The aim of this study was to investigate anticardiolipin (aCL) (IgG and IgM) levels in patients with objectively confirmed venous thromboembolism (VTE). aCL IgG and IgM were measured using an in-house ELISA assay. When aCL levels were ≥10 U, a second measurement was performed using another sample at least 6 weeks after the first test. Patients tested twice with aCL levels ≥10 U were classified as having positive aCL and those with only one measurement ≥10 U had transitory aCL. aCL was evaluated in 282 outpatients with VTE, with median age of 39 years (range:15–94), 66.7% were women. We found 28 patients (9.9%) with positive aCL (IgG in 5.6%, IgM in 1.8% and IgG + IgM in 2.5%) and 29 (10.3%) with transitory aCL (IgG in 5.3%, IgM in 3.2% and IgG + IgM in 1.8%). The association of systemic disease with VTE had a significant effect on the occurrence of transitory aCL (OR= 3.6; 95%CI: 1.5–8.8), but not on positive aCL (OR= 1.7; 95%CI: 0.6–5.0). There was a significant correlation between the first and second measurements for IgG (rs= 0.58; p < 0.0001) and for IgM (rs= 0.49; p = 0.011). Patients with IgG < 30 GPLU showed a higher risk of having transitory aCL (OR= 4.9; 95%CI: 1.2–19.7; p= 0.027), and this effect was more pronounced in patients with IgM < 30 MPLU (OR=15; 95%CI: 1.2 – 113.6; p= 0.008). In conclusion, prevalence of transitory aCL was as high as positive aCL in patients with VTE, mainly among those with IgG or IgM < 30 U. Higher aCL levels were more frequently associated with repeatedly positive aCL. Transitory aCL was more frequent in the presence of systemic disease associated with VTE, which could reflect an epiphenomenon, whereas repeatedly positive aCL could play a more important role in the genesis of VTE.

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