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Estimate of renal function in oldest old inpatients by MDRD study equation, Mayo Clinic equation and creatinine clearance.

BACKGROUND
Patients older than 85 years are increasingly admitted to hospital care settings. Despite this, the clinical employment of equations for estimating glomerular filtration rate (GFR) has been scarcely investigated so far in this age group. Our study compared 2 commonly employed equations to estimate GFR, as well as measured 24-hour creatinine clearance (CrCl), in patients aged >or=85 years.


METHODS
Seventy-three patients consecutively admitted over 4 months to our Internal Medicine Department had an accurate 24-hour urinary collection, as well as serum and urinary creatinine determinations. Measured CrCl was compared with the GFR values estimated by the Modification of Diet in Renal Disease (MDRD) Study and Mayo Clinic quadratic (MCQ) equations.


RESULTS
GFR values derived by MDRD and MCQ equations and CrCl significantly differed from each other in the whole sample. CrCl negatively correlated with age (r=-0.389, p<0.001), at variance with GFR levels obtained by both the MDRD and the MCQ equations. The 3 estimates of renal function significantly correlated with each other, these correlations persisting after correcting for age, serum albumin and 24-hour urinary creatinine. Despite the visual impression of Bland and Altman plots, the overall agreement between methods was poor. Moreover, the proportion of patients classified by the 3 GFR estimates into each stage of kidney disease as specified in the Kidney Disease Outcomes Quality Initiative (K/DOQI) guidelines significantly differed.


CONCLUSIONS
In patients older than 85 years, the tested equations for estimation of GFR and the measured 24-hour CrCl provide significantly different results, so that they may not be used interchangeably in clinical practice.