Orders for manual differentials.
A We know from numerous studies that an automated differential has distinct advantages over a manual differential. Manual differentials are hindered by intra- and inter-observer variation, uneven distribution of cells on the slide, statistical sampling errors due to a lower total number of cells counted, and the possibility of recording errors. In addition, a manual differential adds significantly to the turnaround time of the CBC results. Because automated differentials count many more cells (often 10,000 compared to 100 cells for a manual differential), and are independent of the manual problems that can lead to errors, they produce results that have been shown to be statistically more accurate than manual counts.
Of course, automated differentials cannot completely replace manual differentials; and there are certain situations (failure of the machine to provide a parameter, instrument flags, and other pre-determined laboratory factors) that still require a manual differential. The second important reason for doing manual review is to do a morphologic examination for blasts, immature monocytes, dysplasia, or other abnormal cells (lymphocytes, platelets, and red blood cells).
The goal, obviously, is to maximize efficiency and accuracy by doing manual differentials only in situations where it is helpful or necessary. Some statistics and graphs are available in a 2002 paper. (1) This paper discusses the problems associated with a manual differential and the history of automated cell counters, and provides graphs and charts showing the primary advantages of automated counts.
Despite our efforts, however, there are always going to be clinicians who will order a manual differential up front with their initial CBC. In most cases, they do this merely out of habit, or because they falsely believe that the manual differential is superior to the automated type. Attempts to educate these clinicians, as you are trying to do, are invaluable, and will likely require repeated efforts. In addition to this, another useful strategy (and one we currently use) is to simply no longer allow a manual differential to be ordered up front. In our case, we have removed it from our requisition. The clinician can still request a manual differential by writing it in the "other test" location, or by calling the laboratory to request it. But in these situations, we will do a smear review rather than an actual manual differential. This involves scanning the slide and confirming that it looks consistent with the automated differential. If so, the automated differential is used, and a comment of "smear reviewed" is added. In this way, the clinician still has the possibility of notifying the laboratory that a manual differential may be needed for a particular patient but attempts to avoid its overuse. The manual differential is, therefore, primarily reserved for those situations when the instrument flags or other laboratory policy deems it necessary.
--Kathleen Siechen, MD
--Guang Fan, MD, PhD
Oregon Health and Science University
1. Pierre RV. Peripheral blood film review. Clin Lab Med. 2002;22(1):279-297.
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 firstname.lastname@example.org.
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|Title Annotation:||Answering your questions|
|Author:||Baer, Daniel M.|
|Publication:||Medical Laboratory Observer|
|Date:||Jan 1, 2008|
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