THE ESTIMATION OF RENAL FUNCTION IN SEVERELY OBESE PATIENTS. NEW TOOLS FROM BODY COMPOSITION ANALYSIS

Background

Obesity may concurr to cause kidney disease, thus an accurate evalutation of renal function is important in obese subjects.

Unfortunately, in these patients the accuracy of prediction formulas based on serum creatinine (PCr) and anthropometric data is quite low.

Aim

The aim of this study was evaluate a new method to predict urinary creatinine excretion (UCr) and then creatinine clearance (CCr) by means of a formula which combines the value of body cell mass (BCM) and plasma creatinine (Pcr).

Patients and Methods

Patients:

Seventy-eight patients (57 women), aged 19-66 years

Body Weight 82.5-210, mean 127 kg; BMI 35.2-73.6, mean 45.6 kg/m²

Plasma Creatinine 0.57-1.06 mg/dL

Examined parameters:

Plasma and urine creatinine (urine collection 2 hrs);

CCr measured (m-CCr) with the standard formula;

CCr predicted by CG formula (CG-CCr) and by Salazar&Corcoran formula (S&C-CCr). 

Body Cell Mass (BCM) was measured using a single frequency impedance analyzer (Fig.1). 

Results: Prediction of Urinary Creatinine Excretion

Measured 24h-UCr was 976-3684 mg, m 1801; BCM was 25-74 kg (m 49.4 kg men, 31.7 kg women).

A strict linear correlation was found between 24h-UCr and BCM (r=0.804).

From the realtionship found between measured UCr and BCM, UCr was predicted as:

BCM UCr = 57 + 47.9 * BCM .

The correlation and the agreement betweem predicted an measured UCr were strict (Fig.2).

Results: Prediction of Creatinine Clearance

Creatinine Cl was simply calculated from BCM and Pcr.

BCM CCr (ml/min) = BCM UCr (mg/24 h) 

                                   PCr (mg/ml) *1440

For comparison CCr was predicted by CG formula (CG-CCr) and by Salazar&Corcoran formula (S&C-CCr).

The correlation of the different formulas with measured CCr resulted similar.

However the agreement between BCM-CCr and measured CCr was definitely better than the other formulas (Fig.3)

The mean difference between BCM-CCr and measured CCr was insignificant, differently from the other predictions. BCM-CCr shoiwed also the lowest prediction error (Fig.4).

Conclusion

Urinary creatinine excretion and creatinine clearance can be more accurately predicted in severely obese patients from the measurement of body cell mass combined with serum creatinine, than with other formulas.