Drug Dosing in Kidney Disease — SAQ

Stem

A 72-year-old man with type 2 diabetes and stable CKD was taking metformin, ramipril and furosemide before admission. He now has severe pneumonia, vomiting and hypotension. Creatinine has risen from 1.9 to 2.9 mg/dL over 36 hours, urine output is falling and potassium is 5.8 mmol/L. Empirical vancomycin plus piperacillin-tazobactam was started before cultures were available. He is not receiving dialysis.[1][4]

Questions — 10 marks

a) Explain why a single reported eGFR or Cockcroft-Gault result cannot be treated as the patient's current drug clearance. Give two additional bedside variables needed now. (2 marks)[1][3]

b) State the first two prescribing actions for his chronic medicines, including what must be documented for any temporary hold. (2 marks)[2][7][8]

c) Explain how you would separate loading-dose and maintenance-dose decisions for the time-critical antibiotics. (2 marks)[1][4]

d) State the evidence-based concern about vancomycin plus piperacillin-tazobactam and the current vancomycin exposure target if cultures prove a serious invasive MRSA infection. (2 marks)[5][6]

e) Write four elements of safe discharge sick-day advice. (2 marks)[7][8]

Model Answer and exact marking scheme

a) Changing clearance — 2 marks

• 1 mark: creatinine is changing over 36 hours, so the patient is in non-steady-state AKI. Static creatinine-based equations lag behind changing filtration and must not be converted into an arbitrary fixed “GFR under 30” dose band.[1][3]
• 0.5 mark each, maximum 1 mark: any two of urine-output trend, haemodynamics or volume status, creatinine direction and rate, time and amount of prior doses, correctly timed drug concentrations, or clinical response/toxicity.[1]

b) Chronic medicines — 2 marks

• 1 mark: reconcile each medicine and its current indication; for any proposed pause, state the immediate toxicity or physiological risk in this patient's hypovolaemia, hypotension, hyperkalaemia and evolving AKI, and use the current drug-specific source. No mark for a class-wide stop rule.[1][2]
• 1 mark: for every held medicine, document the alternative or urgent specialist plan if needed, what will be monitored, the review trigger, responsible reviewer and explicit restart condition. Failure to restart beneficial treatment can also cause harm.[2][7][8]

No mark is awarded for writing a universal memorized stop list without a restart plan.[2][7]

c) Loading versus maintenance — 2 marks

Award 0.5 mark for each mandatory component; both loading and both maintenance components are required for 2 marks:[1][4]

• 0.5 mark: connect the loading decision to infection urgency and the required initial target exposure.[1][4]
• 0.5 mark: state that sepsis, fluid resuscitation or oedema may expand volume of distribution, so rising creatinine alone is not a reason to omit or automatically reduce an indicated loading dose.[1][4]
• 0.5 mark: connect maintenance dose or interval to the changing clearance and the antimicrobial's agent-specific PK/PD target.[1][4]
• 0.5 mark: use the current drug- and indication-specific local source and reassess from prior doses, urine output, appropriately timed exposure data and clinical response after meaningful physiological change.[1][4]

d) Antibiotic evidence and target — 2 marks

Award 0.5 mark for each mandatory component:[5][6]

• 0.5 mark: describe the association with more creatinine-defined AKI than some comparator regimens.[6]
• 0.5 mark: qualify causality: observational confounding and inhibited secretion or pseudo-AKI are plausible, and the prospective study found no parallel cystatin C, BUN, dialysis or mortality signal; do not claim proven synergistic nephrotoxicity or automatically substitute cefepime.[6]
• 0.5 mark: restrict AUC24/MIC 400 to 600 mg·h/L to serious invasive MRSA infection.[5]
• 0.5 mark: specify a broth-microdilution MIC of 1 mg/L and target attainment within the first 24 to 48 hours; trough-only 15 to 20 mg/L is not an equivalent target and a pre-fourth-dose trough is not universally required with Bayesian AUC methods.[5]

e) Discharge sick-day advice — 2 marks

Award 0.5 mark each for four elements:[7][8]

1. a clear trigger, such as vomiting, diarrhoea, poor intake, hypotension or suspected AKI;[7][8]
2. the named medicines to pause and the reason for each;
3. whether an alternative or urgent specialist plan is needed for a medicine that cannot safely be paused;
4. when and where to seek clinical review and what to monitor;
5. the restart condition, responsible clinician and follow-up test.

Maximum 2 marks. Evidence for community sick-day protocols is limited, so advice must be individualized and must prevent failure to restart.[7][8]

Examiner summary

A full-mark answer says non-steady state, protects time-critical antimicrobial exposure, uses a drug- and indication-specific current source, qualifies the vancomycin evidence and target, and makes every temporary hold a hold-review-restart plan.[1][2]

References8Show ledgerHide ledger

1. [1]Lea-Henry TN, Carland JE, Stocker SL, et al. Clinical Pharmacokinetics in Kidney Disease: Fundamental Principles Clin J Am Soc Nephrol, 2018.PMID 29934432
2. [2]Levin A, Ahmed SB, Carrero JJ, et al. Executive summary of the KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease: known knowns and known unknowns Kidney Int, 2024.PMID 38519239
3. [3]Chen S. Retooling the creatinine clearance equation to estimate kinetic GFR when the plasma creatinine is changing acutely J Am Soc Nephrol, 2013.PMID 23704286
4. [4]Eyler RF, Shvets K. Clinical Pharmacology of Antibiotics Clin J Am Soc Nephrol, 2019.PMID 30862698
5. [5]Rybak MJ, Le J, Lodise TP, et al. Therapeutic Monitoring of Vancomycin for Serious Methicillin-resistant Staphylococcus aureus Infections: A Revised Consensus Guideline and Review by the American Society of Health-system Pharmacists, the Infectious Diseases Society of America, the Pediatric Infectious Diseases Society, and the Society of Infectious Diseases Pharmacists Clin Infect Dis, 2020.PMID 32658968
6. [6]Miano TA, Hennessy S, Yang W, et al. Association of vancomycin plus piperacillin-tazobactam with early changes in creatinine versus cystatin C in critically ill adults: a prospective cohort study Intensive Care Med, 2022.PMID 35833959
7. [7]Watson KE, Dhaliwal K, McMurtry E, et al. Sick Day Medication Guidance for People With Diabetes, Kidney Disease, or Cardiovascular Disease: A Systematic Scoping Review Kidney Med, 2022.PMID 36046611
8. [8]Whiting P, Morden A, Tomlinson LA, et al. What are the risks and benefits of temporarily discontinuing medications to prevent acute kidney injury? A systematic review and meta-analysis BMJ Open, 2017.PMID 28389482