Background: Estimated glomerular filtration rate (eGFR) based on serum cystatin C (Scys) is useful for patients with decreased muscle mass, but has been also reported to be affected by cancer. The usefulness of Scys in eGFR in terminal cancer patients with decreased muscle mass is unknown. Therefore, we analyzed appropriate eGFR formulae for terminal cancer patients. Methods: Study design was a retrospective observational study. Based on creatinine height index (CHI), 184 terminal cancer patients were stratified into CHI ≥ 90% (normal muscle mass, 59 patients); CHI 60–89% (mildly to moderately decreased muscle mass, 64 patients); and CHI < 60% (severely decreased muscle mass, 61 patients) groups. Twenty-four-hour creatinine clearance was measured and converted to the glomerular filtration rate (GFR) as a renal function measure. To estimate GFR, various eGFR formulae for Japanese were used: eGFRScys, eGFRScr5 and eGFRScr3, eGFRaverage and eGFRScys-Scr, and eGFRCG, based on Scys, serum creatinine (Scr), Scys and Scr combined, and Cockcroft–Gault formula (CG), respectively. Errors between measured and estimated values of renal function were verified using mean prediction errors (ME). When a 95% confidence interval (CI) of ME included 0, the accuracy of the eGFR formula was graded as good. Results: eGFRScys ME was 0.2 (95% CI lower limit − 3.7, upper limit 4.0) mL/min/1.73 m2 in CHI 60–89% group and 9.2 (6.1, 12.9) mL/min/1.73 m2 in CHI < 60% group. eGFRScys was most accurate among the eGFR formulae. Conclusions: eGFR based on Scys was demonstrated as useful in terminal cancer patients with decreased muscle mass.
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