Common Registration Assessment, Part 1
Steady-state concentration and infusion rate
At steady state, concentration depends on infusion rate and clearance. Three original questions.
Published 6 October 2026. Independent revision material, not GPhC questions, not for patient care.
The Pharmaceutical Journal's pharmacokinetics article explains that during a constant rate infusion the concentration rises until it levels off. The steady state concentration depends only on the balance between the infusion rate and the clearance.
Because of that, a steady state level that is too low or too high can be corrected by direct proportion: double the rate to double the level, and halve the rate to halve it. The units must stay consistent.
The method
- Check that the patient is at steady state.
- For a change of level, scale the infusion rate by the ratio of target level to measured level.
- For a level from first principles, divide infusion rate by clearance.
- Keep units matched, for example mg/hour with L/hour giving mg/L.
Three practice questions
Work each one on paper first, then open the answer. All drugs and patients are fictional.
Question 1
A patient is at steady state on an infusion of 40 mg/hour with a measured level of 8 mg/L. The target is 12 mg/L. What new infusion rate is needed? These figures are made up for the exercise.
Show the answer
Answer: 60 mg/hour
Working: Direct proportion: new rate = 40 × (12 ÷ 8) = 60 mg/hour.
Question 2
A drug is infused at 50 mg/hour and the patient's clearance is 5 L/hour. What is the steady state concentration? These figures are made up for the exercise.
Show the answer
Answer: 10 mg/L
Working: Css = infusion rate ÷ clearance = 50 ÷ 5 = 10 mg/L.
Question 3
The target steady state concentration is 15 mg/L and the patient's clearance is 4 L/hour. What infusion rate is needed in mg/hour, and how many mg is that over 24 hours? These figures are made up for the exercise.
Show the answer
Answer: 60 mg/hour; 1,440 mg
Working: Rate = target × clearance = 15 × 4 = 60 mg/hour. Over 24 hours that is 60 × 24 = 1,440 mg.
What the article says
The article describes the steady state concentration as the balance between infusion rate and clearance and says a new dose can be found by direct proportion. The values on this page are invented for practice.
Where marks are lost
- Using the proportion the wrong way up.
- Calling a level steady state before it has levelled off.
- Mixing mg/hour with micrograms/L.
Frequently asked questions
What if the patient is not at steady state yet?
The proportion method assumes steady state, so the level may still be rising.
Does a loading dose change the steady state?
No. It gets there faster, but the steady state depends on rate and clearance.
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
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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
See also the formula sheet, the eight sample questions and the approved calculator page.