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Question and answer links
- W1: A 39-year-old man is on day 9 of cephalexin for leg cellulitis. His creatinine has risen from 0.9 to 1.8 mg/dL; he passe · Answer
- W2: A 66-year-old woman with type 2 diabetes and stable CKD G3a (eGFR 52 mL/min/1.73 m² on three checks over a year) takes m · Answer
- Q3: Dale, 61, day 6 of vancomycin plus piperacillin-tazobactam, after three doses of gentamicin and two days of ketorolac: c · Answer
- Q4: If you were thinking "vancomycin tubular injury (ATN) is the sole explanation for Dale's AKI," and you then learn that o · Answer
- Q5: Dale, day 8: stage 2 AKI (probable tubular injury with suspected drug-induced AIN), creatinine 2.8 mg/dL and rising, euv · Answer
- Q6: Dale, day 9: creatinine 3.1 mg/dL and rising (1.0 → 2.3 → 2.8 → 3.1); printed eGFR 22; admission eGFR 86; Cockcroft–Gaul · Answer
- Q7: Dale, day 15: creatinine 1.7 mg/dL and falling (baseline 1.0; peak 3.2 on day 10); probable nephrotoxic tubular injury p · Answer
- Q8: Weeks later a culture suggests an infection for which gentamicin may be needed. Dale has a prior nephrotoxic AKI. Which · Answer
Session S4 · 2026-10-29 · supplementary · 20 min Anchor: Acute kidney injury — nephrotoxic acute tubular necrosis (ATN) from vancomycin or an aminoglycoside, with acute interstitial nephritis (AIN) in the differential · Integrates: contrast decisions in an injured kidney, drug dosing during AKI, the sediment that separates ATN from AIN, post-AKI follow-up One-line hook: Every hospital floor has a patient on day six of vancomycin. Read that day's creatinine, sediment and medication list and you save a kidney; read only the eGFR and you dose four drugs wrong.
Learning Objectives
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7 · Q8
- Recognize nephrotoxic ATN — a non-oliguric creatinine rise several days into vancomycin or an aminoglycoside — and choose the first steps for the drug and its level.
- Separate ATN from AIN by the urine sediment, and explain why the classic triad and urine eosinophils cannot settle the diagnosis.
- Decide about iodinated contrast during AKI — who is truly at risk, whether the scan can wait, which precautions help — and apply the metformin rule.
- Dose renally cleared drugs while the creatinine is still moving, and name the kidney-function estimate you used.
- Write an AKI discharge plan: what belongs in the summary and on the allergy list, which drugs restart and when, and when the kidney is rechecked.
Warm-Up (individual commit before the case)
Answers for this section: W1 · W2
Two readiness questions. Commit to an answer before you open each callout.
W1What points to interstitial nephritis?
A 39-year-old man is on day 9 of cephalexin for leg cellulitis. His creatinine has risen from 0.9 to 1.8 mg/dL; he passes normal volumes of urine and has no fever, rash or eosinophilia. Which statement about acute interstitial nephritis (AIN) is correct?
With no fever, rash or eosinophilia, AIN is effectively excluded
A positive urine eosinophil stain would confirm AIN
Sterile pyuria with white-cell casts would support AIN
Muddy brown granular casts are the expected AIN sediment
Show answer and explanation · Link to this question
Correct answer: C
Learning point: Drug-induced AIN is an immune-mediated interstitial injury with variable timing and presentation. Sterile pyuria and white-cell casts support it, but are neither required nor specific; the classic fever–rash–eosinophilia triad is often absent. Urine eosinophils neither confirm nor exclude AIN. Why not the others: A falsely excludes AIN without the triad; B assigns certainty to a weak test; D describes a sediment supporting tubular injury rather than a characteristic AIN finding. Course reading: Evidence for this case
W2Metformin before a contrast CT
A 66-year-old woman with type 2 diabetes and stable CKD G3a (eGFR 52 mL/min/1.73 m² on three checks over a year) takes metformin 500 mg twice daily and is booked for an outpatient CT with IV iodinated contrast at a center following the ACR contrast manual. What should happen to her metformin?
