The Cockcroft-Gault(CG) equation creatinine clearance, based on weight, age, serum creatinine is used as a substitute for measured glomerular filtration rate(GFR) in drug calculation. The Calvert Formula for carboplatin dose is (GFR+25)xAUC, was originally developed using the Cr51-EDTA, not the CG equation. Since then the new isotope dilution mass spectrometry(IDMS) traceable reference method has resulted in creatinine readings 10-20% lower than previous levels. Rates of obesity and co-morbidity have also increased. Calculated CG-GFR and Cr51-GFR results will be compared in community oncology patients.
Suitable subjects were obtained from the Medical Imaging Department. Patients included had solid or haematological malignancies. Patient characteristics including age, co-morbidity, body mass index(BMI), tumour type and serum creatinine were obtained. A hypothetical carboplatin (AUC5) dose was obtained by Cr51-GFR versus CG-GFR.
Fifty-five cancer patients underwent Cr51-GFR between mid-2015 and May 2017. 29.7% were male and 70.3% were female. 63% had two or more co-morbidities. In females, mean age was 63(range 32-87), mean creatinine was 63.8micromols(range 36-106), mean BMI was 29.5(range 17.5-47.2). In females mean GC-GFR was 103(range 36-221), mean Cr51-GFR 78.2(range 36-134)-a 31.7% over-estimation. In females, mean carboplatin(AUC5) CG-GFR(carboCG) was 641.7(range 305-1230) versus carboplatin Cr51-GFR(carbo Cr51) was 517.5(range 305-795). In no patient did carboCG match carbo Cr51 exactly. In 89% of females carboCG was higher than carbo-Cr51. In 71% of females carboCG was more than 10% higher than carbo-Cr51. In 31.8% carboCG was more than 30% higher than carbo-Cr51. In 1% of female patients was carboCG lower than carbo Cr51. In males similar, but less prominent results were seen.
In community oncology patients with co-morbidities, increased BMI, within the setting of new IDMS creatinine readings, GC-GFR is inaccurate, particularly over-estimating GFR in females, leading to imprecise and over-estimated carboplatin doses. Wherever possible, Chromium 51-EDTA should be used to measure GFR for accurate drug calculation.