Protective effect of hydroxychloroquine on renal damage may be biased: comment on the article by Pons-Estel et al

To the Editors:
I read with great interest the article by Pons-Estel et al, recently published in Arthritis Care & Research, on hy- droxychloroquine (HCQ) use and its effect on renal dam- age in persons with underlying lupus renal disease (1). The authors found that HCQ use (classified as ever used versus never used) by persons with renal disease, but not yet any renal damage as defined by the Systemic Lupus International Collaborating Clinics/American College of Rheumatology Damage Index renal domain, reduced the risk of subsequent renal damage with an adjusted hazard ratio (HR) of 0.29. As presented, this article has 2 major limitations in study design that make this finding ques- tionable.