Drug Dosing in Kidney Disease — SAQ

Stem

A 72-year-old man with type 2 diabetes, hypertension and known CKD stage 3b (baseline eGFR 35 mL/min/1.73 m²; creatinine 1.9 mg/dL) is admitted to the ward with community-acquired pneumonia. He is volume-depleted (postural drop, dry mucousae). His medications are: metformin 1 g BD, ramipril 10 mg OD, furosemide 40 mg OD, atorvastatin 40 mg OD, aspirin 75 mg OD. The team plans empirical IV vancomycin + piperacillin-tazobactam and fluid resuscitation. On day 3, creatinine has risen to 2.9 mg/dL and potassium is 5.8 mmol/L. [1]

Questions

a) Outline the FIVE-STEP systematic approach to prescribing in kidney disease. (3 marks) [1]

b) For THIS patient on admission, which medications should be HELD immediately and why? Name the specific safety principle that makes Cockcroft-Gault and eGFR invalid at this point. (3 marks) [1]

c) The team chooses vancomycin + piperacillin-tazobactam. State the nephrotoxicity concern with this combination, the dose-adjustment principle for vancomycin, and the monitoring target. (2 marks) [1]

d) State the written patient safety-net ('sick-day rules') that should be communicated at discharge. (2 marks) [1]

Model Answer

a) Five-step systematic approach (1 mark per correctly stated pair, max 3):

1. Estimate kidney function — Cockcroft-Gault for dosing, eGFR for staging.
2. Review every drug on the list — renal-cleared? nephrotoxic? narrow-therapeutic-index (TDM)? interaction?
3. Adjust the dose OR extend the interval; loading dose usually unchanged.
4. Monitor drug levels where indicated (vancomycin AUC, digoxin, lithium, etc.) AND renal function/electrolytes.
5. Reassess at high-risk moments — AKI, declining GFR, polypharmacy, new drugs, contrast, transitions of care. [1]

b) Medications to HOLD immediately (1 mark for the principle + 2 marks for correct drugs, max 3):

• Metformin — rising creatinine + hypoperfusion = metformin-associated lactic acidosis risk; stop and do not restart until eGFR above 30 and the patient is well.
• Ramipril (ACEi) — efferent arteriolar dilation reduces GFR further in volume depletion; also a contributor to hyperkalaemia.
• Furosemide — the patient is volume-depleted; continuing the loop diuretic worsens the pre-renal AKI (hold until euvolaemic, then reassess).
• Aspirin — low-dose aspirin can usually continue, but any NSAID would be held. [1]

Key principle: Cockcroft-Gault and eGFR are INVALID in AKI because they assume steady-state creatinine; during a rising creatinine the formulae OVERESTIMATE true GFR and any dose they produce will be too high. Assume the GFR is low and review the chart within the first hour of recognising AKI. [1]

c) Antibiotic concern and monitoring (1 mark each, max 2):

• Vancomycin + piperacillin-tazobactam carry a recognised synergistic AKI risk (2 to 4 times either agent alone, per the ASHP/IDSA 2020 consensus). Consider switching piperacillin-tazobactam to cefepime if the AKI progresses.
• Vancomycin dosing: weight-based loading 20 to 25 mg/kg (actual body weight), then AUC-guided dosing (AUC/MIC 400 to 600; trough 15 to 20 mg/L for serious MRSA), drawn before the 4th dose and adjusted daily in AKI. [1]

d) Sick-day rules (safety-net) — 1 mark for the trigger, 1 mark for the drug list:

• Trigger: "If you develop vomiting, diarrhoea, or fever and cannot drink normally for 24 hours."
• Action: temporarily STOP metformin, ACEi/ARB (ramipril), diuretics (furosemide), SGLT2 inhibitors, and NSAIDs; keep drinking; restart when eating and drinking normally again for 24 to 48 hours; seek medical help if not improving. [1]

High-yield takeaway: The two errors examiners reward naming — not adjusting a renally-cleared drug (toxicity) and missing a nephrotoxin (AKI) — are both prevented by the systematic five-step review applied at every contact. [1]

References

1. [1]Lea-Henry TN, et al. Clinical Pharmacokinetics in Kidney Disease: Fundamental Principles. CJASN, 2018.PMID 29934432