Background Thrombocytopenia is a common hematological feature of Systemic Lupus Erythematosus (SLE), affecting 10–40% of patients. Our objective was to characterize the frequency of thrombocytopenia in SLE and determine its time of onset during the course of the disease and its severity. We also sought to identify factors predicting the development of thrombocytopenia.

Methods Single centre cohort analysis of 707 patients with SLE followed for up to 40 years. We reviewed the patients’ clinical notes identifying the presence of thrombocytopenia, its time of onset and ascertained other clinical and serological features of the disease. Thrombocytopenia was classified as mild (100–149×10⁹/L), moderate (31–99×10⁹/L) or severe (≤30×10⁹/L platelets). It was also classified as asymptomatic, with minor bleeding or with major bleeding. A case control analysis was conducted, using the patients in the lupus cohort that did not develop thrombocytopenia as a control group.

Results 23% of patients (n=162) had thrombocytopenia prior to or during the course of SLE. Twenty-three patients (14.2%) had isolated Idiopathic Thrombocytopenic Purpura (ITP) before the diagnosis of SLE. Median follow-up time was 19 years (IQR=13). Most patients (N=67, 41.4%) had mild thrombocytopenia. Most patients (n=96, 59.3%) developed asymptomatic thrombocytopenia and only 6 patients (3.7%) had major bleeding events in the context of thrombocytopenia. Patients of South Asian ethnicity had an increased risk of developing of thrombocytopenia (OR=3.33) compared to Caucasian patients (p=0.002).Haemolytic Anaemia (OR=5,49; p<0.001) and kidney involvement (OR=1.81, p=0.004) were also associated with a higher risk of developing thrombocytopenia.

Conclusions Although a common manifestation of SLE, thrombocytopenia may be a marker of severe disease being associated with other organ involvement, notably lupus nephritis and other haematological complications, such as haemolytic anaemia.