9. Antimicrobial Dosing & TDM in Critical Illness

Quick Recap

Cross-cutting pharmacotherapy protocol — companion to Sepsis Bundles & Source Control and the revised Septic Shock protocol (Infectious Diseases System), where the 2026 SSC guideline's upgrade of prolonged beta-lactam infusion to a strong recommendation was noted. This protocol provides the underlying PK/PD rationale and trial evidence for that recommendation, plus broader antimicrobial dosing principles (vancomycin AUC-guided dosing, augmented renal clearance, renal replacement therapy adjustments) not covered in disease-specific protocols.

1. Definition

Pharmacokinetics (PK) in critical illness: critically ill patients have systematically altered drug absorption, distribution, metabolism, and elimination compared to non-critically-ill patients, driven by capillary leak, fluid resuscitation, hypoalbuminemia, altered cardiac output, and variable renal/hepatic function — meaning standard, licensed antibiotic doses (derived from healthy-volunteer or non-critically-ill patient PK studies) are frequently inappropriate for this population, in either direction (under- or over-dosing).

Augmented renal clearance (ARC): a state of supraphysiologic renal drug elimination, typically defined as measured creatinine clearance ≥130 mL/min/1.73m², common in younger, less comorbid critically ill patients with preserved or hyperdynamic cardiac output (e.g., trauma, sepsis without shock, post-neurosurgical patients) — the mirror-image problem to the more familiar concern of renal impairment, and frequently under-recognized because clinicians default to worrying about renal dose reduction rather than the possibility that standard dosing is already too low.

Time-dependent vs. concentration-dependent killing: beta-lactams are the classic example of time-dependent killing — efficacy correlates with the duration serum concentration remains above the pathogen's minimum inhibitory concentration (time above MIC, or fT>MIC), not peak concentration — this is the PK/PD rationale underlying prolonged/continuous infusion strategies (Section 11). Aminoglycosides and fluoroquinolones are concentration-dependent, where peak concentration relative to MIC (Cmax/MIC) drives efficacy, favoring high-dose, extended-interval dosing instead.

Therapeutic drug monitoring (TDM): measurement of serum drug concentrations to guide individualized dosing, most established and actionable in critical care for vancomycin (AUC-guided) and aminoglycosides, with emerging application to beta-lactams in specific circumstances.

2. Pathophysiology

Volume of distribution (Vd) expansion: capillary leak and aggressive fluid resuscitation increase the volume into which hydrophilic antibiotics (beta-lactams, aminoglycosides, vancomycin) distribute, diluting serum concentrations achieved by a standard dose — this is a major, frequently underappreciated driver of subtherapeutic dosing in the acute resuscitation phase specifically, independent of renal function.

Hypoalbuminemia: reduces protein binding for highly protein-bound antibiotics (e.g., ceftriaxone, ertapenem), increasing the free (active) drug fraction and altering clearance — relevant to selecting and dosing protein-bound agents in critically ill patients, who are frequently hypoalbuminemic.

Augmented renal clearance: driven by increased cardiac output and renal blood flow in the early, hyperdynamic phase of critical illness (particularly sepsis without shock, trauma, burns, post-neurosurgical states) — accelerates elimination of renally-cleared hydrophilic antibiotics (beta-lactams, vancomycin, aminoglycosides), risking subtherapeutic exposure with standard dosing even in patients with normal or elevated measured creatinine clearance who might otherwise seem "not at risk" for underdosing.

Renal replacement therapy (RRT): CRRT and intermittent hemodialysis both remove antibiotics to a degree dependent on the specific agent's molecular weight, protein binding, and Vd, as well as the RRT modality/dose itself — standard "renal dosing" tables designed for chronic kidney disease are frequently inadequate for the dynamic, high-clearance environment of modern CRRT, and dedicated RRT-specific dosing guidance should be used rather than extrapolating from CKD dosing.

