Case · Presentation

Ten days after a sigmoid colectomy, the fever is getting worse.

Ms T. Johnson is a 62‑year‑old woman with type 2 diabetes (HbA_1c 9.2%), hypertension, hyperlipidemia and stage 4 chronic kidney disease. She underwent elective sigmoid colectomy 10 days ago for recurrent diverticulitis complicated by localized perforation, and was discharged on hospital day 3 after three days of intravenous piperacillin–tazobactam.

Two days after discharge the pain returned around the incision and became diffuse, with intermittent fevers, chills, poor appetite and nausea. The abdomen is distended and diffusely tender, maximal in the lower quadrants, with mild guarding, no rebound, and hypoactive bowel sounds. The incision is well‑healed with mild surrounding induration but no erythema or drainage.

Age and sex

62, female

Weight · BMI

78 kg · 29.4

Charlson index

≈5

Post‑operative day

10

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Case · Workup

CT finds a 6.8 cm abscess. The PCR names the enzymes.

Contrast‑enhanced CT shows a rim‑enhancing collection of 6.8 × 5.2 × 4.5 cm in the right paracolic gutter with small internal gas bubbles, remote from an intact anastomosis. Both aerobic bottles flagged positive at 7.8 and 8.2 hours; MALDI‑TOF identifies Klebsiella pneumoniae at 99.9% confidence.

Multiplex PCR on early growth from blood and abscess fluid detects bla_NDM and bla_CTX‑M, with no bla_KPC, bla_{OXA‑48‑like}, bla_VIM or bla_IMP. Susceptibility testing agrees: meropenem, ceftazidime–avibactam and aztreonam alone all resistant; aztreonam–avibactam susceptible at MIC 0.25 µg/mL.

WSES sepsis score

7 moderate–high

Creatinine clearance

28 mL/min

Procalcitonin

8.5 ng/mL

Lactate

3.2 mmol/L

Go to the decision

Case · Decision

Which regimen would you start?

Source control is achieved — 45 mL of purulent fluid drained under CT guidance on hospital day 1. Not all carbapenem resistance is the same.

• AAztreonam–avibactam plus metronidazoleLoad 1.8 g IV over 3 h, then 0.9 g IV q8h over 3 h · renal‑adjusted, CrCl 28
• BCeftazidime–avibactam plus metronidazole0.94 g IV q12h · renal‑adjusted, CrCl 28
• CMetronidazole plus cefiderocol1 g IV q8h over 3 h · renal‑adjusted, CrCl 15–29
• DMetronidazole plus colistin≈117 mg CBA q24–36h after a loading dose · renal‑adjusted, CrCl 15–29
• EMetronidazole plus meropenem–vaborbactam1 g/1 g IV q12h over 3 h · renal‑adjusted, CrCl 28
• FPolymyxin B≈58–98 mg IV q12h · standard dosing
• GMetronidazole plus sulbactam–durlobactam1 g/1 g IV q6h over 3 h · standard dosing

Lock in my answer Select a regimen to continue

Case · Verdict

Three are defensible. Four are not.

• AAztreonam–avibactam plus metronidazoleAcceptableAvibactam inhibits the CTX‑M ESBL while aztreonam resists MBL hydrolysis; MIC 0.25 µg/mL. ASSEMBLE: 67% clinical cure against 33% (Daikos GL, et al. 2025).
• BCeftazidime–avibactam plus metronidazoleAcceptableIDSA 2024 says it “may be used” but that aztreonam–avibactam is “preferred”; ceftazidime is inactive against an MBL producer (Tamma PD, et al. 2024).
• CMetronidazole plus cefiderocolAcceptableAn alternative where aztreonam–avibactam is unavailable. CREDIBLE‑CR all‑cause mortality 34% against 18% (Bassetti M, et al. 2021).
• DMetronidazole plus colistinNot appropriateSusceptible at MIC 0.5 µg/mL but last line: 44% mortality against 19%, and CrCl 28 makes nephrotoxicity a real risk (Falcone M, et al. 2021).
• EMetronidazole plus meropenem–vaborbactamNot appropriateVaborbactam does not inhibit metallo‑β‑lactamases, so it is ineffective against an NDM producer (Tamma PD, et al. 2024).
• FPolymyxin BNot appropriateRoughly 50% mortality against 22% for β‑lactam/β‑lactamase inhibitor combinations, with limited penetration into abscesses (Tsuji BT, et al. 2019).
• GMetronidazole plus sulbactam–durlobactamNot appropriateApproved for carbapenem‑resistant Acinetobacter baumannii; minimal activity against Enterobacterales (Xacduro FDA, 2023).

See what settles it

Case · Evidence

Aztreonam survives the metallo‑β‑lactamase. Avibactam covers the ESBL.

Aztreonam alone would fail against the CTX‑M ESBL; aztreonam–avibactam succeeds because avibactam inhibits the ESBL while aztreonam resists MBL hydrolysis. Duration is limited to seven days after source control, with clinical reassessment at 48–72 hours.

• 01Not all carbapenem resistance is the same. MBLs — NDM, VIM, IMP — hydrolyse all β‑lactams except monobactams and require aztreonam‑based combinations. Serine carbapenemases such as KPC and OXA‑48 respond to the newer β‑lactamase inhibitor combinations.
• 02Rapid diagnostics change outcomes. Rapid‑diagnostic‑guided therapy reduced 14‑day mortality from 37% to 16% in carbapenem‑resistant infections.
• 03Source control cannot be delayed. Even with optimal antimicrobial therapy, undrained collections will not resolve.
• 04Dose for this patient. CKD required every 8 hours rather than every 6; the 3‑hour infusion maximised time above MIC; the achieved AUC/MIC exceeded 2,000 against a target of 200.

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