Low Creatinine Potentially Overestimates Glomerular Filtration Rate in Older Fracture Patients: A Plea for an Extensive Use of Cystatin C?

Aim: Muscle mass is frequently reduced in older patients experiencing injurious falls and may further reduce during hospitalization for bone fracture. In these patients, renal function may be overestimated, because it is usually calculated using serum creatinine, which is strictly related to muscle mass. We evaluated if creatinine levels change during hospitalization in older patients with fracture. We also assessed the role of cystatin C as a more appropriate marker of renal function, comparing estimated glomerular filtration rate (eGFR) according to different formulas based on creatinine and/or cystatin C levels. Methods: Patients aged 65+ years, consecutively hospitalized for fracture, were enrolled in a prospective cohort study. Creatinine and cystatin C levels were measured at baseline and in the post-operative period; eGFR was calculated using six equations based on creatinine and/or cystatin C. Results: 425 patients were enrolled (mean age 84 years, mean creatinine 0.97 mg/dL, mean cystatin C 1.53 mg/L). Creatinine levels significantly decreased after surgery (p<0.001), while cystatin C remained stable. According to creatinine-based formulas, eGFR was < 60 mL/min/1.73 m2 in 29–30% at baseline and only in 17% participants in the post-operative period. Conversely, according to equations including cystatin C, eGFR was < 60 mL/min/1.73 m2 in half to three-quarters of the sample at all assessments. Conclusions: In older fractured patients, creatinine levels decline during hospital stay and may possibly overestimate renal function, whereas cystatin C remains stable. Whether cystatin C is a more reliable marker of renal function in this specific population should be further investigated.