🧠 The theory: why each factor is there
Creatinine is a waste product cleared by the kidneys, so a high blood creatinine means poor clearance, which is why it sits on the bottom of the equation. Kidney function falls with age, so (140 minus age) drops as people get older. Larger people produce more creatinine, so weight is on top. The sex factor (1.23 for men, 1.04 for women) reflects the greater average muscle mass in men. The whole equation estimates how fast the kidneys clear the drug, guiding the dose.
CrCl = F x (140 minus age) x weight / serum creatinine
F is 1.23 for men and 1.04 for women. Weight is in kilograms and serum creatinine is in micromol/L.
Worked example
A 65-year-old woman, 60 kg, has a serum creatinine of 80 micromol/L. What is her creatinine clearance?
- Step 1 — Choose the factor: she is female, so F = 1.04.
- Step 2 — Work out the age term: 140 minus 65 = 75.
- Step 3 — Multiply the top line: 1.04 x 75 x 60 = 4680.
- Step 4 — Divide by the serum creatinine: 4680 / 80 = 58.5.
Answer: about 59 mL/min
Worked example
A 70-year-old man, 80 kg, has a serum creatinine of 120 micromol/L. What is his creatinine clearance?
- Step 1 — He is male, so F = 1.23.
- Step 2 — Age term: 140 minus 70 = 70.
- Step 3 — Top line: 1.23 x 70 x 80 = 6888.
- Step 4 — Divide by creatinine: 6888 / 120 = 57.4.
Answer: about 57 mL/min
Worked example
A 50-year-old man, 70 kg, has a serum creatinine of 90 micromol/L. What is his creatinine clearance?
- Step 1 — Male, F = 1.23. Age term: 140 minus 50 = 90.
- Step 2 — Top line: 1.23 x 90 x 70 = 7749.
- Step 3 — Divide: 7749 / 90 = 86.1.
Answer: about 86 mL/min
⚠️ CrCl is not the same as eGFR
Creatinine clearance from Cockcroft-Gault is used for drug dosing. The eGFR reported by labs uses a different equation and is normalised to body size, so for narrow-margin drugs, renally cleared drugs (including DOACs), and patients at extremes of weight, dose from the Cockcroft-Gault creatinine clearance.
Test yourself
An 80-year-old woman, 55 kg, serum creatinine 100 micromol/L. CrCl?
Answer: 1.04 x (140 - 80) x 55 = 1.04 x 60 x 55 = 3432; 3432 / 100 = about 34 mL/min
A 60-year-old man, 90 kg, serum creatinine 75 micromol/L. CrCl?
Answer: 1.23 x (140 - 60) x 90 = 1.23 x 80 x 90 = 8856; 8856 / 75 = about 118 mL/min
A 68-year-old man, 75 kg, serum creatinine 110 micromol/L. CrCl?
Answer: 1.23 x (140 - 68) x 75 = 1.23 x 72 x 75 = 6642; 6642 / 110 = about 60 mL/min