#2370 Creatinine, cystatin C, muscle mass, and mortality: findings from a primary and replication population-based cohort
Authors: Groothof Dion , Shehab Naser , Erler Nicole , Post Adrian , Kremer Daan , Polinder-Bos Harmke , Groen Henk , Pol Robert , Gans Reinold , Bakker Stephan J L 
Published: May 2024

Abstract

Background and Aims
Creatinine measures are used as initial test to derive eGFR and confirmatory testing with circulating cystatin C is recommended when creatinine-based eGFR is considered less accurate due to deviant muscle mass. Low muscle mass is associated with increased risk of premature mortality. However, the associations of circulating creatinine and cystatin C with muscle mass and mortality remain unclear and require further investigation to better inform clinical decision-making. This study aimed to disentangle these complex relationships and better inform clinical decisions on marker reliability in varying settings by investigating whether cystatin C adjustment could clarify the associations of circulating creatinine with muscle mass and mortality.

Method
We conducted a prospective, observational cohort study including 8, 437 community-dwelling adults enrolled in the Dutch PREVEND study and 5, 033 in the U.S. NHANES. Associations of serum creatinine and/or cystatin C with muscle mass surrogates and mortality were quantified with linear and Cox proportional-hazards regression, respectively. Missing observations in covariates were multiply imputed using Substantive Model Compatible Fully Conditional Specification.

Results
Mean (SD) age of PREVEND and NHANES participants (50% and 48% male) were 49.8 (12.6) and 48.7 (18.7) years, respectively. Median (Q1-Q3) serum creatinine and cystatin C were 0.79 (0.69-0.90) and 0.90 (0.70-1.00) mg/dl and 0.87 (0.78-0.98) and 0.91 (0.80-1.10) mg/l, respectively. Higher serum creatinine was associated to greater muscle mass, while serum cystatin C was not associated with muscle mass. Adjusting both markers for each other strengthened the positive relationship between serum creatinine and muscle mass and revealed an inverse association between serum cystatin C and muscle mass. In the PREVEND cohort, 1, 636 (19%) deaths were registered over a median follow-up of 12.9 (5.8-16.3) years with a 10-year mortality rate (95% CI) of 7.6% (7.1-8.2%). In the NHANES, 1, 273 (25%) deaths were registered over a median follow-up of 17.9 (17.3-18.5) years with a 10-year mortality rate of 13.8% (12.8-14.7%). Both markers were associated with increased mortality (Fig. panels A and B). Notably, when adjusted for each other, higher serum creatinine was associated with decreased mortality (Fig. panel C), while the association between serum cystatin C and increased mortality strengthened (Fig. panel D). The shapes of the associations in the PREVEND study and NHANES were almost identical.

Conclusion
The strong association between serum creatinine and muscle mass challenges its reliability as GFR marker, necessitating a more cautious approach in its clinical use. The minimal association between serum cystatin C and muscle mass supports its increased use as a more reliable alternative in routine clinical practice.