The question
Work it with the BNF open before you show the answer. At exam pace this item has 6 minutes.
A 66-year-old man is brought to the emergency department with two days of worsening weakness and palpitations. He has chronic kidney disease, and three days ago his general practitioner started an antibiotic for a urinary tract infection. He is on a cardiac monitor in the resuscitation area with resuscitation staff present.
PMH: Chronic kidney disease; hypertension; benign prostatic enlargement.
DH: ramipril 5 mg PO daily; trimethoprim 200 mg PO twice daily (12-hrly), day 3 of 7; atorvastatin 20 mg PO nightly; tamsulosin 400 micrograms PO daily. No known drug allergies.
SH: retired; lives with his wife; non-smoker.
On examination: Temperature 36.9°C, HR 88/min and regular, BP 138/84 mmHg, RR 16/min, O2 sat 97% breathing air. Alert and orientated. Proximal muscle weakness in both legs; reflexes present. Chest clear, JVP not raised, no oedema. Abdomen soft; bladder not palpable. A 22-gauge cannula is sited in the back of the left hand and flushes easily. He is attached to a cardiac monitor.
Investigations: Na 134 mmol/L (135–145) K 7.1 mmol/L (3.5–5.3) Bicarbonate 17 mmol/L (22–29) Urea 21.4 mmol/L (2.5–7.8) Creatinine 187 µmol/L (60–120), eGFR 34 mL/min/1.73 m² Glucose 6.8 mmol/L (4.0–7.8) 12-lead ECG: peaked T waves in the chest leads, flattened P waves, QRS duration 136 ms
Answer and marking
calcium gluconate 10% solution for injection · 30 mL · intravenous (IV) · once only, given by slow intravenous injection over 10 minutes; repeat if no improvement in the ECG within 5–10 minutes
Marks5 for the drug choice, 5 for the written prescription (dose, route, frequency).
Also creditedcalcium gluconate; calcium chloride; cal gluconate; ca gluconate
Drug choiceSevere hyperkalaemia with ECG changes is two problems in sequence, and the marks here are for knowing the sequence. Problem one is that his myocardium is about to arrest: peaked T waves, flattened P waves and a QRS of 136 ms are the walk to a sine wave. Intravenous calcium fixes none of the potassium and all of the immediate danger — it re-stabilises the membrane within minutes, which is why the BNF's instruction for ECG-change hyperkalaemia is calcium first, with the ECG rechecked at 5–10 minutes and the dose repeated if the trace has not improved. Problem two is the potassium itself, and that is where insulin comes in: 10 units of soluble insulin with 25 g of glucose over 5–30 minutes drives potassium into cells from about 15 minutes. Prescribed FIRST, insulin–glucose leaves the myocardium bare for exactly the quarter of an hour in which this ECG is at its most dangerous — which is why it collects partial credit for drug knowledge and nothing for the prescription. Salbutamol is an adjunct to that second step; the binders (sodium zirconium cyclosilicate, patiromer, the old resin) act over hours in the gut and belong to the non-emergency presentation; bicarbonate treats the acidaemia, not the emergency; furosemide is an afterthought for later.
Dose, route and frequencyThe dose question is the salt question, and the MHRA wrote to every UK prescriber about it in June 2023 after deaths from getting it wrong. Thirty millilitres of calcium GLUCONATE 10% provides approximately 6.8 mmol of calcium — the same calcium as TEN millilitres of calcium CHLORIDE 10%. Two traps follow. First, the 10 mL ampoule of gluconate — the hypocalcaemia dose everyone has seen given — delivers a third of what this ECG needs, and underdosing is the specific error the MHRA advice names. Second, 30 mL written against calcium chloride is a threefold calcium overdose. Give the gluconate by slow intravenous injection over 10 minutes, into a monitored patient, and recheck the ECG at 5–10 minutes: the effect lasts only 30–60 minutes, so a repeat is often needed. Calcium chloride at its correct 10 mL dose loses a single mark here: the BNF prefers gluconate outside cardiac arrest, and chloride is the more irritant salt to push through a 22-gauge cannula in the back of a hand.
