Common Registration Assessment, Part 1
Salt correction factors
The active drug is only part of the salt that is given. Three original questions on converting between the two.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's article on pharmacokinetics uses phenytoin to show a salt correction factor. Phenytoin is given as the sodium salt, which contains 92 mg of phenytoin per 100 mg of salt, so the factor is 0.92.
Going from active drug to salt means dividing by 0.92. Going from salt to active drug means multiplying by it. Check which way you are going before you press equals.
The method
- Decide whether the quantity you have is the active drug or the salt.
- Active drug from salt: multiply by the factor.
- Salt from active drug: divide by the factor.
- Round only at the end, as the question instructs.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient weighing 60 kg needs 14 mg/kg of phenytoin as the active drug. The salt contains 92 mg phenytoin per 100 mg. How many mg of phenytoin sodium are needed, to the nearest mg? These figures are made up for the exercise.
Show the answer
Answer: 913 mg
Working: Active drug = 14 × 60 = 840 mg. Salt = 840 ÷ 0.92 = 913.04 mg, which is 913 mg to the nearest mg. The salt amount is larger than the active amount.
Question 2
A vial contains 1,000 mg of phenytoin sodium. How much phenytoin is that, using a factor of 0.92?
Show the answer
Answer: 920 mg
Working: Active drug = salt × 0.92 = 1,000 × 0.92 = 920 mg.
Question 3
A patient weighing 75 kg is to receive a loading dose to reach 20 mg/L. The volume of distribution is 0.7 L/kg and the dose is supplied as phenytoin sodium with a factor of 0.92. What mass of phenytoin sodium is needed, to the nearest mg?
Show the answer
Answer: 1,141 mg
Working: V = 0.7 × 75 = 52.5 L. Active drug = 52.5 × 20 = 1,050 mg. Salt = 1,050 ÷ 0.92 = 1,141.3 mg, which is 1,141 mg.
What the article says
The article explains that phenytoin sodium contains 92 mg phenytoin per 100 mg salt, so the 0.92 factor converts between them. Its worked loading dose example is for phenytoin. The numbers on this page are different and are not dosing guidance.
Where marks are lost
- Multiplying by 0.92 when you need to divide.
- Applying the factor twice.
- Leaving out the weight.
Frequently asked questions
Why is the salt dose larger?
Only 92% of the salt by mass is the active drug, so more salt is needed to supply the same active amount.
Is the factor the same for every salt?
No. Each salt has its own factor, so always use the one given or a current reference.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- 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
- Creatinine clearance (Cockcroft-Gault) calculations
- Moles and millimoles calculations
- Displacement volume calculations for reconstitution
- Quantity to supply calculations
- Using a provided formula in calculations
- Dose information from packaging and labels
- Diluting a stock to a lower strength
- Medicine cost and switch calculations
- Rounding rules in calculations
- Enteral feed rate calculations
- Electrolyte content over time
- Infusion dose per kg per hour
- Rounding doses to measurable volumes
- Relative risk and relative risk reduction
- Absolute risk reduction and number needed to treat
- Odds ratio and confidence intervals
- Relative versus absolute risk reduction
- Loading dose and volume of distribution
- Top-up doses from measured drug levels
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
See also the formula sheet, the eight sample questions and the approved calculator page.