Background:
Carboplatin remains integral for treatment of gynaecological malignancies and dosing is based on glomerular filtration rate (GFR). Measurement via radiotracer decay (nmGFR) is ideal. However, this may be unavailable. Therefore, GFR is often estimated using formulae that have not been validated in patients with cancer and/or specifically for gynaecological malignancies, leading to debate over optimal estimation. Suboptimal GFR estimation may affect efficacy or toxicity.

Methods:
We surveyed several UK National Health Service Trusts to assess carboplatin dosing practise. We then explored single-centre accuracy, bias and precision of various formulae for GFR estimation, relative to nmGFR, before validating our findings in an external cohort.

Results:
Across 18 Trusts, there was considerable heterogeneity in GFR estimation, including the formulae used (Cockcroft-Gault (CG) vs Wright), weight-adjustment and area under the curve (5 vs 6). We analysed 274 and 192 patients in two centres. Overall, CamGFR v2 (a novel formula for GFR estimation developed at Cambridge University Hospitals NHS Foundation Trust) excelled, showing the highest accuracy and precision. This translated into accuracy of hypothetical carboplatin dosing; nmGFR-derived carboplatin dose fell within 20% of the Cam GFR v2-derived dose in 86.5% and 87% of patients across the cohorts. Amongst the CG formula and its derivatives, using adjusted body weight in those with BMI ≥25 kg/m2 (CG-AdBW) was optimal. The Wright and unadjusted CG estimators performed most poorly.

Conclusions:
When compared with nmGFR assessment, accuracy, bias and precision varied widely between GFR estimators, with the newly developed Cam GFR v2 and CG-AdBW perfoming best. In general, weight (or body surface area)-adjusted formulae performed best, while the unadjusted CG and Wright formulae or the use of AUC6 (vs. nmGFR AUC5) produced risk of significant overdose. Thus, individual centres should validate their GFR estimation methods. In the absence of validation, CG-AdBW or CamGFR v2 are likely to perform well while unadjusted CG /Wright formulae or AUC6 dosing should be avoided.