A narrow therapeutic index is a characteristic feature of cytotoxic agents. Some of these agents are almost entirely eliminated renally in unchanged active form. As a consequence, assessment of the individual glomerular filtration rate (GFR) may help to predict the pharmacokinetic behaviour of cytotoxic agents in plasma more precisely. In addition, GFR-adapted individualization of cancer chemotherapy may have an enormous impact on the severity of side effects. Several methods are available to determine GFR or creatinine clearance (CrCl). GFR-measurement based on experimental methods with radiolabelled isotopes, contrast media or inulin helps to reflect the real situation very closely. In addition, 24-h urine collection is a convenient and feasible method to measure creatinine clearance. Finally, several mathematical equations exist to estimate GFR or CrCl based on serum creatinine and other parameters. Only a few of these equations have been developed in oncologic patients. However, some of these equations are routinely used in clinical practice, because they allow a rapid estimation of GFR. Based on the fact that clinically relevant differences have been assessed between calculated values and the real situation, mathematical calculation of GFR or CrCl does not seem to be appropriate to assess individual renal function precisely enough over a broad range of individual GFR or CrCl. Whether the measurement of low-molecular-weight proteins, such as cystatin C and ss-trace protein, may help to reflect the real situation more precisely is a matter of controversial debate.

These observations suggest that individual GFR values over a broad range cannot be calculated accurately enough with only one selected formula. It may be useful to classify renal function of patients with cancer according to the novel algorithm by using MDRD first and then to subsequently calculate GFR in higher and lower ranges with the Wright and modified Salazar-Corcoran formulas, respectively. This algorithm should be validated using larger numbers of patients.

These results suggest that the concept of a non cystatin C-based novel algorithm including three different formulas rather than one single equation may improve accurate estimation of GFR over a broad range of constitutive values, including patients with low constitutive renal function as well as overweight patients.

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