Hold it on the morning of the scan and for 48 hours after, restarting after a repeat creatinine
Continue it unchanged; with an eGFR above 30 and no AKI, no hold or repeat creatinine is needed
Stop it for good, because metformin with iodinated contrast is unsafe at any eGFR below 60
Halve the dose for a week and give a liter of IV saline before the scan to protect the kidney
Show answer and explanation · Link to this question
Correct answer: B
Learning point: ACR Category I permits continuation for IV iodinated contrast when there is no AKI and eGFR is at least 30. ACR Category II requires interruption for AKI or eGFR below 30, with reassessment before restart after 48 hours. Metformin itself is not nephrotoxic; accumulation during illness or kidney failure can contribute to lactic acidosis. Why not the others: A follows the stricter US product label for eGFR 30–60, but the stem explicitly specifies ACR policy; C invents a permanent contraindication; D supplies neither indicated prophylaxis nor a useful metformin strategy. Course reading: Evidence for this case
Part 1 — Presentation
Answers for this section: Q3 · Q4
Dale, 61, drives a long-haul truck and has type 2 diabetes (A1c 8.6 %), neuropathy, peripheral artery disease and hypertension. On day 0 he arrived with a painless three-week ulcer under his right great toe; a probe touched bone, and the X-ray showed osteomyelitis of the first metatarsal head. Fever 38.6 °C, white count 16.8. The emergency department started vancomycin, piperacillin-tazobactam and gentamicin; gentamicin stopped after three once-daily doses (days 0–2) when the blood cultures stayed negative. On day 1 he fasted for an operative debridement and bone biopsy, then received ketorolac for two days. The bone culture, final on day 4, grew MRSA (methicillin-resistant Staphylococcus aureus) alone; the antibiotics were not changed. Baseline creatinine 1.0 mg/dL two months ago and on admission (eGFR 86 at steady state).
Day 6, morning: creatinine 2.3 mg/dL. Dale feels well, the foot is healing, and he made 1.8 L of urine yesterday. Vascular surgery has booked a CT angiogram of the leg for tomorrow. His enoxaparin, gabapentin and metformin are running at the doses written on admission.
Vitals: T 37.3 °C · heart rate 88 · BP 128/74 · respiratory rate 16 · SpO₂ 97 % on room air · weight 82.0 kg (84.0 on admission). Exam: comfortable; mucous membranes slightly dry; lungs clear; no edema; no rash; clean, granulating wound over the right first ray; faint right foot pulses.
| Drug | Dose | Route | Frequency | Indication |
|---|---|---|---|---|
| Vancomycin | 1,250 mg | IV | every 12 hours since day 0 | MRSA osteomyelitis |
| Piperacillin-tazobactam | 4.5 g | IV | every 6 hours since day 0 | Empiric gram-negative and anaerobic cover |
| Gentamicin | 420 mg (5 mg/kg) | IV | once daily, days 0–2 (stopped) | Empiric cover until cultures |
| Ketorolac | 15 mg | IV | every 6 hours, days 1–2 (stopped) | Post-operative pain |
| Metformin | 1,000 mg | PO | twice daily | Type 2 diabetes |
| Gabapentin | 600 mg | PO | three times daily | Diabetic neuropathy |
| Enoxaparin | 40 mg | SC | once daily | VTE prophylaxis |
Also insulin, aspirin, atorvastatin and amlodipine.