3. Immediate Stabilization (ABCDE) — Dosing-Relevant Considerations

Not an acute stabilization scenario; this protocol's "stabilization" equivalent is ensuring the first dose of antimicrobial therapy is adequately dosed and delivered without unnecessary delay, since underdosing at initiation cannot be fully corrected by later TDM-guided adjustment in a rapidly evolving infection.

Checklist:

4. Focused History

• Renal function trend (not a single value) — acute changes matter more than a static number
• RRT modality and settings if applicable
• Fluid balance/resuscitation volume received (relevant to Vd expansion)
• Serum albumin (relevant to protein-bound agent dosing)
• Body habitus (obesity affects Vd and dosing weight selection for several agents)
• Known drug allergies/prior adverse reactions
• Concurrent nephrotoxic or interacting medications

5. Comprehensive System-wise Examination

Not a primary examination-driven protocol; relevant findings are indirect — volume status (fluid overload vs. depletion affecting Vd), signs of nephrotoxicity, and clinical response to therapy (source-specific signs of infection resolution or persistence) inform ongoing dosing decisions.

6. Syndrome Identification — Reframed as Dosing-Risk Classification

• Standard renal function, standard Vd: standard dosing likely adequate, though PK/PD principles (Section 11) should still guide infusion strategy for beta-lactams
• Augmented renal clearance: risk of subtherapeutic dosing with standard regimens; consider higher doses and/or TDM, particularly for vancomycin, beta-lactams, and aminoglycosides
• Renal impairment (not on RRT): standard renal dose-reduction principles apply, though these are more established than the ARC/Vd-expansion side of this spectrum
• On RRT (CRRT or intermittent HD): requires RRT-specific dosing guidance, not standard CKD dosing tables
• Significant capillary leak/large positive fluid balance: consider Vd expansion as a distinct contributor to subtherapeutic exposure, independent of renal clearance

7. Differential Diagnosis (of Apparent Treatment Failure)

Must-not-miss:

• Inadequate source control (cross-reference Sepsis Bundles & Source Control protocol) — no antibiotic dosing strategy compensates for an undrained source
• Resistant organism not covered by current empiric regimen
• Subtherapeutic dosing due to ARC, Vd expansion, or RRT under-dosing (this protocol's specific focus)

Common:

• Inadequate source control masquerading as "antibiotic failure"
• Line-associated or other new secondary infection

Iatrogenic:

• Standard/licensed dosing applied without adjustment for critical-illness-specific PK alterations
• TDM not performed or not acted upon when available

8. Severity/Risk Assessment

ARC risk factors: younger age, lower severity of illness (preserved/hyperdynamic physiology), trauma, burns, post-neurosurgical state, sepsis without overt shock, absence of significant comorbidity — counterintuitively, the "less sick-looking" ICU patient is often at higher ARC risk than the more critically unstable patient, an important clinical pattern to recognize since it runs against intuition.

Formal ARC assessment: measured (not estimated) creatinine clearance via timed urine collection is more reliable than estimated GFR equations (which were not validated in this population and can misclassify ARC patients).

9. Investigations

Immediate bedside: not applicable

Routine labs: renal function trend, serum albumin, measured creatinine clearance (timed urine collection) where ARC is suspected and consequential (e.g., narrow-therapeutic-index or critical agents)

Therapeutic drug monitoring:

• Vancomycin: AUC-guided dosing (target AUC24/MIC 400–600 mg·h/L) is now the guideline-preferred monitoring strategy over trough-only monitoring, using Bayesian estimation software where available for more accurate individualized PK modeling than simple trough-based nomograms
• Aminoglycosides: peak and trough monitoring for extended-interval dosing regimens
• Beta-lactams: TDM is emerging but not yet universally available/standardized; most valuable in specific high-risk scenarios (ARC, RRT, obesity, treatment failure) rather than routine use for all patients

Repeat frequency: vancomycin TDM should be obtained early (first 24–48 hours) in ARC patients specifically, since ARC patients are more likely to have a higher AUC discrepancy between initial dosing design and follow-up TDM than non-ARC patients — early reassessment matters more in this subgroup, not less.

