Prevalence of Antiphospholipid Anitbodies in Idiopathic Venous Thromboembolism.

The presence of antiphospholipid antibodies (APLA), which include lupus anticoagulant (LA) and anticardiolipin antibodies (ACA), increases the risk of venous thromboembolism (VTE). Although, the association demonstrated between LA and VTE is strong, that between ACA and VTE is weak with the exception of high titers. This suggestion has been that this may be related to the induction of ACA by acute illness. This may be the explanation for the widely varying frequency of ACA positivity in patients with acute VTE in the current medical literature. The prevalence of LA in patients with acute VTE is more consistent but the value of testing for ACA and LA at presentation of acute VTE remains unknown. We routinely test all idiopathic VTE patients at presentation for LA and ACA. It had been our impression that most positive results revert to normal with subsequent testing. We sough to test this hypothesis. The objective was to determine the prevalence of abnormal LA and ACA results and the frequency of subsequent normalization in patients presenting with a first idiopathic venous thromboembolic event. A retrospective chart review was conducted at the Thrombosis Unit at the Ottawa Hospital. 278 charts of unrelated, consecutive patients with idiopathic VTE were reviewed. 232 patients with single idiopathic venous thromboembolic events with documented LA and ACA results were included. The patients were divided into two groups based on the time between the acute VTE and initial APLA test. The frequency of abnormal LA and ACA on initial and repeat tests was determined in each group. 170 and 177 patients were screened for LA and ACA within one month of the acute VTE. On initial tests, LA and ACA were detected in 14.1% and 7.9% of patients, respectively. However, LA and ACA were present in only 1.2% and 1.7% of patients, respectively, on subsequent tests. In patients screened more than one month after an acute VTE 3.8% of the 52 patients had evidence of LA on initial testing and all were positive on repeat tests. ACA were present in 8% of patients on initial tests and in 6% (75% of the initial positive results) of patients on subsequent tests. A high frequency of positive LA and ACA tests results is observed when these tests are performed at the time of presentation of acute VTE. Repeat testing beyond one month from presentation demonstrates most of the results return to normal suggesting false positive results are common at presentation. However, when the tests are performed one month or longer after the acute VTE, the frequency of positive APLA test is much lower and false positives are uncommon. Screening for APLA should not be performed until at least one month after the diagnosis of idiopathic.