Effect of weight, sex, age, clinical diagnosis, and thromboplastin reagent on steady-state intravenous heparin requirements

Background: There is a significant direct relationship between steady-state intravenous heparin dose requirements and total body weight. Less is known about whether sex, age, clinical diagnosis, and the thromboplastin used to measure the activated partial thromboplastin time (aPTT) affect heparin dose requirements. Methods: Four cohorts of patients treated with intravenous heparin were gathered from 3 hospitals: 2 cohorts with deep vein thrombosis (DVT) and 2 cohorts with coronary artery disease (CAD). For each clinical diagnosis, half the patients were monitored using one aPTT reagent and the remainder were monitored using a second reagent. Heparin doses and aPTT measurements were recorded, and the dose necessary to achieve an aPTT ratio of 2.0 was calculated using a computer software program. Results: We analyzed the records of 340 patients: 165 with DVT and 175 with CAD. Using analysis of variance, there was a significant difference in the steady-state heparin requirements among patients with DVT compared with patients with CAD (P<.001). For each clinical diagnosis, the use of a different thromboplastin reagent did not affect heparin close requirements (P>.42). Linear regression modeling disclosed that the steady-state heparin dose for patients with DVT was a function of weight plus an effect modifier involving weight and age, whereas for patients with CAD there was only a weak relationship with weight. Conclusions: Steady-state heparin dose requirements were significantly different in patients with DVT compared with patients with CAD, suggesting that different dosing nomograms are needed for each condition. For patients with DVT, the accuracy of the initial heparin dose estimate may be improved by considering the patient's age and weight.