Indeterminate lupus anticoagulant results: Prevalence and clinical significance

BACKGROUND Reports of indeterminate lupus anticoagulant (LAC) results are common; however, no published data on their prevalence or clinical significance are available. We investigated the prevalence and clinical characteristics of patients with indeterminate LAC. METHODS We retrospectively reviewed the clinical and serologic characteristics of 256 unselected patients with LAC results. RESULTS Indeterminate results were observed in 32.7% of LAC profiles that were least frequent (25.4%) when activated partial thromboplastin time (aPTT) was normal, most frequent (39.8%) when aPTT was elevated, and were observed in 35% of patients taking warfarin. The final indeterminate LAC cohort included 65 patients with a mean follow-up of 18 months. Malignancy and autoimmune disease were present in 29% and 25% of patients, respectively. The most common thrombotic events were deep vein thrombosis (DVT) (28%), cerebral ischemic stroke (14%) and pulmonary embolism (14%). Patients with indeterminate results were more likely to be men, older, and with a history of DVT, superficial thrombosis, or myocardial infarction than patients with negative tests (N=106). Concurrent warfarin therapy was more prevalent in the indeterminate group, but was not statistically significant. In the multivariate analysis, none of the variables showed statistical significance. During follow-up, 10 of 16 patients with indeterminate results showed change in classification upon retesting. CONCLUSION Patients with indeterminate LAC results were common, and their clinical characteristics differed from those with negative results. There is a need for a prospective study of the clinical history of patients with indeterminate LAC results.