Kidney function is most commonly monitored using estimated glomerular filtration rate (eGFR) and albuminuria, which are markers of glomerular function.1 Tubular secretion is an important nonglomerular kidney function and is critical for toxin excretion and excretion of nonfiltered endogenous metabolites.2,3 Creatinine is filtered by the glomerulus and secreted by the proximal tubule such that creatinine clearance (CLcr) overestimates GFR by 10% to 20%. Whether measuring secretory function provides additional insights into kidney tubule health is uncertain.