Hydroxychloroquine (HCQ) has been promoted as a potential treatment of coronavirus disease 2019 (COVID-19), but there are safety concerns.

The purpose of this study was to determine the effects of HCQ treatment on QT interval.

We retrospectively studied the electrocardiograms of 819 patients treated with HCQ for rheumatologic diseases from 2000 to 2020. The primary outcome was corrected QT (QTc) interval, by Bazett formula, during HCQ therapy.

Mean patient age was 64.0 ± 10.9 years, and 734 patients (90%) were men. Median dosage of HCQ was 400 mg daily, and median (25th-75th percentile) duration of HCQ therapy was 1006 (471-2075) days. Mean on-treatment QTc was 430.9 ± 31.8 ms. In total, 55 patients (7%) had QTc 470-500 ms, and 12 (1.5%) had QTc >500 ms. Chronic kidney disease (CKD), history of atrial fibrillation (AF), and heart failure were independent risk factors for prolonged QTc. In a subset of 591 patients who also had a pretreatment electrocardiogram, mean QTc increased from 424.4 ± 29.7 ms to 432.0 ± 32.3 ms (P <.0001) during HCQ treatment. Of these patients, 23 (3.9%) had either prolongation of QTc >15% or on-treatment QTc >500 ms. Over median 5.97 (3.33-10.11) years of follow-up, 269 patients (33%) died. QTc >470 ms during HCQ treatment was associated with a greater mortality risk (hazard ratio 1.78; 95% confidence interval 1.16-2.71; P = .008) in univariable but not in multivariable analysis.

HCQ is associated with QT prolongation in a significant fraction of patients. The risk of QT prolongation is higher among patients with CKD, AF, and heart failure, who may benefit from greater scrutiny.