10. Point-of-Care Ultrasound

Not applicable to this protocol.

11. Evidence-Based Management

Beta-Lactams — Prolonged/Continuous Infusion: The Evidence Base Behind the 2026 SSC Upgrade

• PK/PD rationale: beta-lactams exhibit time-dependent killing — efficacy correlates with time above MIC (fT>MIC), not peak concentration, making prolonged or continuous infusion (which maximizes time above MIC for a given total daily dose) mechanistically rational, particularly in critically ill patients with expanded Vd and/or ARC who are most likely to fail to sustain adequate concentrations with standard intermittent bolus dosing
• BLING III trial (Dulhunty et al., JAMA 2024, n=7,202, 104 ICUs, 7 countries): continuous vs. intermittent infusion of piperacillin-tazobactam or meropenem in critically ill sepsis patients — 90-day mortality 24.9% (continuous) vs. 26.8% (intermittent), odds ratio 0.91 (95% CI 0.81–1.01, p=0.08) — did not reach statistical significance on the primary outcome, but the confidence interval is consistent with both no effect and a clinically important benefit; more clinical cures were observed with continuous infusion (a secondary outcome that did show benefit)
• Pooled meta-analysis incorporating BLING III and prior trials (including MERCY): pooled risk ratio for 90-day mortality with prolonged/continuous vs. intermittent infusion 0.86 (95% credible interval 0.72–0.98, high certainty) — this pooled estimate does reach statistical significance, and is the evidentiary basis most directly supporting the 2026 SSC guideline's upgrade of this recommendation to strong (cross-reference Septic Shock and Sepsis Bundles protocols)
• Practical synthesis: while BLING III itself was individually non-significant on its primary mortality endpoint, the totality of evidence (BLING III + prior trials, pooled) supports prolonged/continuous beta-lactam infusion as beneficial with high-certainty pooled evidence — this is a case where interpreting a single large trial's non-significant primary result in isolation would understate the actual weight of evidence; the pooled analysis is the more informative summary
• Practical implementation: always give an initial loading dose regardless of infusion strategy (a bolus loading dose ensures early adequate concentration before steady-state continuous infusion is achieved), then transition to prolonged (3–4 hour) or continuous (24-hour) infusion of the maintenance dose

Vancomycin — AUC-Guided Dosing

• Current guideline consensus (ASHP/IDSA/PIDS/SIDP 2020) favors AUC-guided dosing (target AUC24/MIC 400–600 mg·h/L, using Bayesian estimation) over traditional trough-only monitoring, reflecting better correlation with both efficacy and nephrotoxicity avoidance
• Augmented renal clearance substantially affects vancomycin exposure: ARC patients require meaningfully higher doses to achieve target concentrations — studies report needing daily doses in the range of ~45–46 mg/kg/day (vs. ~35–36 mg/kg/day in non-ARC patients) to achieve equivalent target trough concentrations, and a majority of ARC patients remain subtherapeutic on standard weight-based dosing without early TDM-guided adjustment
• ARC patients are also more likely to show a rising AUC discrepancy between initial dose design and follow-up TDM (i.e., initial dosing underestimates true exposure needs even more than expected) — reinforcing the value of early, not delayed, TDM in this specific subgroup

Augmented Renal Clearance — General Principles

• Consider ARC specifically in younger, less comorbid, hemodynamically preserved/hyperdynamic critically ill patients — do not assume normal or high creatinine clearance means "no dosing concern"; it may mean the opposite for renally-cleared, hydrophilic agents
• Measured (timed urine) creatinine clearance is more reliable than estimated GFR equations for ARC identification
• Consider higher-than-standard doses and/or early TDM for vancomycin, beta-lactams (via prolonged/continuous infusion plus dose escalation where PK/PD targets are not met), and aminoglycosides in confirmed or suspected ARC

