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PSA Calculations & Units: The Survival Guide

A practical, exam-safe system for dose calculations, unit conversions and infusion rates—plus a double-check loop that prevents silly errors.

Review date
Feb 2026
Reading time
13 min read
Author
Dr. Kola Tytler MBBS CertHE MBA MSt MRCGP
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Key points

  • PSA calculation marks are often lost to <strong>units</strong>, not pharmacology. Treat units as the main task.
  • Use a fixed workflow: <strong>write given → write required → convert → calculate → sense-check → document</strong>.
  • Never memorise doses from blogs: use <strong>BNF/BNFC</strong> in the PSA environment and focus on the maths + safety logic.
The PSA explicitly assesses calculation skills (and your ability to record outcomes accurately). Your edge is not being “good at maths”—it is using a repeatable safety workflow that stops preventable errors under time pressure.

The only workflow you need

<strong>1) Given</strong> (what you know) → <strong>2) Required</strong> (what you must output) → <strong>3) Units</strong> (convert early) → <strong>4) Formula</strong> (write it) → <strong>5) Calculate</strong> → <strong>6) Sense-check</strong> (range/route/time) → <strong>7) Document</strong> clearly.
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Step 1 — Convert units before doing anything else

Make everything one consistent unit system (e.g., mg, mL, hours). Common conversions: micrograms ↔ mg; mmol ↔ mg (only when appropriate and using provided conversion factors); minutes ↔ hours. Most mistakes happen when you “carry” mixed units through the calculation.
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Step 2 — Write the formula (yes, every time)

Do not calculate in your head. Write a single line formula. Example patterns: • <strong>Dose</strong> = weight × mg/kg • <strong>Volume</strong> = required dose / concentration • <strong>Rate</strong> = volume / time This reduces “panic algebra”.
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Step 3 — Use the PSA double-check loop

After calculating, do two checks: <strong>Check A (units):</strong> do the units cancel correctly? <strong>Check B (sense):</strong> does it look plausible (route, frequency, magnitude)? If either check fails, re-do from Step 1.
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Step 4 — Document like a prescribing risk professional

Your final output must be explicit: dose + unit + route + frequency + duration (if asked) + rate (if infusion) + any monitoring caveat. Avoid ambiguous shorthand.

High-frequency calculation types (drill these)

  • mg/kg dosing (single dose and daily dose split into divided doses).
  • Concentration conversions (mg/mL, micrograms/mL).
  • Infusion rates (mL/hour) from total volume over time.
  • Dose from infusion rate and concentration (reverse calculation).
  • Renal adjustment scenarios (the maths + the safety behaviour: check renal function, then confirm dosing guidance).

Important PSA behaviour

In the PSA, you are assessed on safe prescribing competence—not bravado. If a calculation result feels implausible, that is a signal to re-check units and assumptions. <strong>Speed without verification is the exam’s silent killer.</strong>
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The 30-minute drill protocol (twice weekly)

Set a timer. Do 8–12 calculations. For every error, label it as one of: • unit conversion • wrong denominator • arithmetic slip • documentation ambiguity After 2 weeks, your error pattern will shrink dramatically because it’s usually one dominant category.
SourcePSA: Blueprint (14.07.2025) — official PDF
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SourcePSA: Item Writing Manual (July 2025) — official PDF
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SourcePrescribing Safety Assessment — official site
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  • Make everything one consistent unit system (e.g., mg, mL, hours). Common conversions: micrograms ↔ mg; mmol ↔ mg (only when appropriate and using provided conversion factors); minutes ↔ hours. Most mistakes happen when you “carry” mixed units through the calculation.

    Return to this section

  • Do not calculate in your head. Write a single line formula. Example patterns: • Dose = weight × mg/kg • Volume = required dose / concentration • Rate = volume / time This reduces “panic algebra”.

    Return to this section

  • After calculating, do two checks: Check A (units): do the units cancel correctly? Check B (sense): does it look plausible (route, frequency, magnitude)? If either check fails, re-do from Step 1.

    Return to this section

  • Your final output must be explicit: dose + unit + route + frequency + duration (if asked) + rate (if infusion) + any monitoring caveat. Avoid ambiguous shorthand.

    Return to this section
  • Return to this section
  • Return to this section
  • Return to this section
  • Return to this section
  • Return to this section

  • Set a timer. Do 8–12 calculations. For every error, label it as one of: • unit conversion • wrong denominator • arithmetic slip • documentation ambiguity After 2 weeks, your error pattern will shrink dramatically because it’s usually one dominant category.

    Return to this section

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