Background: It is well known that plasma creatinine concentration is affected by muscle mass, while some studies have suggested cystatin C is affected by body mass index (BMI). Our aim was to assess the effects of lean versus fat mass on cystatin C and creatinine derivative equations in estimating glomerular filtration rate (GFR) in healthy young men. Methods: Three groups of participants were studied: those classified as normal (BMI 18-25 kg/m 2 with body fat ,30%); mus-

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Age and lean mass were strong determinants of GFR. Estimates of GFR should therefore be indexed to an estimate of lean mass.

The CKD-EPI without normalisation (CKD-EPI_noBSA) equation was superior to the CKD-EPI equation in estimating raw-measured Tc-DTPA GFR (mL/min).

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The inclusion of cystatin C in the CKD-EPI equations improved the prediction of kidney function in the overall population, although probably not sufficiently for it to be favored over radioisotopic GFR for guiding chemotherapy. More research is warranted to further improve estimated GFR equations for these purposes.

AIMSWe aimed to compare the performances of contemporary cystatin C (Cys)-based GFR equations, and the creatinine only Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation for predicting gentamicin clearance. METHODSThe bias and imprecision of the CKD-EPI, CKD-EPI_Cys and creatinine-cystatin C CKD-EPI (CKD-EPI_CrCys) equations for predicting gentamicin clearances, were assessed in 260 patients treated with gentamicin during 2012-2013. The creatinine-cystatin C Berlin Initiative Study equation (BIS_CrCys) was examined in the ≥70 year subgroup. The reference gentamicin clearance was calculated using post-dose plasma concentrations. RESULTSThe CKD-EPI_CrCys equation had the highest percentage of estimates within 30% of the reference gentamicin clearance (70%, P = 0.003) and lowest root mean square error (95% CI) of 29 (25, 23) ml min −1 of the three equations for the entire cohort. There was no significant improvement in the performances of the equations with the exclusion of 41 patients with abnormal thyroid function tests or steroid co-prescription at the time of the index gentamicin dose. Of the remaining 219 patients, adjustment for individual BSA improved the performances of all GFR equations (P ≤ 0.003) in those with body mass indices (BMI) <18.5 or ≥30 kg m CONCLUSIONSThe CKD-EPI_CrCys equation provided the best estimate of gentamicin clearance. If used for guiding gentamicin dosing, the results from GFR equations should be adjusted for individual BSA at the extremes of body size.

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