Evaluation of the capillary microhematocrit as a screening test for anemia in pediatric office practice

The capillary microhematocrit test is widely used to screen pediatric patients for anemia. Recently, it has been suggested that this method produces spuriously elevated values compared with venous hematocrits measured by a Coulter electronic counter and might consequently fail to detect children who are truly anemic. To address this issue we studied 66 white children 9 months to 14 years of age whose capillary hematocrits were either below, equal to, or one or two points above the lower limit of normal for age. Venous specimens were obtained simultaneously with the capillary sample; hemoglobin, hematocrit, and mean corpuscular volume results were obtained using a Coulter electronic counter. Using published standards of venous hemoglobin, we determined the sensitivity, specificity, and predictive values of the capillary microhematocrit in this population of patients with low or borderline values. Twenty of the 66 patients had venous hemoglobin values less than the lower limit of normal. The sensitivity of the microhematocrit was 90.0%; the specificity was 43.5%. The predictive values for a normal (negative) hematocrit was 90.1%; the predictive value for a low (positive) hematocrit was 40.9%. We conclude that the microhematocrit method using capillary blood will miss very few patients with significantly low venous hemoglobin values and is thus an acceptable screening test for anemia. Because it does not require expensive equipment or special skill to obtain the specimen or perform the test, it is ideal for physicians' offices or nonhospital-based clinics.