Immune thrombocytopenia is rare in patients with antiphospholipid syndrome without systemic lupus erythematosus: insights from a series of 351 patients.

OBJECTIVES: To describe the aetiologies of thrombocytopenia in patients with antiphospholipid syndrome (APS) and to assess the frequency and characteristics of immune thrombocytopenia (ITP), including its association with SLE. METHODS: We retrospectively analysed patients with APS who experienced at least one episode of thrombocytopenia (platelet count <100 x 10ˆ9/L) in a tertiary referral cohort enriched in severe cases, particularly catastrophic antiphospholipid syndrome (CAPS). Individual medical records were systematically reviewed to determine the cause of thrombocytopenia. ITP was stringently defined by exclusion of alternative diagnoses and by a documented response to ITP-specific therapies. Clinical, biological and therapeutic characteristics were analysed. The risk of severe haemorrhagic events was evaluated. RESULTS: Among 351 patients with APS, 102 (29.1%) experienced thrombocytopenia and were included in the study. The median platelet nadir was 34.5 x 10ˆ9/L, and 33 (32.4%) had severe thrombocytopenia (<20 x 10ˆ9/L). The most frequent cause of thrombocytopenia was CAPS (n=75, 73.5%), followed by ITP (n=12, 11.8%), including two patients who had both CAPS and ITP. 11 (10.8%) patients had other identified causes, while thrombocytopenia remained unexplained in six patients (5.9%).Patients with ITP more frequently had associated SLE than those with CAPS (80.0% vs 35.6%, p=0.013). Overall, isolated ITP without SLE accounted for only 0.6% of the entire APS cohort. In survival analysis, a graded increase in the risk of severe haemorrhagic events was observed with increasing thrombocytopenia severity, although the association did not reach statistical significance (HR 2.55, 95% CI 0.94 to 6.94 for platelet nadir <20 x 10ˆ9/L vs >100 x 10ˆ9/L). CONCLUSION: In patients with APS, the underlying cause of thrombocytopenia can be identified in most cases through careful review of medical records. True ITP is uncommon and appears to be exceptionally rare in patients with APS without concomitant SLE. These findings highlight the importance of thoroughly investigating alternative causes of thrombocytopenia and of systematically screening for SLE before diagnosing ITP in patients with APS. TRIAL REGISTRATION NUMBER: NCT02782039.