A Retrospective Analysis of Creatinine-Based Kidney Function With and Without Sex Assigned at Birth Among Transgender Adults

BACKGROUND: Clinicians use sex-based kidney function estimating equations, but the appropriate sex modifier for transgender adults undergoing hormone therapy (HT) is undetermined. OBJECTIVES: Compare median estimated creatinine clearance (eCrCL; Cockcroft-Gault) and estimated glomerular filtration rates (eGFRs; Modification of Diet in Renal Disease [MDRD] and Chronic Kidney Disease Epidemiology Study [CKD-EPI]) before and during HT when estimated with and without sex assigned at birth. METHODS: Single-system retrospective cohort study of transgender adults (2007-2017) prescribed ≥90 days HT (index date = first order) and measured serum creatinine ≤6 months pre-index date (baseline) and ≤12 months post-index date. We grouped patients based on testosterone or estrogen treatment and compared eCrCL and eGFRs at baseline up to 6-12 months post-index date using equations based on sex assigned at birth (female or male modifier in testosterone or estrogen groups, respectively) or gender identity (male or female modifier in testosterone or estrogen groups, respectively). We used Wilcoxon signed-rank tests (Bonferroni correction) for all comparisons. RESULTS: In total, 29 (median age 26 years, follow-up 259 days) and 41 patients (29 years, 250 days) were prescribed testosterone or estrogen, respectively. In the testosterone group, the maximum eCrCL and eGFR changes based on sex assigned at birth were −14%, P = 0.0181; −18%; P = 0.0009, respectively, and based on gender identity were +5%, P > 0.025 and +11%, P = 0.0094, respectively. In the estrogen group, eCrCL or eGFRs based on sex assigned at birth did not change from baseline but based on gender identity were −17%, P < 0.0001 and −26%, P < 0.0001, respectively. CONCLUSION AND RELEVANCE: Female-based equations may underestimate kidney function in transgender adults undergoing testosterone or estrogen treatment. Prospective cohort studies are needed to confirm the clinical significance of these findings.