| Test | Day 0 | Day 2 | Day 4 | Day 6 | Reference range |
|---|---|---|---|---|---|
| Creatinine (mg/dL) | 1.0 | 1.0 | 1.3 | 2.3 | 0.7–1.3 |
| BUN (mg/dL) | 18 | 16 | 22 | 32 | 8–20 |
| BUN:creatinine | 18:1 | 16:1 | 17:1 | 14:1 | 10–20:1 |
| Potassium (mEq/L) | 4.4 | 4.2 | 4.5 | 4.9 | 3.5–5.0 |
| Bicarbonate (mEq/L) | 23 | 24 | 23 | 21 | 22–28 |
| Vancomycin trough (mg/L) | — | 14 | — | 27 | above 20 raises AKI risk |
| WBC (×10⁹/L) / eosinophils | 16.8 / 1 % | 12.1 / 1 % | 10.4 / 2 % | 9.2 / 2 % | 4.0–11.0 / 0–5 % |
Urinalysis, day 6: specific gravity 1.011 · pH 5.5 · protein trace · blood, leukocyte esterase and nitrite negative · WBC 0–2 and RBC 0–2 per high-power field · several muddy brown granular casts and renal tubular epithelial cells · no white-cell casts.
The AKI began on day 4 — a rise of 0.3 mg/dL within 48 hours meets the KDIGO definition — and nobody noticed. Today 2.3 ÷ 1.0 = 2.3 times baseline: stage 2. eGFR not valid — creatinine is moving.
Q3What is happening, and what do you do today?
Dale, 61, day 6 of vancomycin plus piperacillin-tazobactam, after three doses of gentamicin and two days of ketorolac: creatinine 1.0 → 1.3 → 2.3 mg/dL; urine 1.8 L/day; vancomycin trough 27 mg/L; BUN:creatinine 14:1; urine specific gravity 1.011 with muddy brown casts and tubular cells, no white-cell casts. Which assessment, and which plan for the vancomycin, are right?
Prerenal from fasting for surgery: give 2 L of saline, keep the vancomycin dose, and recheck in 48 hours
AIN from the piperacillin-tazobactam: stop it, start prednisone, and keep the vancomycin unchanged
Vancomycin tubular injury: keep giving it at the same dose, and add furosemide so a brisk urine flow flushes the tubules
Vancomycin tubular injury: hold it, recheck the level, and let pharmacy and infectious disease redose or switch
Show answer and explanation · Link to this question
Correct answer: D
Learning point: Muddy brown casts and tubular cells favor acute tubular injury after stacked nephrotoxic exposures. The high vancomycin level can contribute to injury and also result from reduced clearance. Hold the next dose and obtain pharmacy/ID review of levels, kidney trend and culture-directed treatment. Current vancomycin guidance favors AUC-guided monitoring; students need to recognize toxicity and request that review, not calculate an AUC. Why not the others: A ignores the toxic level and delays review; B prematurely proves AIN and starts steroids while leaving vancomycin unchanged; C uses diuresis as a substitute for managing toxicity. Specific gravity, BUN:creatinine and absent white-cell casts cannot exclude concurrent causes. Course reading: Evidence for this case
Day 8 — a new finding
Pharmacy held the vancomycin for 30 hours, until the level fell to 16; infectious disease kept it — MRSA in bone needs about six weeks — and pharmacy now picks each dose from levels. Two trauma scans bumped the CT angiogram from the day-7 list; it is rebooked for day 9. Nobody has reopened the rest of the medication list.
On day 8 a new examination and urine panel become available.
Concordance-style practice — no expert panel stands behind the key. "Unchanged" means the finding does not move the hypothesis.
Q4Evidence update
If you were thinking "vancomycin tubular injury (ATN) is the sole explanation for Dale's AKI," and you then learn that on day 8 he has a new rash, a temperature of 38.1 °C, 8 % eosinophils, and white-cell casts with sterile pyuria, this hypothesis becomes:
Much less likely
Less likely
Unchanged
More likely
Much more likely
Show answer and explanation · Link to this question
Correct answer: B
Learning point: The new inflammatory findings raise concern for superimposed drug-induced AIN, so ATN as the sole explanation becomes less likely. A or B is defensible with that reasoning; the day-6 evidence can still support tubular injury. Neither the rash nor sediment proves two biopsy-defined lesions. Stop the unnecessary beta-lactam, review all possible culprits including vancomycin, and involve nephrology if recovery is delayed. Why not the others: C underweights the new findings; D and E strengthen a sole-cause explanation despite evidence for another process. Do not score A versus B as a validated probability difference. Course reading: Evidence for this case
On the morning of day 8 the nurse finds a pink, blotchy rash across his trunk and thighs. Temperature 38.1 °C. He feels fine; the foot looks better than ever.
