Evaluation of renal function in elderly patients: performance of creatinine-based formulae versus the isotopic method...

Measurement of the glomerular filtration rate (GFR) is recognized worldwide as the most accurate way of assessing kidney function. The prevalence of impaired renal function increases with advancing age. In this study we compared the clinical formulae Cockcroft-Gault (CG), isotope dilution mass spectrometry-modification of diet in renal disease (IMDS-MDRD) and chronic kidney disease epidemiology collaboration (CKD-EPI) with (99m)Tc-diethylene triamine pentaacetic acid ((99m)Tc-DTPA) in elderly patients over and under the age of 70 years in an attempt to establish which formula produces the best measurement of renal function in this population.

Patients were randomly selected from two age groups [<70 years (n = 37) and ≥ 70 years (n = 39)]. Two plasma samples were collected at 60 and 180 min after injection of (99m)Tc-DTPA, and the GFR was calculated applying Charles D. Russell's two-sample method.

In patients younger than 70 years, no statistically significant difference was found between GFR evaluated with (99m)Tc-DTPA and GFR obtained using the other methods.In patients aged at least 70 years, no statistically significant difference was found between GFR evaluated with (99m)Tc-DTPA and GFR evaluated using the CG real weight formula. Conversely, statistically significant differences were found between GFR evaluated with (99m)Tc-DTPA and GFR obtained using the CG normalized weight (P = 0.002), IMDS-MDRD (P = 0.024) and CKD-EPI (P = 0.028) formulae.

In patients older than 70 years, the use of the two 'classical' formulae (IMDS-MDRD and CKD-EPI) overestimated GFR in stage III CKD (GFR 30-59 ml/min) when compared with the gold standard (99m)Tc-DTPA method. Thus, in patients aged 70 years and above only the CG real weight formula provided unbiased results comparable to (99m)Tc-DTPA. In conclusion, in elderly patients, GFR measured using CKD-EPI and IMDS-MDRD serum creatinine-based formulae may be overestimated compared with that measured using (99m)Tc-DTPA GFR.