White cell counts that are reported on the CBC are directly counted from an automated instrument or by manual method. The age of the patient directly influ-
138 Part III • White Cell Disorders ences whether this number is within or outside of the reference range (Table 9.2). Pediatric reference ranges show more variability than do ranges for adults. Some of the peculiarities of the newborn include highest white counts at 3 months.
The WBC differential is an evaluation of the types of mature white cells in the peripheral circulation. Although only a snapshot of the white cell concentration at a particular moment in time, the differential offers valuable information as to the hematological status of an individual and their response to any circumstances which may alter hematological status. In general terms, the differential is performed on a well-stained, well-distributed peripheral smear.
The peripheral smear is evaluated for distribution at X10 and then a white cell estimate is performed at X40 (refer to Chapter 20 for procedures). Following this, a differential count is performed. One hundred white cells are counted and the percentage and identification of each type of white cell is recorded. These percentages are compared to the reference ranges for an individual according to age (Table 9.3). White cell estimates provide important quality control data for the technologist performing the differential. If the white cell estimate fails to agree with the automated count, then perhaps the wrong smear was pulled and an investigation should proceed to correct this error. In most cases, one hundred white cells are carefully counted and identified, but there are circumstances which may warrant counting two hundred white cells. Students will need to refer to the Standard Operating Procedure at each clinical site for recommendations for counting a 200-cell differential. If this is the case, the physician should be aware that a 200-cell count was performed. Table 9.4 lists general conditions when a 200 cell count differential may be desirable. Critical values outside of the reference range have been established for each clinical facility regarding the CBC and the differential. These values are usually flagged by the automated instrument and must be reported to the physician and/or the pathologist in a timely fashion. Laboratory personnel keep careful
Birth 4 to 40
4 Years 5 to 15
Adult 4 to 11
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