Creatinine Clearance (Cockcroft-Gault)

Donald Cockcroft and Henry Gault published this in 1976 from 249 hospitalised men, and it remains the equation most drug monographs are written against. The logic is a mass balance. Creatinine comes from muscle at a rate that tracks lean body mass, which falls with age, and it leaves almost entirely through the kidney, so a steady serum level tells you how fast the kidney is clearing it. The $(140 - a)$ term is the age decline, the mass term is the muscle bulk, and the 0.85 for women reflects lower muscle mass at the same weight. A 70 kg man of 70 with a creatinine of 1.0 mg/dL gets $(140-70) \ imes 70 / 72 = 68$ mL/min.

Every part of that story is also where it breaks. Because it uses total body weight, it overestimates clearance substantially in obesity, where the extra kilograms are not extra muscle, and many services substitute ideal or adjusted body weight for exactly that reason. It underestimates in very muscular people and in anyone with amputation or wasting, where the creatinine is low for a reason that has nothing to do with the kidney. It assumes the serum creatinine is at steady state, so in acute kidney injury, where the level is still climbing, it reads far too optimistic and does so at the moment it matters most.

The equation also depends on how the laboratory measures creatinine. Modern IDMS-standardised assays read roughly 5 percent lower than the older Jaffe methods Cockcroft and Gault used, so the same patient scores slightly higher clearance today than in 1976. None of this makes the equation useless. It makes it an estimate with a known bias, and renal drug dosing is a clinical decision that weighs this number against the patient in front of you, not a lookup.