Common Registration Assessment, Part 1
Creatinine clearance (Cockcroft-Gault) calculations
Three original questions using the Cockcroft-Gault equation with the constants given in the BNF.
Published 4 October 2026. Independent revision material, not GPhC questions, not for patient care.
Many renal dose adjustments depend on creatinine clearance. The BNF's renal impairment guidance gives the Cockcroft-Gault equation with a constant of 1.23 for men and 1.04 for women, and serum creatinine in micromol per litre.
The BNF also notes that ideal body weight can be preferred where fat is likely to be a major contributor to body mass. If a question names the weight to use, use that one.
The method
- Subtract age from 140.
- Multiply by the weight the question tells you to use.
- Multiply by 1.23 (men) or 1.04 (women).
- Divide by serum creatinine in micromol/L.
- Compare the result with the threshold in the question.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A man aged 65 weighs 72 kg with serum creatinine 90 micromol/L. Estimate creatinine clearance (Cockcroft-Gault). Use actual body weight.
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Answer: 73.8 mL/min
Working: (140 − 65) = 75. 75 × 72 = 5,400. 5,400 × 1.23 = 6,642. 6,642 ÷ 90 = 73.8 mL/min.
Question 2
A woman aged 78 weighs 58 kg with serum creatinine 105 micromol/L. Estimate creatinine clearance. Use actual body weight.
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Answer: 35.6 mL/min
Working: (140 − 78) = 62. 62 × 58 = 3,596. 3,596 × 1.04 = 3,739.84. 3,739.84 ÷ 105 = 35.6 mL/min.
Question 3
A man aged 82 weighs 66 kg with serum creatinine 130 micromol/L. Drug X must be reduced if creatinine clearance is below 50 mL/min (fictional threshold). Does it need reducing?
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Answer: 36.2 mL/min, so yes
Working: (140 − 82) = 58. 58 × 66 = 3,828. 3,828 × 1.23 = 4,708.44. 4,708.44 ÷ 130 = 36.2 mL/min, which is below 50.
A reminder about weight
The BNF renal impairment guidance gives the Cockcroft-Gault constants used above and says ideal body weight can be preferred where fat is likely to be a major contributor to body mass. In the exam you use the weight the question tells you to use. Serum creatinine itself comes from a blood test, which is why a lab photo sits on this page; it is a stock image, not a result.
Where marks are lost
- Using 1.23 for a woman or 1.04 for a man.
- Using the wrong weight type.
- Reading the threshold the wrong way round.
Frequently asked questions
Where do the constants come from?
The BNF prescribing in renal impairment guidance, which states 1.23 for men and 1.04 for women.
Is this a clinical tool?
No. These are revision exercises. Real renal dosing decisions use the product information, the BNF and clinical judgement.
Sources
- BNF, prescribing in renal impairment
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional drugs. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
Try 15 free questions with worked answers. No sign-up required.
More calculation topics
- Dose by weight calculations for the GPhC assessment
- Volume to give: liquid medicine calculations
- Percentage strength calculations: w/v, w/w and ratios
- Dilution calculations with C1V1 = C2V2
- Infusion rate calculations in mL per hour
- Drip rate calculations: drops per minute
- Body surface area calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
See also the formula sheet, the eight sample questions and the approved calculator page.