The accurate assessment of coagulation status is an important part of interventional procedures performed in the cardiac catheterization laboratory. While the traditional clinical means of assessing heparin anticoagulation has been with the activated partial thromboplastin time (APTT), the activated coagulation time (ACT) has come into widespread use in the catheterization laboratory as an assay of whole blood clotting time which can be performed rapidly at the bedside. The purpose of the present study was to (1) assess the anticoagulant effect of a 10,000 U bolus of heparin in PTCA patients and (2) document the relationship between ACTs and APTTs in a subset of these patients. Baseline and postheparin ACTs were measured using a HemoTec coagulation timer in 545 unselected PTCA patients. The average baseline ACT was 120 +/- 22 sec. After a 10,000 U bolus of heparin the average ACT was 249 +/- 44 sec; 58% of patients had an ACT less than 250 sec, 17% had an ACT between 250 and 275 sec, 12% had an ACT between 275 and 300 sec, and 13% had an ACT greater than 300 sec. A total of 175 paired ACT and APTT measurements were obtained in a random subset of these patients at baseline, after heparinization, and at 4-6 hr intervals after the procedure. The APTT was limited by absolute upper and lower limits of 150 and 22 sec; there were no such limits on the ACT. When limiting values were excluded, there was a strong overall correlation between ACT and APTT measurements (r = 0.92, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)