• glomerular filtration rate;
  • serum cystatin C;
  • Cockcroft-Gault;
  • Modification of Diet in Renal Study Group (MDRD) formula;
  • older people

OBJECTIVES: To assess serum cystatin C, compared with other markers of renal function, as a marker of renal function in the old old (aged 85 and older).

DESIGN: A cross-sectional analysis of data obtained in medically stable people aged 70 and older in a geriatric ward at a university hospital.

SETTING: University hospital in Belgium.

PARTICIPANTS: Forty-eight patients (17 men, 31 women) mean age ± standard deviation 84.4 ± 6.3 without acute illness or overt malignancy 7 days after admission were included. Twenty-five patients were aged 85 and older.

MEASUREMENTS: Blood samples and 24-hour urine collections were obtained from each patient to determine serum creatinine, serum cystatin C levels, serum albumin, and creatinine clearance. Glomerular filtration rate (GFR) was estimated using the Cockcroft-Gault formula and the Modification of Diet in Renal Study Group (MDRD) formula. On the same day, clearance of 51chromium ethylenediamine tetraacetic acid was performed in all patients as the criterion standard of GFR.

RESULTS: Serum creatinine (r = 0.68), serum cystatin C (r = 0.62), urinary creatinine clearance (r = 0.57), the Cockcroft-Gault formula (r = 0.82), and the MDRD-formula (r = 0.65) correlated significantly with GFR (P < .0001). Regression analysis showed that serum cystatin C and serum creatinine were comparable markers of renal function (Y = 0.442 ± 0.007 × GFR and Y = 0.494 ± 0.01 × GFR respectively). Receiver operating characteristic analysis showed a similar area under the curve for serum cystatin C and serum creatinine (P = .5) in detecting renal impairment (GFR <80 mL/min). The Cockcroft-Gault formula provides a good estimation of GFR when the GFR is less than 60 mL/min (Y = 1.11 ± 1.04 x GFR). When the GFR is greater than 60 mL/min, the Cockcroft-Gault formula underestimates GFR (Y = 11.01 ± 0.66 x GFR). In patients aged 85 and older, a slight decrease in GFR (51.8 ± 21.3 mL/min vs 65.2 ± 34.3 mL/min in patients aged 70–84; P = .10) is observed. This is reflected by a nonsignificant increase in serum cystatin C (P = .06), whereas serum creatinine is identical in both groups (P = .88).

CONCLUSION: Serum cystatin C, serum creatinine, the Cockcroft-Gault formula, the MDRD formula, and urinary creatinine clearance are comparable markers of renal function in the overall older population. The Cockcroft-Gault formula underestimates renal function in older people with GFR greater than 60 mL/min. In our study, serum cystatin C was not superior to serum creatinine in the detection of renal impairment.