Creatinine Clearance and Carboplatin Dose Calculator

Why kidney function matters in chemotherapy

Many cancer drugs leave the body through the kidneys: carboplatin and cisplatin, methotrexate, capecitabine, some immunotherapies and many supportive medicines. If the kidneys clear a drug slowly, more of it stays in the blood for longer, raising side effects. That is why a creatinine blood test is taken before every cycle and why your team talks about your clearance.

Creatinine is a waste product of muscle. Its level in the blood rises when the kidneys filter less, but it also depends on how much muscle you have, which is where the formulas come in.

Cockcroft-Gault, eGFR and measured GFR

Cockcroft and Gault published their equation in 1976 from 249 men; the 0.85 factor for women was added later. It estimates creatinine clearance in millilitres per minute from age, weight, sex and serum creatinine, and remains the equation most chemotherapy protocols were written for.

The eGFR printed on your lab report usually comes from the CKD-EPI equation and is normalised to a standard body size of 1.73 m², so it is not directly interchangeable with Cockcroft-Gault, although the two are close for people of average build. Both overestimate function in people who have lost muscle, and both can mislead in obesity. When a decision is finely balanced, hospitals measure GFR directly with a tracer or a timed urine collection.

European labs report creatinine in µmol/L, US sources in mg/dL; dividing µmol/L by 88.4 converts between them.

How the Calvert formula sets a carboplatin dose

Carboplatin is unusual: instead of mg/m², the dose is chosen to reach a target exposure, the area under the concentration-time curve (AUC). Calvert and colleagues showed in 1989 that dose (mg) = AUC × (GFR + 25), where 25 represents the small amount of drug cleared by routes other than the kidneys.

Protocols typically target an AUC of 5 or 6 for treatment given with curative intent and 4 to 5 when carboplatin is combined with other drugs or given to frailer patients. Because modern standardised creatinine assays read lower than the older ones the formula was built on, the US National Cancer Institute advised in 2010 that the GFR used should be capped at 125 mL/min to avoid overdosing, and most European centres do the same.

Why your dose may differ from this calculation

• Your centre may use measured GFR, eGFR or a different weight (ideal or adjusted) in the equation.
• Doses are rounded to practical amounts, sometimes into fixed bands.
• Low platelets after a previous cycle usually lead to a lower AUC or a delay.
• Protocols set maximum doses regardless of the formula.
• Kidney function changes: dehydration, contrast dye or anti-inflammatory painkillers can raise creatinine temporarily, and a repeat test may be requested before dosing.

Looking after your kidneys during treatment

Drink enough fluid on treatment days and the days after, as your team advises, and tell them about every medicine and supplement you take, especially anti-inflammatory painkillers such as ibuprofen, which reduce kidney blood flow. Report vomiting or diarrhoea that stops you keeping fluids down, and any swelling of the legs or a sharp fall in how much urine you pass.

Questions to ask your team

Bring the result with you and ask:

• Which measure of kidney function do you use for my dosing, and what was my last value?
• What target AUC is my carboplatin set to, and why?
• Has my dose been capped, banded or reduced?
• Are any of my other medicines hard on the kidneys?