Common Registration Assessment, Part 1
Enoxaparin dose calculations by weight
Three original questions on weight-based enoxaparin doses and the volume to give, with the working shown.
Published 14 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Clexane summary of product characteristics on emc gives treatment of deep vein thrombosis and pulmonary embolism as either 1.5 mg/kg (150 IU/kg) once daily or 1 mg/kg (100 IU/kg) twice daily, by subcutaneous injection. The choice is made by the prescriber, and the once daily dose is for uncomplicated patients with a low risk of recurrence.
Clexane prefilled syringes come in 20 mg/0.2 mL, 40 mg/0.4 mL, 60 mg/0.6 mL, 80 mg/0.8 mL and 100 mg/1 mL, which is 100 mg/mL. When the dose depends on weight, the SmPC says to use graduated syringes and discard the excess, and that where an exact dose is not possible the volume is rounded up to the nearest graduation. The volumes below are the exact arithmetic.
The method
- Find the regimen and the mg/kg dose.
- Multiply the weight in kg by the dose to get the dose in mg.
- Divide the dose in mg by the strength in mg/mL to get the volume.
- If the dose is over 100 mg, a single 100 mg/1 mL syringe is not enough, so follow the product and the prescriber's instruction.
- Multiply by the number of doses a day to compare total daily doses.
Three practice questions
Work each one on paper first, then open the answer. All patients and prescriptions are fictional.
Question 1
A patient weighs 72 kg and has 1 mg/kg twice daily. What is each dose in mg, and what volume of a 100 mg/mL syringe?
Show the answer
Answer: 72 mg, which is 0.72 mL
Working: 72 × 1 = 72 mg. 72 ÷ 100 = 0.72 mL.
Question 2
A patient weighs 60 kg and has 1.5 mg/kg once daily. What is the dose, and what volume of a 100 mg/mL syringe?
Show the answer
Answer: 90 mg, which is 0.9 mL
Working: 60 × 1.5 = 90 mg. 90 ÷ 100 = 0.9 mL.
Question 3
A patient weighs 65 kg. How much more enoxaparin is given in 24 hours with 1 mg/kg twice daily than with 1.5 mg/kg once daily?
Show the answer
Answer: 32.5 mg more
Working: Twice daily: 65 × 1 × 2 = 130 mg. Once daily: 65 × 1.5 = 97.5 mg. 130 - 97.5 = 32.5 mg.
Exam approach
Work out mg from the weight, then the volume from the strength, and check the total for 24 hours.
Where marks are lost
- Confusing mg with IU, where 100 IU/kg is 1 mg/kg.
- Using the once daily mg/kg for a twice daily regimen.
- Dividing by the wrong strength, for example using the syringe size in mL instead of mg per mL.
- Ignoring renal function: the SmPC gives a different dose for severe renal impairment.
Frequently asked questions
What is the treatment dose of enoxaparin in DVT and PE?
The SmPC gives 1.5 mg/kg once daily or 1 mg/kg twice daily by subcutaneous injection, chosen by the prescriber.
Is the dose changed in severe renal impairment?
The SmPC has a separate table for severe renal impairment, and for treatment of DVT and PE it gives 1 mg/kg once daily. Check the definition in the SmPC and the patient's renal function.
Sources
- emc, Clexane prefilled syringes SmPC
- GPhC, Common Registration Assessment specification and permitted items for 2026
The questions above are original and use fictional patients and prescriptions. PreRegExamPrep is not affiliated with or endorsed by the General Pharmaceutical Council.
Practise until the method is automatic
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See also the formula sheet, the eight sample questions and the approved calculator page.