Sarcopenia, characterized by progressive loss of muscle mass and strength, is highly prevalent in patients with Type 2 Diabetes Mellitus (T2DM) and significantly contributes to functional decline and poor outcomes. However, diagnosis remains challenging due to limited access to specialized equipment in routine practice. The Sarcopenia Index (SI = creatinine x eGFRcys), derived from common renal function blood tests in T2DM, has been proposed as a simple and cost-effective surrogate marker of muscle health. Nevertheless, evidence regarding its diagnostic accuracy and its added value to established prediction models is still limited in outpatient diabetic populations. This study aimed to evaluate the incremental predictive value of the SI for sarcopenia in patients with T2DM.

This cross-sectional study included 74 T2DM patients aged ≥50 years, consecutively recruited from an outpatient clinic in southern Vietnam between August and October 2025. Sarcopenia was defined according to the updated AWGS 2025 criteria, using low ASM/BMI plus low handgrip strength; severe sarcopenia additionally required low gait speed. SI was calculated as serum creatinine multiplied by cystatin C–based eGFR. Two prediction models were constructed: Model 1 included standard muscle assessments; Model 2 incorporated SI. Diagnostic performance was evaluated using ROC analysis.

The prevalence of sarcopenia and severe sarcopenia was 41.9% and 12.2%, respectively. SI showed significant correlations with ASM/BMI (r = 0.38, p < 0.001) and handgrip strength (r = 0.52, p < 0.001), and an inverse correlation with gait speed (r = –0.31, p < 0.05). While Model 1, including the three standard muscle assessments, achieved good predictive performance (AUC = 0.8492, 95% CI: 0.760–0.938), the inclusion of SI in Model 2 significantly enhanced the model's accuracy (AUC = 0.8807, 95% CI: 0.796–0.965). This indicates that SI provides independent predictive value beyond traditional muscle parameters.

SI is a useful biochemical marker that enhances the diagnostic accuracy for sarcopenia when combined with conventional muscle assessments. Because it is derived from routine laboratory tests, SI may serve as a practical and low-cost screening tool for early identification of sarcopenia in high-risk T2DM outpatient populations. Larger, multicenter studies are needed to validate these findings and establish clinically relevant cut-off values.