Assessing renal function is especially important when designing a drug dosing regimen for patients with impaired kidney function. Patient weight is required for applying standard renal function estimation formulas, and using ideal or adjusted body weight is recommended for obese patients. Estimating the renal function of obese patients using actual body weight may overestimate their true renal function; however, the extent of this overestimation in Japanese patients remains unclear. We thus examined the impact of body weight when assessing renal function in obese patients with impaired renal function.

Our study included patients with eGFR <60 mL/min/1.73 m² and body mass index（BMI）>25 kg/m², who underwent 24-hour urine storage under hospitalization at Tokyo Dental College Ichikawa General Hospital from January 1, 2017, to December 31, 2022. Cockcroft-Gault creatinine clearance（CCr）was compared with measured CCr（mCCr）.

The total number of subjects was 107. Mean absolute error（MAE）for measured, ideal, adjusted, and standard weights were 10.1, 14.2, 12.0, and 14.5, respectively. The percentages of estimates within mCCr ± 30％ were 74.8％ for measured weight, 53.3％ for ideal weight, 70.1％ for adjusted weight, and 54.2％ for standard weight, with measured weight showing the highest accuracy. In contrast, patients with BMI >30 kg/m² and eGFR <30 mL/min/1.73 m² displayed the highest accuracy when adjusted weights were used. The MAE of 6.5 for measured weight and 6.2 for adjusted weight, with 58.8％ and 64.7％ of estimates within ±30％ of mCCr, respectively.

When estimating renal function in obese patients with eGFR <60 mL/min/1.73 m², caution should be exercised as using measured weight may overestimate renal function. However, it remains more accurate than using adjusted weight. It is also recommended to use adjusted weight in the estimation formula for patients with a BMI > 30 kg/m² and an eGFR < 30 mL/min/1.73 m².