| Test | Day 6 | Day 8 | Reference range |
|---|---|---|---|
| Creatinine (mg/dL) | 2.3 | 2.8 | 0.7–1.3 |
| WBC (×10⁹/L) / eosinophils | 9.2 / 2 % | 9.0 / 8 % (0.72 ×10⁹/L) | 4.0–11.0 / 0–5 % |
| Urine leukocyte esterase / nitrite | negative / negative | 2+ / negative | negative |
| Urine WBC (per high-power field) | 0–2 | 25–50 | 0–5 |
| Urine casts | muddy brown granular | white-cell casts; a few granular | none |
| Urine protein (dipstick) | trace | 1+ | negative |
| Urine culture | — | no growth | no growth |
| Urine output (L/day) | 1.8 | 1.7 | — |
Part 2 — Data
Answers for this section: Q5
Day 8, 17:00. The piperacillin-tazobactam has stopped. Vascular surgery has re-examined the foot — right ankle-brachial index 0.52, no rest pain, a clean wound — and wants a CT angiogram of the leg to plan a bypass or angioplasty: "It can wait some days, but I want the pictures this admission." The scan is on tomorrow's list, and radiology is calling about his kidneys.
Now: creatinine 2.8 mg/dL and still rising · urine 1.7 L/day · BP 132/78 · heart rate 84 · weight 82.5 kg · lungs clear · no edema · metformin 1,000 mg twice daily still being given.
The nephrotoxin ledger — what this kidney has absorbed since admission:
| Exposure | Days | Kidney hazard | Status |
|---|---|---|---|
| Gentamicin | 0–2 | tubular toxin (ATN) | stopped day 3 |
| Ketorolac | 1–2 | NSAID: constricts the afferent arteriole | stopped day 3 |
| Vancomycin | 0 onward | tubular toxin; trough 27 on day 6 | pharmacy-dosed from levels |
| Piperacillin-tazobactam | 0–8 | interstitial nephritis; no target after day 4 | stopped day 8 |
| Iodinated contrast | proposed day 9 | contrast-associated AKI | decision pending |
Q5The CT angiogram
Dale, day 8: stage 2 AKI (probable tubular injury with suspected drug-induced AIN), creatinine 2.8 mg/dL and rising, euvolemic, urine 1.7 L/day, metformin still running. Vascular surgery wants a CT angiogram of the leg (IV iodinated contrast) tomorrow; the foot can wait some days. What is the plan for the scan?
Defer while feasible; reassess with radiology, use an adequate diagnostic dose and indicated saline prophylaxis, and hold metformin
Scan tomorrow as booked with N-acetylcysteine and a bicarbonate drip, and keep the metformin going
Cancel it for this admission, because iodinated contrast is contraindicated in AKI, and plan from the exam
Scan tomorrow as booked without changes, since controlled studies found no contrast risk at any kidney function
Show answer and explanation · Link to this question
Correct answer: A
Learning point: Because the limb assessment permits delay, defer and reassess the diagnostic benefit against the evolving AKI. If IV contrast remains needed, use a dose sufficient for diagnosis and isotonic saline prophylaxis when appropriate for his volume status. Hold metformin during AKI and follow restart criteria. Urgent imaging can proceed when the expected benefit outweighs risk. Why not the others: B leaves metformin running and relies on N-acetylcysteine, which is not recommended; bicarbonate appears similar to saline but is not preferred. C treats AKI as an absolute contrast prohibition; D assumes zero risk in every patient. Course reading: Evidence for this case
Part 3 — Course and Decisions
Answers for this section: Q6 · Q7 · Q8
Day 9. The angiogram is deferred, and metformin has been held since last evening. The rash is fading. Creatinine 3.1 mg/dL — 3.1 ÷ 1.0 = 3.1 times baseline, stage 3 — with 1.6 L of urine and a potassium of 5.1. The BMP prints "eGFR 22"; the admission eGFR of 86 still heads the order-entry screen, and the enoxaparin and gabapentin are at their admission doses. Cockcroft–Gault on today's creatinine (82 kg) gives a clearance of about 29 mL/min. Nephrology advises waiting several days off the piperacillin-tazobactam before any talk of biopsy or steroids.
