16.4 Antimicrobial Pharmacokinetics, Allergy, Toxicity & Infection Prevention
Key Takeaways
Beta-lactams are time-dependent; the 2026 Surviving Sepsis Campaign recommends prolonged (extended or continuous) infusion for maintenance dosing after a loading dose.
Augmented renal clearance (creatinine clearance above about 130 mL/min) occurs in young trauma, burn, and sepsis patients and causes beta-lactam underdosing at standard doses.
Vancomycin dosing now targets an AUC/MIC of 400 to 600 rather than a trough of 15 to 20 mg/L, which reduces nephrotoxicity.
A PEN-FAST score of 0 to 2 indicates a very low risk of true penicillin allergy and supports direct oral challenge in appropriate settings.
Cefepime causes nonconvulsive status epilepticus and myoclonus when dosed without adjustment in kidney impairment, and carbapenems reduce valproate levels sharply.
Why Standard Doses Fail in Critical Illness
| Change in critical illness | Consequence |
|---|---|
| Capillary leak and aggressive fluid resuscitation | Expanded volume of distribution for hydrophilic drugs (beta-lactams, aminoglycosides, vancomycin): subtherapeutic peaks unless a full loading dose is given |
| Hypoalbuminemia | Higher free fraction of highly bound drugs (ceftriaxone, ertapenem, teicoplanin) with faster clearance |
| Augmented renal clearance (creatinine clearance above about 130 mL/min) | Seen in younger trauma, burn, neurocritical and sepsis patients: standard doses undershoot |
| Acute kidney injury and kidney replacement therapy | Accumulation if doses are not reduced; conversely, CRRT clears many antimicrobials substantially |
| ECMO circuits | Sequestration of lipophilic and highly protein-bound drugs (fentanyl, propofol, voriconazole) in the oxygenator and tubing |
| Altered gut absorption and interrupted feeds | Unreliable enteral absorption; use intravenous routes in shock |
The rule: never reduce the loading dose for organ dysfunction, because it is determined by the volume of distribution; adjust the maintenance dose for clearance.
Pharmacodynamic Targets
| Class | Killing pattern | Optimization |
|---|---|---|
| Beta-lactams | Time-dependent (time above MIC) | Extended (3 to 4 hour) or continuous infusion after a loading dose; higher frequency. The BLING III trial found no mortality difference but a favorable secondary signal, and meta-analysis supports continuous infusion; the 2026 SSC recommends prolonged infusion |
| Aminoglycosides | Concentration-dependent (peak/MIC) | High once-daily dosing (gentamicin 5 to 7 mg/kg, amikacin 15 to 20 mg/kg); monitor trough for toxicity |
| Vancomycin | AUC-dependent | AUC/MIC of 400 to 600, using two-level or Bayesian estimation; the old trough target of 15 to 20 mg/L caused excess nephrotoxicity |
| Fluoroquinolones | Concentration-dependent (AUC/MIC) | Adequate single daily dose |
| Daptomycin | Concentration-dependent | 6 to 10 mg/kg daily; inactivated by pulmonary surfactant, so never use it for pneumonia; monitor creatine kinase |
| Linezolid | AUC-dependent | Good lung penetration; causes thrombocytopenia, serotonin syndrome risk with serotonergic drugs, and lactic acidosis with prolonged use |
| Polymyxins (colistin) | Concentration-dependent | Loading dose required; nephrotoxic and neurotoxic; poor lung penetration |
Drug-Specific Toxicity Traps
- Cefepime neurotoxicity: myoclonus, encephalopathy, and nonconvulsive status epilepticus, usually from unadjusted dosing in kidney impairment. Stop the drug; dialysis helps in severe cases.
- Vancomycin plus piperacillin-tazobactam: associated with a higher incidence of acute kidney injury (at least by creatinine-based definitions) than vancomycin with cefepime or meropenem.
- Aminoglycosides: nephrotoxicity (nonoliguric, usually reversible) and irreversible ototoxicity and vestibulotoxicity; avoid in myasthenia gravis.
- Carbapenem plus valproate: carbapenems drop valproate levels dramatically and can precipitate seizures; choose another antibiotic or another antiepileptic.
- Linezolid plus serotonergic drugs: serotonin syndrome risk, particularly with SSRIs.
- Fluoroquinolones: QT prolongation, tendinopathy, aortic aneurysm association, glucose dysregulation, and delirium.
- Trimethoprim-sulfamethoxazole: hyperkalemia (ENaC blockade), a creatinine rise from tubular secretion blockade without true injury, myelosuppression, and hypersensitivity.
- Amphotericin B deoxycholate: infusion reactions, nephrotoxicity, and potassium and magnesium wasting; liposomal formulations are less nephrotoxic.
- Voriconazole: visual disturbance, hepatotoxicity, encephalopathy, and extensive CYP interactions, which demand therapeutic drug monitoring.
Antibiotic Allergy Assessment
Up to 10% of hospitalized patients carry a penicillin allergy label, but fewer than 5% of those are truly allergic. The label drives use of broader, less effective, more toxic alternatives.
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PEN-FAST scores the history:
- PEN: penicillin allergy reported.
- F: five years or less since the reaction (2 points).
- A: anaphylaxis or angioedema, or S: severe cutaneous adverse reaction (2 points).
- T: treatment required for the reaction (1 point).
A score of 0 to 2 indicates a very low risk of a positive penicillin allergy test and supports direct oral challenge in an appropriate setting.
