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.