Renal Replacement Therapy Dosing

• Use dedicated CRRT/RRT-specific dosing references rather than extrapolating from standard CKD dosing tables — CRRT clearance is dynamic and dependent on modality, effluent rate, and filter characteristics in ways that static CKD-stage dosing does not capture
• Reassess dosing with any change in RRT modality, dose, or filter

Novel Beta-Lactams and Beta-Lactam/Beta-Lactamase Inhibitor Combinations (MDR Pathogen Coverage)

• Agents such as ceftolozane/tazobactam, ceftazidime/avibactam, cefiderocol, ceftobiprole, imipenem/relebactam, and meropenem/vaborbactam are increasingly used against multidrug-resistant pathogens
• PK/PD target attainment for these novel agents is minimally affected by obesity or ECMO, but more substantially affected by renal function status (either ARC or RRT) — the same renal-function-driven dosing principles above apply to these newer agents, not just legacy beta-lactams

12. Organ Support

Interacts directly with renal replacement therapy (Section 11 RRT dosing) and with overall fluid balance/resuscitation strategy (Vd expansion, cross-reference Vasopressor & Inotrope Selection & Titration and Nutrition Support protocols for the broader fluid-balance context this dosing consideration sits within).

13. Disease-Specific Therapy — Practical Dosing Principles Summary

• Beta-lactams (piperacillin-tazobactam, meropenem): loading dose, then prolonged (3–4h) or continuous (24h) infusion of maintenance dose in sepsis/septic shock
• Vancomycin: loading dose 25–30 mg/kg, then AUC-guided maintenance dosing (target AUC24/MIC 400–600); consider higher initial doses and early (24–48h) TDM in suspected ARC
• Aminoglycosides: extended-interval, high-dose regimens guided by peak/trough monitoring, leveraging concentration-dependent killing

14. Consultation Matrix

15. Monitoring Framework

• Clinical: response to therapy (fever curve, hemodynamic trajectory, inflammatory marker trend, source-specific clinical signs)
• Laboratory: renal function trend, vancomycin AUC-guided TDM per Section 9, aminoglycoside peak/trough as applicable
• Escalation triggers: apparent treatment failure despite adequate source control → consider subtherapeutic dosing (ARC, Vd expansion, inadequate RRT dosing) before assuming resistant organism or escalating empiric breadth alone

16. ICU Bundle Checklist

17. Complications

Early:

• Subtherapeutic exposure → treatment failure, resistance selection pressure
• Supratherapeutic exposure → nephrotoxicity (particularly vancomycin, aminoglycosides), other agent-specific toxicity

Late:

• Antimicrobial resistance development from prolonged subtherapeutic exposure
• Prolonged infection course/ICU stay from inadequately dosed initial therapy

Prevention: loading doses, PK/PD-informed infusion strategy, early recognition and dose adjustment for ARC, appropriate RRT-specific dosing

Rescue: early TDM-guided dose correction; infectious disease/pharmacy consultation for complex dosing scenarios or apparent treatment failure

18. Escalation & De-escalation

Escalation: apparent treatment failure → reassess dosing adequacy (ARC, Vd, RRT) before broadening empiric coverage reflexively; obtain TDM if not already available.

De-escalation: narrow antimicrobial spectrum per culture data and clinical response (cross-reference Sepsis Bundles & Source Control protocol's antimicrobial stewardship principles); reassess dosing strategy as renal function/hemodynamics evolve over the course of illness (ARC in the early hyperdynamic phase may resolve as illness progresses or resolves, requiring dose reassessment in either direction).

19. ICU Discharge Criteria (Dosing-Relevant Context)

Cross-reference ICU Discharge Criteria & Step-Down protocol. Dosing-specific consideration: any ICU-specific dosing strategy (continuous infusion, ARC-adjusted high-dose vancomycin) explicitly communicated at transfer, since receiving ward teams may default to standard dosing/administration practices without explicit handoff of the rationale for a non-standard regimen.

20. Documentation & Medicolegal Checklist

• Renal function assessment (including ARC consideration where relevant) documented at antimicrobial initiation
• Loading dose administration documented
• