This study was undertaken to evaluate the precision and sensitivity of three different commercial APTT reagents containing the activators kaolin, micronized silica, or ellagic acid. These reagents varied greatly in their ability to detect mild coagulopathies. The ellagic acid reagent was able to detect the mildest deficiencies for the most common coagulopathies. This reagent was sensitive to 50% levels of Factor VIIIC, whereas the APTT with the kaolin reagent was not prolonged until levels of 35% or less were attained. The micronized silica reagent was the least sensitive to Factor IX deficiency, detecting levels of 12% or less. Precision was similar for all reagents when tested with normal and slightly abnormal plasmas. Since methods and instrumentation vary, each laboratory should evaluate their APTT reagent to determine its precision and sensitivity.