Q6Dosing on day 9
Dale, day 9: creatinine 3.1 mg/dL and rising (1.0 → 2.3 → 2.8 → 3.1); printed eGFR 22; admission eGFR 86; Cockcroft–Gault about 29 mL/min. Enoxaparin 40 mg daily and gabapentin 600 mg three times daily are unchanged since admission; metformin is held; pharmacy doses the vancomycin from levels. Which order set is right?
Leave the doses alone: they were set from an eGFR of 86, and the creatinine should soon turn around
Re-dose everything to today's printed eGFR of 22, and restart metformin at half dose for his glucose control
Keep metformin held, reduce gabapentin and enoxaparin for a clearance under 30, leave vancomycin to pharmacy
Stop gabapentin and enoxaparin outright, and hold every renally cleared drug until the creatinine is 1.0
Show answer and explanation · Link to this question
Correct answer: C
Learning point: Neither creatinine estimate is reliable during changing kidney function. Review each medicine with pharmacy using its label, toxicity risk, current trend and drug levels where available; do not apply a universal lowest-estimate rule. This patient needs reduced gabapentin and renal-adjusted prophylaxis, with reassessment as function changes; metformin remains held. Why not the others: A uses obsolete clearance; B treats the printed estimate as exact and restarts metformin during evolving AKI; D indiscriminately stops necessary therapy. Severe gabapentin toxicity can justify a hold, while routine withdrawal should account for the label taper advice. Course reading: Evidence for this case
Days 10–15. The creatinine peaked at 3.2 mg/dL on day 10, then fell to 2.2 by day 13; the rash cleared by day 11 without steroids. On day 13 he had the CT angiogram — isotonic saline before and after, the lowest dose, metformin still held — and the creatinine kept falling, to 1.7 on day 15. The angiogram showed an occluded superficial femoral artery; angioplasty is planned in six weeks. He goes home on day 15 to finish the planned antibiotic course with pharmacy-directed frequent levels and creatinine checks while renal function is changing, spacing only after stability.
Q7The discharge plan
Dale, day 15: creatinine 1.7 mg/dL and falling (baseline 1.0; peak 3.2 on day 10); probable nephrotoxic tubular injury plus suspected piperacillin-tazobactam AIN; CT angiogram on day 13 without a creatinine rise; metformin held; gabapentin and enoxaparin renally adjusted. The discharge BMP prints eGFR 45. Which discharge plan is right?
Write "AKI, resolved"; restart every home medicine at its usual dose; leave the next labs to his routine diabetes visit
Write "contrast nephropathy"; list iodinated contrast as an allergy; check a single creatinine in three months' time
Write "CKD stage G3a" from today's eGFR of 45; order a low-protein renal diet; refer him for dialysis education
Document stage 3 AKI and suspected causes; record the reaction; set drug restart rules and early labs plus a 3-month kidney review
Show answer and explanation · Link to this question
Correct answer: D
Learning point: Document the AKI stage, incomplete recovery, probable tubular injury and suspected AIN, naming the suspected culprit and reaction rather than asserting a biopsy-proven or blanket class allergy. Arrange early labs during antibiotic treatment and a three-month recovery/CKD review. Restart held medicines only after individualized reassessment; metformin requires stable function and an eligible eGFR. Why not the others: A falsely calls recovery complete; B assigns causation to a later contrast exposure without supporting trajectory; C labels CKD from one nonsteady-state eGFR. Future beta-lactam choices need specialist review of the recorded reaction. Course reading: Evidence for this case
Q8If gentamicin is considered again
Weeks later a culture suggests an infection for which gentamicin may be needed. Dale has a prior nephrotoxic AKI. Which prescribing step best limits avoidable harm?
