The activated clotting time (ACT) is routinely used for monitoring of heparin effects during cardiopulmonary bypass (CPB). However, ACT is not a specific assay for heparin and may be influenced by several other factors, which may be misleading with regard to the proper administration of heparin and protamine. In this pilot study, we compared a new test, the high-dose thrombin time (HiTT), with the conventional ACT test for both in vitro and in vivo heparin-induced anticoagulation. Our in vitro results showed that there were heparin dose-dependent increases in ACT and HiTT. Data on 30 adult cardiac patients indicated that HiTT correlated well with heparin concentration both after initial heparin administration and during CPB (r = 0.645 and 0.515). Hypothermia and hemodilution occurring during CPB did not alter HiTT results. ACT also correlated well with both heparin concentration and HiTT before CPB, but the linear relationship was lost during CPB. Our results suggest that HiTT is a useful assay for monitoring heparin effects during cardiac surgery, even during hypothermia and hemodilution.

The dilute tissue thromboplastin inhibition (DTTI) test (Schleider et al, 1976) is a sensitive but non-specific test for lupus anticoagulant (LA). False positive results are seen in patients with clotting factor deficiency involving the extrinsic pathway and also in some patients with specific factor inhibitors (Triplett et al, 1983; Rosove et al, 1986). Since the effect of LA is phospholipid dependent but those of factor deficiency and specific inhibitors are not, we analyse the test results by comparing the degree of inhibition using different dilutions of tissue thromboplastin and express it as the DTTI index. This is defined as the clotting time ratio with 0.2% tissue thromboplastin divided by the clotting time ratio with 2% tissue thromboplastin. We also perform a dilute tissue thromboplastin time with platelet substitution to see if this could neutralize the inhibition caused by LA. Both of these modifications can reliably distinguish LA from other conditions associated with prolonged APTT better than the original DTTI test.