The Calvert formula doses carboplatin by target exposure rather than body surface area: dose (mg) = target AUC × (GFR + 25). Because carboplatin is cleared almost entirely by the kidneys, tying the dose to kidney function gives more consistent exposure than a flat mg/m² dose. This tool caps GFR at 125 mL/min, a common safeguard when GFR is estimated.
What the Formula Means
The Calvert formula multiplies the target exposure by the patient's total carboplatin clearance:
- Target AUC (area under the concentration-time curve, in mg·min/mL) is the drug exposure the oncologist wants, typically about 4 to 6 depending on the regimen and intent.
- GFR (mL/min) is the patient's glomerular filtration rate, the main route by which carboplatin leaves the body.
- + 25 is a fixed constant (mL/min) representing carboplatin's non-renal clearance. Adding it to GFR gives total drug clearance. Exposure (AUC) equals dose ÷ clearance, so rearranging gives dose = AUC × clearance.
The total dose comes out in milligrams, not mg/m², because exposure already accounts for how fast the individual patient eliminates the drug.
Worked Example
Suppose the oncologist sets a target AUC of 5 and the patient's estimated GFR is 90 mL/min. Apply the formula: 5 × (90 + 25) = 5 × 115 = 575 mg. If instead the estimated GFR came back as 140 mL/min, the cap applies and GFR is treated as 125. That gives 5 × (125 + 25) = 5 × 150 = 750 mg rather than the 825 mg an uncapped value would have produced, so the cap trims roughly 75 mg of potential overdose.
Dose by Target AUC and GFR
| GFR (mL/min) | AUC 4 | AUC 5 | AUC 6 |
|---|---|---|---|
| 50 | 300 mg | 375 mg | 450 mg |
| 75 | 400 mg | 500 mg | 600 mg |
| 100 | 500 mg | 625 mg | 750 mg |
| 125 (cap) | 600 mg | 750 mg | 900 mg |
Every GFR at or above 125 mL/min collapses to the capped row, so the formula stops increasing the dose for a higher estimated GFR beyond that point.
Reading the Calculated Dose
The number this formula returns is a total milligram dose for a single cycle, not a daily or per-kilogram figure. It already accounts for how fast the individual patient clears the drug, which is why two patients of identical size can receive quite different doses. Treat the result as a starting point. The prescribing team still checks it against the protocol, the patient's blood counts, any toxicity from prior cycles, and the overall goals of treatment before it is given. A dose that looks unusually high or low for the regimen is a prompt to re-examine the GFR that produced it.
Getting the GFR
The GFR used in Calvert is often a Cockcroft-Gault creatinine clearance or a measured GFR. Estimate it with the calculator on the home page, then enter the value above. See carboplatin renal dosing for context, and the renal drug dosing hub for other agents. For another dose calculation that depends on kidney function, see the vancomycin AUC dosing calculator.
Why Carboplatin Is Dosed This Way
Most chemotherapy is dosed by body surface area, on the assumption that a larger body needs a proportionally larger dose. Carboplatin is an exception because its toxicity (chiefly a drop in platelets and white cells) tracks far more tightly with total drug exposure than with body size. Two patients of the same height and weight can clear carboplatin at very different rates depending on their kidney function, so a flat mg/m² dose would over-treat the slow clearer and under-treat the fast one. Calvert and colleagues showed in 1989 that targeting a chosen AUC, with the dose calculated from the patient's own clearance, produced far more predictable blood-count nadirs. Dosing to exposure rather than to size makes carboplatin unusually controllable for a cytotoxic drug.
Measured vs Estimated GFR in Calvert
A Calvert dose is only as accurate as the GFR entered into it. The original work used a measured GFR from a radioisotope study, which is precise but slow and not always available. In day-to-day practice an estimated GFR, often a Cockcroft-Gault creatinine clearance, is used instead. Modern, standardised creatinine assays read lower than the older assays the formula was built on, which tends to inflate the estimated GFR and, in turn, the calculated dose. The 125 mL/min cap is the practical response to that drift. By treating any higher estimated GFR as 125, the cap limits how far an overstated clearance can push the dose. Where precision is critical, a measured GFR remains the gold standard.
How AUC Relates to Toxicity and Effect
A higher target AUC delivers more drug exposure, which means a greater anti-tumour effect but also a deeper fall in blood counts. Lower AUC targets (around 4) are common when carboplatin is combined with other myelosuppressive agents, while higher targets (5 to 6) may be used for single-agent or curative-intent regimens. The treating oncologist chooses the target by weighing efficacy against marrow toxicity, using the patient's prior treatment, blood counts, and goals of care. The formula does not make that judgement. It translates a chosen target into a milligram dose once the patient's clearance is known.
A Second Worked Example
Consider a frailer patient with reduced kidney function: estimated GFR 40 mL/min and a target AUC of 5. The dose is 5 × (40 + 25) = 5 × 65 = 325 mg, markedly lower than the 575 mg calculated earlier for a patient with a GFR of 90. The target exposure is identical in both cases. The formula scales the dose down for a patient who eliminates the drug slowly, a safety step that body-surface-area dosing would miss.
Limitations and Cautions
- The GFR cap of 125 mL/min only partly protects against overdosing; the original validation used a measured, not estimated, GFR, so estimated values carry extra uncertainty.
- Target AUC is a clinical decision set by the prescribing oncologist; this tool does not choose it.
- Unit consistency matters: target AUC is in mg·min/mL and GFR in mL/min, giving a dose in mg.
- The result is a starting estimate. Final dosing depends on the protocol, prior toxicity, blood counts, and clinical judgement.