Use the usual fixed dose and check creatinine only after the antibiotic course
Confirm the indication and alternatives, then arrange kidney-function and drug-level monitoring with individualized dosing
Add routine furosemide to prevent gentamicin from accumulating in the kidney
Use once-daily dosing without monitoring because the longer interval eliminates nephrotoxicity
Show answer and explanation · Link to this question
Correct answer: B
Learning point: Aminoglycosides can injure the kidney and inner ear. Reassess whether the drug is necessary, avoid unnecessary nephrotoxic combinations, and have pharmacy individualize dosing and monitoring. Extended intervals do not eliminate toxicity and are not automatically appropriate during unstable AKI. Why not the others: A delays detection; C adds avoidable toxicity risk; D mistakes one dosing strategy for protection from all harm. Course reading: Evidence for this case
Resolution and Teaching Points
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7 · Q8
At one week Dale's creatinine was 1.4; at two weeks it was 1.2 on two checks (eGFR 69), so metformin restarted at its usual 1,000 mg twice daily. Gabapentin went back up in steps. The vancomycin finished at six weeks with no further injury. At three months: creatinine 1.1 mg/dL (eGFR 76, now at steady state), UACR 22 mg/g, BP 128/78 — resolved AKI, no CKD. His allergy list reads piperacillin-tazobactam — suspected acute interstitial nephritis, not biopsy-confirmed, and with diabetes and two kidney injuries behind him, his eGFR and UACR are checked yearly.
Clinical Pearl — Several days in, still making urine Vancomycin and aminoglycoside injury arrives after several days of therapy (classically days 5–7) and is usually non-oliguric. Duration, a high trough, a dry day and a second nephrotoxin amplify it. Hold the drug, check the level, let pharmacy and infectious disease redose or switch.
Clinical Pearl — Two lesions can share one creatinine The changing sediment suggests overlapping processes: tubular injury on day 6 and possible AIN on day 8. Do not wait for the triad or order urine eosinophils; stop the likeliest culprit.
Clinical Pearl — Contrast and the injured kidney AKI or eGFR below 30 calls for individualized IV-contrast planning. Delay when clinically acceptable; urgent benefit can outweigh risk. Use adequate diagnostic contrast, indicated saline prophylaxis and the applicable metformin policy. N-acetylcysteine is not recommended; bicarbonate has no clear advantage over saline.
Pitfall — The order set on autopilot Every drug written on day 0 was dosed for an eGFR of 86. The vancomycin got attention on day 6, but the metformin ran until day 8 and the gabapentin and enoxaparin until day 9. The admission eGFR goes stale the day the creatinine moves: re-dose the whole list, and de-escalate an antibiotic the cultures no longer justify — piperacillin-tazobactam had no target after day 4, and it is a suspected contributor to the later inflammatory findings.
Concept Links
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7 · Q8
Assigned reading: the explanations in this case and the question-linked course evidence guide. The objective map distinguishes tested decisions from concepts taught in explanations.
Concepts: aki-02, aki-06, aki-09, aki-10, aki-12, aki-25, aki-26, ckd-18, structural-15, structural-21, structural-23.
Source verification — 2026-10-12
Answers for this section: W1 · W2 · Q3 · Q4 · Q5 · Q6 · Q7 · Q8
The US enoxaparin label gives 30 mg subcutaneously once daily for medical VTE prophylaxis when CrCl is below 30 mL/min. This is distinct from treatment dosing. In unstable AKI, reassess bleeding risk and prophylaxis choice with pharmacy daily. The gabapentin label uses CrCl bands; 200–700 mg once daily applies to CrCl above 15 through 29, while the 15 mL/min row is 100–300 mg daily with proportional reduction below 15. Toxicity may require holding the drug; a cautious discharge dose is not a universal renal-dose band.