Carboplatin dosing by the Calvert formula depends on an estimate of glomerular filtration rate, and every creatinine-based estimate assumes a normal relationship between muscle mass and creatinine production. In a cachectic patient, an older patient with little muscle, or someone who has lost weight rapidly during treatment, serum creatinine can sit in the normal range while true filtration is well below it. The estimate then reads high, and the calculated dose is too large.
So look at the patient before you accept the number. Where muscle mass is clearly low, cap the estimated filtration rate — many units cap at 125 mL/min — recalculate at each cycle rather than carrying forward the figure from the first, and reweigh rather than assuming the recorded weight still applies. Where the decision is finely balanced and the facility exists, a measured clearance is worth the delay.
The error is quiet. It does not show as a dosing mistake; it shows as unexpected grade 4 thrombocytopenia in cycle two, and it is usually attributed to the disease.
- Recalculate the filtration estimate and reweigh at every cycle, not just the first.
- Cap the estimated GFR in a patient with obviously low muscle mass.
- Treat a normal creatinine in a cachectic patient as uninformative, not reassuring.
- Investigate unexpected grade 3 or 4 cytopenia after carboplatin as a possible dosing error.
- Where the margin is narrow and the facility exists, use a measured clearance.
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