ExplanationAlso part of the plan, though not marked in the boxes: insulin–glucose immediately after the calcium, a potassium recheck, holding the ramipril and trimethoprim, and senior renal input at an eGFR of 34.
calcium gluconate 10% solution for injection — 30 mL (approximately 6.8 mmol), intravenous (IV) by slow injection over 10 minutes, once only, repeat if the ECG has not improved at 5–10 minutes. Full marks. Fifteen to 29 mL: 5 + 1. Ten millilitres: 5 + 0. Above 30 mL: 5 + 0. calcium chloride 10% — 10 mL (1 g) IV once only: 5 + 4. Thirty millilitres of the chloride: 5 + 0. insulin–glucose (any spelling) — 2 + 0. 'calcium' with no salt — 2 + 0. salbutamol — 1 + 0. sodium zirconium cyclosilicate, patiromer, calcium polystyrene sulfonate, sodium bicarbonate, furosemide — 0/10. (BNF hyperkalaemia treatment summary and calcium gluconate monograph, including the MHRA/CHM advice of June 2023, read 22 August 2026.)
How this item is marked
Every answer is matched to a marking tier. The first rows below are from this item's real scheme (38 tiers in total).
| Medicine | Dose and route | Marks | Marking note |
|---|---|---|---|
| calcium gluconate | 30 mL · intravenous (IV) | 10 / 10 | optimal. BNF, acute severe hyperkalaemia: 'By slow intravenous injection, Adult: 30 mL for 1 dose, calcium gluconate 10% (providing approximately 6.8 mmol of ca… |
| calcium gluconate | 30 mL · intravenous (IV) | 10 / 10 | the same prescription with the administration time written in the frequency box, which is where a once-only chart's rate instruction often lands. The scorer com… |
| calcium gluconate | 3 g · intravenous (IV) | 10 / 10 | the same dose in grams: 30 mL of a 10% solution is 3 g (BNF: calcium gluconate 1 g contains Ca2+ 2.23 mmol; 3 × 2.23 ≈ 6.7 mmol, the monograph's 'approximately … |
| calcium gluconate | 6.8 mmol · intravenous (IV) | 10 / 10 | the same dose written as millimoles of calcium, the MHRA advice's own figure. Credited identically. |
| calcium gluconate | 3 ampoules · intravenous (IV) | 10 / 10 | three 10 mL ampoules of the 10% solution are 30 mL. Credited identically. |
| calcium gluconate | 15-29 mL · intravenous (IV) | 6 / 10 | between the hypocalcaemia dose and the hyperkalaemia dose — most often a remembered '10–20 mL'. Under the 6.8 mmol the ECG needs, on a warning the MHRA issued p… |
| calcium gluconate | 1.5-2.9 g · intravenous (IV) | 6 / 10 | the same intermediate underdose written in grams (15–29 mL of 10%). Deduct 4. |
| calcium gluconate | 1-14 mL · intravenous (IV) | 5 / 10 | the classic error the MHRA's June 2023 advice exists for: the 10 mL hypocalcaemia ampoule given for hyperkalaemia, a third of the calcium the ECG needs. Right d… |
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How to approach a prescribing item
The repeatable method from the Prescribing section guide.
- Name the indicationSay in one phrase what the drug is for. If you cannot, you are not ready to choose one.
- Choose the class, then the drugThe BNF treatment summary gives the class and first-line agent; the case's modifiers pick the drug within it.
- Find the dose under the right indicationMonographs list several indications with different doses. Read the heading, not the first number.
- Apply the patientWeight-based? Renal impairment? Age band? Pregnancy? Each can change the dose or the drug.
- Write it as the chart needs itDose in a unit, route from the accepted list, frequency in words the chart uses, once-only or regular as instructed.
Before you submit
- Is the drug I chose the first-line agent for this indication, in this patient?
- Is the dose taken from the right indication in the monograph?
- Have I written a dose in a unit, not a volume?
- Does the route exist for this drug and this form?
- Is there a frequency, and is it in the chart's words?
- Have I checked renal function, allergy and the interaction the case mentions?
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