Clinical use of the heparin nomogram

To the Editor.— I would like to elaborate further on an issue that Cruickshank and colleagues 1 briefly mention regarding the general use of their recently published heparin nomogram. There are at least eight different sources of thromboplastin and 14 different laboratory systems currently in use in North America for activated partial thromboplastin time (APTT) determination. The College of American Pathologists issues proficiency survey plasma samples to participating hospitals across the nation on a quarterly basis and determines the mean APTT and the standard deviation for each method employed by participating laboratories. In 1990, the APTT determined from a single specimen (H2-01) ranged from 30.9±0.9 to 46.8±1.8 seconds, depending on the method used. 2 At McMaster University, the site of Cruickshank's study, a highly sensitive thromboplastin reagent is used, and APTTs ranging from 60 to 85 seconds correlate with therapeutic heparin levels (0.2 to 0.4 U/mL). At our institution, plain