Plasma factor VII and thrombin–antithrombin III levels indicate increased tissue factor activity in sickle cell patients

Although the mechanisms involved in the persistent clinical complications of sickle cell disease have not yet been fully delineated, previous studies suggest that sickle cell (HbSS) patients have a disposition to generate more thrombin and plasma in vivo than normal subjects. The reasons for the impaired regulation of haemostasis in HbSS patients is poorly understood. We report studies evaluating the extent to which in vivo coagulation and fibrinolysis are altered in HbSS patients in steady state. The concentrations of total factor VII (F(VII)t), factor VII zymogen (F(VII)z), thrombin-antithrombin III (TAT), fibrinopeptide A(FPA), and fibrin D-dimer in plasmas of 50 normal controls (HbAA) and 45 HbSS steady state patients, were measured using sensitive and specific enzyme-linked immunoassays. The average plasma concentration of F(VII)t, in sickle cell plasma was significantly lower than that of the control subjects (0.70 +/- 0.19 U/ml versus 1.16 +/- 0.41 U/ml), whereas F(VII)z in the patients and controls were 0.47 +/- 0.15 U/ml and 1.15 +/- 0.33 U/ml respectively, P < 0.001. Both measures of factor VII suggest a higher factor VII turnover in sickle cell disease. The mean concentration of TAT in the plasma of HbSS patients were significantly higher than those of HbAA controls (371 +/- 44 pM versus 42 +/- 2 pM) (P < 0.001), a difference that is strongly indicative of higher rates of in vivo thrombin generation by HbSS patients. Plasmas of HbSS patients had significantly higher concentrations of FPA compared to those of the control subjects (12.85 +/- 1.96 ng/ml versus 4.22 +/- 0.37 ng/ml) (P < 0.001). The D-dimer levels were also higher in the HbSS than control plasmas (1029.6 +/- 58.6 ng/ml versus 224.3 +/- 27.6 g/ml) (P < 0.001), with the patients' values being indicative of enhanced fibrinolysis. These results strongly suggest accelerated in vivo coagulation and fibrinolysis in HbSS patients even during steady state. They are consistent with the hypothesis that haemostasis is less tightly regulated in the HbSS patients than in HbAA controls. The altered regulation of haemostasis may contribute to the initiation of vaso-occlusive processes associated with sickle cell painful episodes.