Normal levels of protein C and protein S tested in the acute phase of a venous thromboembolic event are not falsely elevated

99 patients with acute idiopathic VTE who had normal PC and PS determination within the first 24 hours of presentation and who subsequently had their oral anticoagulation discontinued after six months of therapy. PC and PS determinations were repeated ≥6 months after starting treatment and ≥ 14 days after stopping warfarin. Proportions of patients who tested abnormal on the second test were calculated and 95% confidence intervals obtained using the Wilson's score method. Data from a previously published study on patients with abnormal initial tests was included for comparison.None of the 99 patients who had normal PC and PS initially had an abnormal result on repeated testing (0%; 95% CI 0 - 3.7%). Data from the previous study showed that, among patients who initially had abnormal results, 40% (95%CI 35.4-84.8%) were confirmed to have low PC and 63.6% (95%CI 16.8-68.7%) low PS on repeated testing. The difference between proportions was statistically significant (χ2 p-value < 0.001).Our results suggest that PC and PS can be determined during the acute phase of VTE and whereas abnormal results need to be confirmed with repeat testing at a later date, a normal result effectively rules out deficiency with only one test.Venous thromboembolism (VTE) is a common event, often precipitated by surgery, immobility or active malignancy[1]. Many cases, however, have no clear precipitant and are defined as idiopathic VTE [2-4]. The diagnostic work up for these patients includes testing for inherited and acquired hypercoagulable conditions, usually including functional quantitative assays for proteins C and S, and antithrombin, as well as testing for lupus anticoagulant, antiphospholipid antibodies, activated protein C resistance (with or without genetic testing for Factor V Leiden) and determination of the G20210A Prothrombin gene mutation[4]. Although from a practical standpoint this group of tests is most conveniently performed at the time of acute VTE diagnosis, concerns have