Common Registration Assessment, Part 1
Oral maintenance dose and dosing interval
Work through average steady-state concentration calculations using clearance, bioavailability and the interval between doses.
Published 7 October 2026. Independent revision material, not GPhC questions, not for patient care.
A maintenance regimen replaces the amount being cleared. In the simplified linear model, dose = target average concentration × clearance × dosing interval ÷ F. Clearance gives a rate; the interval turns the rate into an amount.
Use the same time unit for clearance and the dosing interval. If clearance is in L/hour, an interval in minutes needs converting to hours. The target is an average steady-state concentration, not a peak or trough. These fictional exercises do not determine a safe clinical regimen.
The method
- Write D = C × Cl × interval ÷ F.
- Match the concentration and clearance volume units.
- Convert the interval to the time unit used by clearance.
- Use F as a fraction, then calculate the amount per dose.
- Check the units: mg/L × L/hour × hours gives mg.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
Use D = C × Cl × interval ÷ F. Target average C is 5 mg/L, Cl is 3 L/hour, interval is 8 hours and F = 0.75. Calculate D.
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Answer: 160 mg
Working: 5 × 3 × 8 ÷ 0.75 = 160 mg per dose. The daily total would be a different answer.
Question 2
Use the same equation with C = 4 mg/L, Cl = 2.5 L/hour, interval = 720 minutes and F = 0.8. Calculate D.
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Answer: 150 mg
Working: 720 minutes = 12 hours. D = 4 × 2.5 × 12 ÷ 0.8 = 150 mg.
Question 3
In this model, a 120 mg dose with F = 0.8 is given every 6 hours. Cl is 4 L/hour. Calculate the average steady-state concentration using C = D × F ÷ (Cl × interval).
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Answer: 4 mg/L
Working: 120 × 0.8 ÷ (4 × 6) = 96 ÷ 24 = 4 mg/L.
Compare rates before amounts
With the other inputs unchanged, doubling the interval doubles the dose needed in this average-concentration model. That is a calculation check, not advice to change a regimen.
Where marks are lost
- Using the daily number of doses where the equation needs the interval.
- Combining minutes with clearance per hour.
- Reading an average concentration as a trough target.
Frequently asked questions
Is the interval the number of doses per day?
No. It is the time between doses. Four evenly spaced doses per day give a six-hour interval.
Can I use this model for every drug?
No. It assumes the linear model stated in the exercise. Non-linear kinetics and clinical dosing decisions need drug-specific guidance.
Sources
- The Pharmaceutical Journal, back to basics: pharmacokinetics
- GPhC, 2026 assessment specification and permitted items
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
- Salt correction factors
- Mass and molar drug concentrations
- Half-life and falling drug levels
- Steady-state concentration and infusion rate
- Bioavailability and equivalent oral doses
- Elimination rate constant and half-life
- Accumulation towards steady state
- Ratio strengths and one in X
- Serial dilution factors
- Mixing solutions of different strengths
- Tablet quantities for reducing regimens
- Equivalent doses from provided conversion tables
- Thresholds and dose-banding algorithms
See also the formula sheet, the eight sample questions and the approved calculator page.