Surgery, venous thrombosis and anti-Xa.

Deep venous thrombosis is a common and unpredictable complication of surgery. In this study it is proposed that patients who develop this complication may be predicted by a low preoperative level of a naturally occurring inhibitor of coagulation, anti-Xa. Two groups of patients were investigated. Women taking the oral contraceptive pill had lower preoperative anti-Xa levels than their non-pill controls (P less than 0.01) and in addition had a significantly higher incidence of deep venous thrombosis (DVT) following emergency surgery (P less than 0.05). In 90 patients undergoing total hip replacement, the mean preoperative anti-Xa level of those patients who developed DVT was significantly lower than those who did not (P less than 0.001). Ninety-four per cent of patients with a preoperative anti-Xa level of less than 80 per cent developed DVT. The effect of low dose heparin on anti-Xa was studied. The results suggest a mechanism for the cause of postoperative thrombosis which also permits selection of individual patients who will develop this complication.