A method and an apparatus for determining or approximating a patient's glomerular filtration rate or a patient's creatinine clearance are disclosed. The method comprises the following steps: determining a serum creatinine concentration of the patient, determining a lean tissue mass of the patient, and determining the glomerular filtration rate of the patient or the creatinine clearance of the patient based on the serum creatinine concentration of the patient and the lean tissue mass of the patient.