Evaluation of albumin-based proteinuria selectivity index as a substitute for transferrin-based proteinuria selectivity index.

Background: The proteinuria selectivity index (SI) is a valuable marker of glomerular permeability. Traditionally, SI has been calculated using the clearance ratio of immunoglobulin G (IgG) to transferrin-SI (Tf)-due to historical limitations in albumin measurement accuracy. However, recent advances have enabled precise quantification of albumin, raising the possibility of using an albumin-based SI-SI (Alb)-in clinical practice. This study aimed to evaluate the correlation between SI (Alb) and SI (Tf) and to compare their diagnostic utility in patients with proteinuria.

Methods: We retrospectively analyzed 265 patients with proteinuria > 0.15 g/g·Cr who visited Ehime University Hospital between January 2014 and April 2024. SI (Alb) and SI (Tf) were calculated as the clearance ratios of IgG to albumin and IgG to transferrin, respectively. Linear regression was used to assess their correlation. Diagnostic performance for minimal change disease (MCD) in patients with nephrotic-range proteinuria (≥ 3.5 g/g·Cr) was evaluated using ROC curves, and AUCs were compared using DeLong's test.

Results: SI (Alb) strongly correlated with SI (Tf) in patients with proteinuria ≥ 3.5 and < 3.5 g/g·Cr. Among patients with nephrotic-range proteinuria, both indices effectively identified MCD, yielding comparable areas under the ROC curve. Sensitivity and specificity at optimal thresholds were similarly high.

Conclusion: SI (Alb) shows a high degree of concordance with SI (Tf) and offers comparable diagnostic accuracy for identifying MCD. Given its practical advantages, SI (Alb) may serve as a reliable and convenient alternative to SI (Tf) for assessing glomerular selectivity in patients with proteinuria.