Data collection
Data for demographic, clinical, and laboratory information of patients were gathered. Demographic data included sex and birth date. Clinical data included the onset time of ITP, symptoms associated with autoimmune diseases, the start and end time of treatments, side effects of medication, and progression of autoimmune diseases. Laboratory data included repeatedly measured platelet counts, the baseline ANA level, and other markers associated with autoimmune diseases (including lupus anticoagulant, anti-β2-glycoprotein I antibody (IgG/IgM), anticardiolipin antibody (IgG/IgM), and the level of complement components C3/C4). ANA titers ≥ 1:160 were considered positive.
HCQ usage
HCQ was prescribed by rheumatologists or hematologists in the setting of previously first-line or second-line treatment failure, suffering from symptoms associated with autoimmune diseases, or presence of positive markers associated with autoimmune diseases. The daily HCQ dosage was 4 ~ 6.5 mg/kg, usually starting at a low dosage. But the actual dosage of HCQ for each individual involved in this study during the course was unavailable due to the retrospective nature of this study. In this study, patients who received HCQ were continuously treated with this drug for a minimum duration of 3 months. HCQ would be stopped for side effects or no response. Considering longer time to response, HCQ was sometimes used in combination with other drugs when patients experienced low platelet counts and high-risk of bleeding.
Outcomes
The primary outcome was platelet counts measured in the follow-ups. Secondary outcome was responses to treatment at 3 months and 1 year from HCQ initiation. Complete response (CR) was defined as platelet count ≥ 100×109/L and absence of bleeding. Response (R) was defined as platelet count ≥ 30×109/L, at least 2-fold increase of the baseline count, and absence of bleeding. No response (NR) was defined as platelet count < 30×109/L, less than 2-fold increase of the baseline count, or bleeding. Loss of CR was defined as platelet count < 100×109/L or bleeding from CR. Loss of R was defined as platelet count < 30×109/L, less than 2-fold increase of the baseline count, or bleeding from R.