual counts vary beyond statistics. Election polls are reported with a margin of error but not manual differentials.  Hm.

Which brings me to white cell estimates. Since these do not simply count ratios but an absolute number, they can be significantly inaccurate. (Should we run a control with an estimate? I wonder.) Here’s our laboratory formula from a text on a shelf:

WBC Estimate = # cells in 10 fields (50x) where RBCs “slightly overlap” / 10 x 3

The College of American Pathologists recommends establishing an “estimation factor” comparing the number counted per field to an automated count, essentially calibrating a field of view (FOV). This still leaves a minor problem of standardizing the RBC field, which can be solved by counting RBCs on a radius, e.g. count 8 cells in a straight line from center to edge to estimate about 200 cells in the FOV.

So, I’m curious. How does your lab estimate WBC counts from peripheral smears?

2 comments

Sorry, I served on a MASH unit as well as a field hospital corpsman with the U.S. Marines, there are other considerations besides the number of bands, patient history and where the pain is centered. Would you rather remove a useless organ like the appendix or let it rupture and have the pateint die on the table.

Jay Straley November 7, 2011 10:36 PM

WOW, that is amazing, sounds like a MASH surgeon appealing to an ear shot Lab tech, ala Alan Alda.

We abide by the CAP or pathologist recommendations for tech review or path smear review referrals and band denf. (No biggie for a trained hematologist, shifts vary slightly, and prior history comments are effective may apply)

- A review is contingent upon defined parameters and the method of measurement which is Vendor Specific Technology that varies like suspect, definitive or cellular interference flagging which requires a diff (some vendors provide a smear equivalent conversion table for their instrument WBC/PLT estimating)

Smear Estimate Evaluation has a Quality and Validation component which necessitates an enumeration and differential for instance:

1) Oncology patients may require a Buffy Coat Prep contingent upon treatments with estimate and differential possibly reflexing a 200 count/ diff utilizing several prepped slides.

2) Recent or prior history may assist with smear quality or CBC tube vs. smear comparability.

3) Critical instrument downtimes that require immediate interim reporting of a cell count with diff may be important in Women’s and Children’s hospitals

Anyway, often times a good rule when performing a more critical evaluation in hematology like the trained "Eyes On" approach of peripheral smears, we like to say count and identify what you see prior to a final report. And you have some nicely valid points just to throw the controversy out there, especially when hospitals may move several hundred to thousands of CBC results in a 24 HR session. Yes, sounds like we need to difine some criteria with an...all go rhythm.

Rob Hetzer, Hospital - MT, Contractor November 7, 2011 8:49 PM
Honolulu HI

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About this Blog


    Scott Warner, MLT(ASCP)
    Occupation: Laboratory Manager
    Setting: Critical Access Hospital
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