Tissue factor activity of blood mononuclear cells is increased after total knee arthroplasty.

Tissue factor (TF) is present in small quantities in normal blood and is reported to be elevated in arterial and venous thrombosis. Patients undergoing total knee arthoplasty (TKA) are at high risk of post-operative venous thromboembolism (VTE). To evaluate the possible contribution of elevated blood TF to VTE risk, we performed serial studies of peripheral blood mononuclear cell (PBMC) functional TF procoagulant activity (PCA) in 19 patients after TKA. PBMC and platelet TF PCA were measured by a functional, clot-based assay following decryption with a calcium ionophore. Plasma TF antigen levels were measured by ELISA. All subjects received chemoprophylaxis and none had VTE. After TKA total TF PCA of PBMC was elevated in 19 of 19 subjects. The peak increase above preoperative levels was 1.1-13.6 fold (>two-fold in 58% and >three-fold in 42%). Median TF PCA of PBMC was not elevated following tourniquet removal, but it was significantly elevated on postoperative days 1 and 2. Thereafter, it decreased to near preoperative values at day 6. Neither platelet TF PCA nor plasma TF antigen levels increased significantly. Since the PBMC count did not rise, the increase in TF PCA was attributable to cell synthesis. The increase in blood TF PCA preceded the median time of diagnosis of venous thromboembolism after TKA established previously. These observations indicate a) TKA stimulates synthesis of encrypted PBMC TF PCA which is likely to contribute to the pathophysiology of VTE; b) TF antigen is not a reliable indicator of TF PCA.