The effect of capillary tube diameter on microhematocrit value.

Changes in percentages of donor deferrals associated with changes in the capillary tube size used for microhematocrit determinations led us to study the variables which influence microhematocrit values. Duplicate microhematocrit determinations were performed on a normal donor population with capillary tubes of different diameter, as well as with different anticoagulants, sample sites, centrifugation times, and degrees of operator skill. Microhematocrit values varied directly with capillary tube diameter over the range of diameters tested, and varied independently of sample site, centrifugation time, anticoagulant, and operator. The magnitude of the difference in microhematocrit values between small and large capillary tubes was approximately one volume percent in the microhematocrit range. Although differences of this magnitude are without clinical significance, they can be of substantial importance in attempts to minimize donor deferrals in a blood center. In our center, increasing the diameter of the capillary tube used for the microhematocrit determination allowed the collection of 6500 additional units of blood annually, or an approximate 3.5 percent increase in total collections.