Development of a new equation to estimate creatinine clearance in cancer patients

Determining renal function is important for chemotherapy eligibility and dosing. Measured creatinine clearance (mCrCl) is the gold standard but is cumbersome. Equations estimating CrCl (eCrCl) based on serum creatinine (SCr) produce widely varying estimates. Considering that SCr is derived from skeletal muscle, this study prospectively developed a new eCrCl equation in cancer patients using CT-defined muscle surface area (MSA) and evaluated its utility in a separate, retrospective series.

In a prospective, observational cohort study of cancer patients, mCrCl by 24-h urine collection was correlated with CT-determined MSA to create an equation for eCrCl [muscle surface area (cm(2)) × 42/SCr]. eCrCl by Wright, Cockcroft-Gault (CG), CKD-EPI, MDRD, and MSA was compared to mCrCl to determine fit. MSA-eCrCl was used to simulate carboplatin dosing in a retrospective series of advanced non-small cell lung cancer (NSCLC).

Prospectively, 22 patients were accrued and evaluable (12 males; median age 69). MSA-eCrCl correlated stronger (r (2) 0.80) than current equations (r (2) 0.47-0.69) with mCrCl. In calculating carboplatin doses for 89 NSCLC patients with MSA and CG-eCrCl, median error of CG-determined carboplatin dose was 5.5 % (range -19.0 to 44.2 %), assuming that MSA was better at estimating CrCl. Forty-two patients (47 %) received doses that varied ≥10 % of what was calculated by MSA.

We propose a new formula for eCrCl in patients that appears more accurate than current formulae and may have implications for chemotherapy efficacy and toxicity. Studies to validate this formula are under way.