Abstract P177: Urine Creatinine Based Estimates of Fat Free Mass: The Rancho Bernardo Study

Abstract

Background: Sarcopenia is difficult to identify without imaging. An accurate, easily obtainable screening tool would allow identification of older persons who might benefit from interventions to increase muscle mass. Urine creatinine excretion rate (CER) on 24-hour urine collections is highly correlated with muscle mass but collection is cumbersome. Spot urine creatinine (UCr) may also mark muscle mass, but is influenced by transient changes in urine tonicity. We previously developed a prediction equation to estimate CER (eCER) on 24-hour urine specimens. Whether this equation is associated with measures of muscle mass and whether spot UCr provides similar estimates to eCER is unknown. Methods: We measured spot UCr and fat free mass (FFM) by dual energy x-ray absorptiometry (DXA) among 1372 community-dwelling individuals. We used the equation developed in our prior study to estimate CER: eCER (mg/day) = 879.89 + 12.51*weight (kg) - 6.19*age + 34.51 if black - 379.42 if female. Spearman correlation coefficients and linear regression were used to determine strengths of association of eCER and spot UCr with FFM by DXA. Results: Mean (SD) age was 70 (11) years, 58% were women, and mean eGFR was 69 (15) ml/min/1.73m 2 . Mean values for FFM by DXA, eCER, and spot UCr were 47 (11) kg, 1107 (339) mg/day, and 94 (55) mg/dL, respectively. eCER was more strongly correlated with FFM by DXA (r = 0.93, p<0.001) than spot UCr (r = 0.39, p<0.001). Figure 1 shows the mean regression lines and 95% limits of agreement for the association of eCER (A) and spot UCr (B) with FFM; the 95% limits of agreement were narrower for eCER compared to spot UCr. The regression equations were: FFM (kg) = 13.21 + 0.03*eCER (mg/day) and FFM (kg) = 39.3 + 0.08* spot UCr (mg/dL). Conclusions: An equation incorporating age, weight, sex, and race to estimate CER was highly correlated with FFM by DXA and was more precise than spot UCr in community-dwelling older adults. Future studies should evaluate whether eCER provides a simple method to predict frailty and mortality in older persons.