Canterbury DHB

Calculating the Dose of Cytotoxic Drugs

Creatinine clearance

There are various ways of assessing renal function before chemotherapy. The most appropriate method will depend on the drug and schedule to be used. In general, we recommend the Cockcroft and Gault formula.

Normal range for the estimated GFR and creatinine clearance is >90 mL/min.

Estimated GFR reported by Canterbury Health Laboratories gives an indication of renal function, but should not be used to determine drug dosing. It is only reliable under steady state conditions (stable creatinine for more than four days) and does not account for body size. This leads to over and underestimates of renal function at the extremes of body size.

Note: A 24-hour urine collection for creatinine clearance is not routinely recommended. A nuclear medicine scan is also rarely done. It may be indicated when chemotherapy drugs with a high risk of renal toxicity (e.g. cisplatin) are used.

Weight and body surface area (BSA)

Chemotherapy drug dosage is usually calculated using BSA or per kg body weight. Some schedules simply require a set dose/day regardless of size.

The relationship between body weight and the dose of the chemotherapy drug that should be administered is controversial. We recommend measuring the actual body weight, recording the ideal body weight, and the patient’s height. Calculate the patient’s BSA using their actual body weight.

If the patient’s actual body weight is >30% above the ideal body weight, some dose reduction may be recommended. Take the mid-point between these two weights and use this adjusted body weight for per kg dosage or for BSA calculations.

Example: actual body weight 125 kg, ideal body weight 85 kg, adjusted body weight = 105 kg.

Note: This adjustment method is not entirely satisfactory but it is simple to use, and is recommended until some better method of dose adjustment is found.

Note:

• The dosage of some drugs may not need to be reduced merely because the patient is overweight. Page the Haematology Pharmacist on 8209.
• There will need to be some flexibility around the 30% cut-off level. This is because a patient just below the 30% mark would get a bigger dose of drug than someone just above it. Some arbitrary adjustment will be required here and the dose to be given should be discussed with the Haematology Pharmacist and the consultant concerned.

Determine ideal body weight

See the Ideal Body Weight table or use the Ideal Weight Calculator.

Calculate surface area

Use the Online BSA Calculators:

• Mosteller
• Du Bois method

Topic Code: 11668