Cockcroft-Gault and Renal Dosing for the GPhC Calculations Paper

The maths in a Cockcroft-Gault question is the easy bit. Marks go on choosing the right weight, using CrCl instead of eGFR when it matters, and reading the dosing table. We cover all three, with 3 worked examples and the mistakes to avoid.

By Umar Majid · 9 Nov 2026

Cockcroft-Gault estimates creatinine clearance (CrCl) as (140 − age) × weight (kg) × 1.23 for men or 1.04 for women, divided by serum creatinine in micromol/L. In the GPhC registration assessment the maths is the easy part. Marks are lost on choosing the right weight, using CrCl rather than eGFR when it matters, and reading the dosing table correctly.

The GPhC's feedback on the June 2026 sitting named pharmacokinetics, including the effect of creatinine clearance on dosing decisions, as an area to work on. So it's worth getting this right.

The Cockcroft-Gault equation

CrCl (mL/min) = (140 − age) × weight (kg) × constant ÷ serum creatinine (micromol/L)

Age is in years. Weight is in kg, and could be actual, ideal or adjusted weight (more on that below). The constant is 1.23 for men and 1.04 for women. Serum creatinine has to be in micromol/L, so convert it if you're given mmol/L.

The 2026 CRA framework says simple formulae such as BMI won't be provided, but more complex formulae will. Expect Cockcroft-Gault to be given, but know it well enough that you never have to think about it.

Should you use eGFR or CrCl?

For most drugs and most adults of average build, the MHRA says eGFR can be used for dose adjustment. Use Cockcroft-Gault CrCl instead in these situations:

Use CrCl when	Examples
The patient is on a DOAC	Apixaban, dabigatran, edoxaban, rivaroxaban
The drug is nephrotoxic	Vancomycin, amphotericin B
The drug is renally excreted with a narrow therapeutic index	Digoxin, sotalol
The patient is 75 or older
The patient is at extremes of muscle mass	BMI under 18 or over 40 kg/m²

The MHRA's reason for DOACs is that eGFR can overestimate renal function in these groups, which risks giving too high a dose and causing bleeding. The units differ too: eGFR is reported in mL/min/1.73 m² and CrCl in mL/min. They aren't interchangeable.

Some guidance thresholds really are written as eGFR. NICE's UTI guideline, for example, uses nitrofurantoin only if eGFR is 45 ml/minute or more. Always use the measure the table or guideline asks for.

Which body weight do you use?

This is where trainees lose marks, and it's also where real-world guidance varies.

Ideal body weight (IBW) is the one used most often. IBW = 50 kg (men) or 45.5 kg (women) + 0.91 × (height in cm − 152.4). In imperial units, it's 2.3 kg for every inch over 5 feet.

If actual weight is less than IBW, use actual weight. Using IBW in an underweight patient overestimates renal function.

In obesity, actual weight overestimates renal function. Local guidelines differ: some use IBW, others use adjusted body weight (IBW + 0.4 × (actual − IBW)) for a BMI of 30 or more. There's no single national consensus, and the MHRA recommends a calculator that shows results for actual, ideal and adjusted weight.

In the exam, follow the question. If it tells you which weight to use, use it. If it gives you height and weight, it probably wants you to work out IBW and compare it with actual weight.

Worked example 1: a DOAC dose

An 81-year-old man with atrial fibrillation weighs 70 kg (normal BMI). Serum creatinine is 140 micromol/L. He is starting rivaroxaban. What dose?

(140 − 81) = 59
59 × 70 = 4,130
4,130 × 1.23 = 5,079.9
5,079.9 ÷ 140 = 36.3, so CrCl = 36 mL/min

The rivaroxaban SmPC gives 20 mg once daily for stroke prevention in AF, reduced to 15 mg once daily at a CrCl of 15 to 49 mL/min. So the answer is 15 mg once daily. He's over 75 and on a DOAC, so CrCl rather than eGFR is correct on two counts.

Worked example 2: using ideal body weight

A 58-year-old man is 180 cm tall and weighs 110 kg (BMI 34). Serum creatinine is 160 micromol/L. The question tells you to use IBW and gives this table for Drug X:

CrCl (mL/min)	Dose
Over 50	500 mg twice daily
30 to 50	250 mg twice daily
Under 30	Avoid

IBW = 50 + 0.91 × (180 − 152.4) = 50 + 25.1 = 75.1 kg
(140 − 58) = 82
82 × 75.1 × 1.23 = 7,574.6
7,574.6 ÷ 160 = 47.3, so CrCl = 47 mL/min

The answer is 250 mg twice daily. Had you used his actual weight, you'd get 69 mL/min and choose the full dose. Same patient, different band.

Worked example 3: when actual weight is lower than IBW

An 84-year-old woman with AF is 165 cm tall and weighs 45 kg (BMI 16.5). Serum creatinine is 92 micromol/L. She takes apixaban.

IBW = 45.5 + 0.91 × (165 − 152.4) = 57.0 kg
Actual weight (45 kg) is lower than IBW, so use 45 kg
(140 − 84) = 56
56 × 45 × 1.04 = 2,620.8
2,620.8 ÷ 92 = 28.5, so CrCl = 28 mL/min

The apixaban SmPC says patients with a CrCl of 15 to 29 mL/min should receive 2.5 mg twice daily. She also meets two of the separate dose-reduction criteria (age 80 or over, weight 60 kg or less). Either way, the answer is 2.5 mg twice daily. Using IBW would have given 36 mL/min and could have sent you down the wrong route.

Reading the dosing table

Read every column. Tables often include age, weight or interacting drugs as well as CrCl, as the apixaban criteria show. Check the boundaries: is 50 in the "30 to 50" row or the "over 50" row? The table will tell you. Check the indication, because DOAC doses differ between AF and venous thromboembolism. And if the question supplies an SmPC extract, use that rather than your memory.

Where the marks go

Wrong constant. 1.23 is for men, 1.04 for women.
Creatinine in the wrong units. 0.092 mmol/L is 92 micromol/L.
Using the lab eGFR when the question or drug needs CrCl.
Using IBW when actual weight is lower.
Height conversions. 1 inch = 2.54 cm, so convert before you calculate IBW.
Rounding too early. Keep full figures and round CrCl at the end, unless told otherwise.

For the other eight calculation types, see the 9 GPhC calculation types, and for pacing, read finishing 40 calculations in 2 hours.

Common questions

Will Cockcroft-Gault be given in the exam? The framework says more complex formulae will be provided, while simple ones like BMI won't. Learn it anyway so you can work quickly.

Should I use eGFR or CrCl for a DOAC? CrCl, calculated with Cockcroft-Gault. The MHRA says this should always be used for DOACs.

Which weight should I use? Whatever the question specifies. Otherwise, use IBW unless actual weight is lower, and remember that guidance in obesity varies.

What units does creatinine need to be in? Micromol/L, to go with the 1.23 and 1.04 constants.

Should I round the CrCl? Round at the end, usually to a whole number, and follow any rounding instruction in the question.

Practise renal dosing until it's automatic

Renal dosing combines a formula, a judgement about weight and a dosing table, so it rewards practice. Our Calculations Course has 400+ GPhC-style questions, every one worked through step by step, so you can see exactly where a mistake happens.

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