Impact of various body weights and serum creatinine concentrations on the bias and accuracy of the Cockcroft-Gault equation

Study objective: To evaluate the impact of various body weights and serum creatinine (S(c) (r) ) concentrations on the bias and accuracy of the Cockcroft-Gault creatinine clearance (C-G Cl(c) (r) ) equation compared with measured 24-hour Cl(c) (r) .

Design: Retrospective analysis.

Setting: Tertiary care hospital.

Patients: A total of 3678 patients with stable renal function and who underwent a 24-hour urine collection between July 1, 1996, and June 30, 2010.

Measurements and main results: For each patient, C-G Cl(c) (r) was calculated and compared with a measured 24-hour Cl(c) (r) . Body weight adjustments to the calculation were performed based on the following weight classifications: underweight, normal weight, overweight, obese, and morbidly obese. In addition, C-G Cl(c) (r) was calculated by using rounded S(c) (r) values based on two S(c) (r) thresholds-0.8 mg/dl and 1 mg/dl-for patients with measured S(c) (r) values below those thresholds. Those patients were then evaluated after stratification into two age groups: all ages and a subgroup of patients aged 65 years or older. The S(c) (r) -rounded C-G Cl(c) (r) values were compared with the C-G Cl(c) (r) values using actual S(c) (r) values. Mean differences were calculated, and accuracy was evaluated. Use of actual body weight in the calculations for underweight patients resulted in an unbiased Cl(c) (r) of -0.22 ml/minute (p=0.898). Use of ideal body weight in the calculations of patients of normal weight returned an unbiased Cl(c) (r) of -1.3 ml/minute (p=0.544). An unbiased C-G Cl(c) (r) could not be calculated for other weight categories. In those patients, adjusted body weight using a factor of 0.4 (ABW(0.4) ) was the least biased and most accurate. In patients aged 65 years or older with an S(c) (r) less than 0.8 mg/dl and less than 1 mg/dl, actual S(c) (r) was unbiased (-3 ml/min [p=1] and -9 ml/min [p=0.279], respectively) and more accurate than rounded S(c) (r) . In patients of all ages with an S(c) (r) less than 0.8 mg/dl and less than 1 mg/dl, actual S(c) (r) proved less biased (-4.5 ml/min [p=0.038] and -5.5 ml/min [p<0.001], respectively) and more accurate than rounded S(c) (r) .

Conclusion: An unbiased C-G Cl(c) (r) can be calculated using actual body weight in underweight patients and ideal body weight in patients of normal weight. Using ABW(0.4) for overweight, obese, and morbidly obese patients appears to be the least biased and most accurate method for calculating their C-G Cl(c) (r) . Rounding S(c) (r) in patients with low S(c) (r) did not improve accuracy or bias of the Cl(cr) calculations.