Observational studies in glomerular diseases suggest that proteinuria reduction translates to improved outcomes. Because of a dearth of high-quality studies supporting this association, proteinuria is not universally recognized as a surrogate end point. Barbour et al. report on the longitudinal measure of proteinuria that best predicts hard renal outcomes. Their finding that time-varying proteinuria is the most reliable proteinuria metric is a necessary step in establishing proteinuria as a surrogate end point.