Characterization of the WBC Differential Count
A physician ordering serial WBC differential counts on the typical hospital/clinic patient assumes that changes in the percentage representation of the various cell types will be due to disease.
The validity of this assumption was examined by analyzing 117,000 differential counts performed over an 18-month period. On each patient who had more than one differential during the study period, consecutive counts were paired and the variance in each cell type was analyzed.
The patient population included both hematologically normal and hematologically sick individuals. All of the differentials from the latter group were excluded and the remaining variance determined. Ascribing this variance to a combination of analytical and sampling variability plus physiological changes, it was possible to determine how much of the total variance in WBC differentials as commonly ordered and performed was due to disease. More than 75% of the total variance encountered in neutrophils, monocytes, basophils, and lymphocytes was due to factors other than disease. Only in the case of eosinophil counts was the proportion of variance due to disease as high as 50%.
Abnormalities which had been used to identify slides from hematologically ill patients were then analyzed. Those definitions of abnormality which could be applied automatically by a whole blood analyzer identified only 63% of the abnormal blood films.
It was concluded that: (a) statistically, an observed change in a patients’s routinely ordered WBC differential is much more likely to be due to factors other than disease, and (b) the only way presently available to identify all of the recognized abnormalities which may be present in a large group of peripheral blood films is to examine each film visually.
Key WordsLeukocytes White cells Differentials Automation Variance
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