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Cross-reactivity: the shared R1 side chain, not the beta-lactam ring, drives most cross-reactivity. Cephalosporins with dissimilar side chains (cefazolin in particular) can usually be given to patients with a penicillin allergy history. Carbapenem cross-reactivity is well under 1%.
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Never rechallenge after anaphylaxis to that same drug class without specialist input, or after severe cutaneous adverse reactions: Stevens-Johnson syndrome, toxic epidermal necrolysis, DRESS, or acute generalized exanthematous pustulosis.
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Desensitization is an option when the drug is essential (for example, in neurosyphilis in pregnancy); it induces temporary tolerance only.
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Vancomycin infusion reaction (flushing of the upper body) is a direct mast cell effect, not an allergy. Slow the infusion and give an antihistamine.
Resistance Patterns Worth Memorizing
- MRSA: vancomycin, daptomycin (not for pneumonia), linezolid, ceftaroline.
- VRE: linezolid or daptomycin (high dose).
- ESBL-producing Enterobacterales: carbapenems are preferred over piperacillin-tazobactam for invasive infection (MERINO trial).
- AmpC inducers (Enterobacter, Serratia, Citrobacter, Providencia, Morganella): avoid third-generation cephalosporins; use cefepime or a carbapenem.
- Carbapenem-resistant Enterobacterales: ceftazidime-avibactam or meropenem-vaborbactam for KPC; cefiderocol or ceftazidime-avibactam plus aztreonam for metallo-beta-lactamase producers.
- Difficult-to-treat Pseudomonas: ceftolozane-tazobactam, ceftazidime-avibactam, imipenem-relebactam, or cefiderocol.
- Acinetobacter baumannii: ampicillin-sulbactam at high dose, sulbactam-durlobactam, cefiderocol, polymyxins.
- Stenotrophomonas maltophilia: trimethoprim-sulfamethoxazole, minocycline, levofloxacin, cefiderocol.
- Fungi: Candida glabrata and C. krusei have reduced azole susceptibility (an echinocandin is first-line for candidemia); Candida auris is multidrug-resistant and demands contact isolation; Aspergillus terreus is amphotericin-resistant; mucorales are voriconazole-resistant.
Stewardship and Infection Prevention
- Stewardship basics: obtain cultures before antibiotics, start broad in septic shock then de-escalate on day 2 or 3, use the shortest effective duration, and set a stop date at the time of ordering. Procalcitonin plus clinical assessment can shorten duration (the 2026 SSC supports this use) but should not delay initial therapy.
- Short courses with evidence: 7 days for ventilator-associated pneumonia (most cases) and for gram-negative bacteremia (BALANCE and earlier trials), 4 days after source control in intra-abdominal infection (STOP-IT), and 5 to 7 days for community-acquired pneumonia with clinical stability.
- Prevention bundles:
- Hand hygiene remains the single highest-impact intervention.
- Central line insertion bundle: hand hygiene, chlorhexidine skin preparation, full barrier precautions, avoiding the femoral site when possible, ultrasound guidance, and daily review of necessity.
- Chlorhexidine bathing reduces bloodstream infection in ICU patients.
- Ventilator bundle: head elevation, sedation interruption with daily extubation assessment, oral care, and subglottic secretion drainage.
- Catheter-associated urinary tract infection: avoid placement, remove early, and do not treat asymptomatic bacteriuria.
- Selective digestive decontamination reduced mortality in units with low antimicrobial resistance, and the 2026 SSC suggests it in that setting.
- Isolation: contact precautions for resistant organisms and C. difficile (with soap and water rather than alcohol gel), droplet for influenza and plague, and airborne for tuberculosis, measles, varicella, and smallpox.
A 27-year-old man with polytrauma and Pseudomonas aeruginosa ventilator-associated pneumonia is receiving cefepime 2 g every 8 hours as a 30-minute infusion. His measured creatinine clearance is 185 mL/min, and he remains febrile with persistent positive cultures at a susceptible MIC. Which change is most likely to improve bacterial killing?
Switch to a 30-minute infusion of cefepime 1 g every 6 hours to maintain the same total daily dose
Add once-daily gentamicin for synergy and discontinue the cefepime to avoid further resistance
Give a loading dose then administer cefepime as an extended or continuous infusion with a higher total daily dose
Switch to daptomycin at 10 mg/kg daily, which achieves better pulmonary penetration than beta-lactams
A 64-year-old woman needs cefazolin for methicillin-sensitive Staphylococcus aureus bacteremia. She reports a penicillin "allergy" at age 12 consisting of a non-urticarial rash that resolved without treatment, more than 40 years ago, and she has no history of anaphylaxis or severe skin reaction. What is the most appropriate approach?
Proceed with cefazolin, because her PEN-FAST risk is very low and cefazolin has a dissimilar side chain
Use vancomycin for the entire treatment course to avoid any beta-lactam exposure
Perform a formal 12-step intravenous desensitization protocol before giving cefazolin
Use daptomycin because cephalosporin cross-reactivity with penicillin exceeds 20%
A 71-year-old man with an eGFR of 22 mL/min/1.73 m² receives cefepime 2 g every 8 hours for a urinary source of sepsis. On day 3 he develops myoclonus and fluctuating consciousness; EEG shows generalized periodic discharges without a clinical seizure, and head imaging is unremarkable. What is the most likely explanation and appropriate action?
Hepatic encephalopathy, managed with lactulose and rifaximin
Cefepime neurotoxicity from inadequate renal dose adjustment, managed by stopping the drug
Nonconvulsive status epilepticus from an unrecognized stroke, managed with levetiracetam alone
Serotonin syndrome from the antibiotic, managed with cyproheptadine
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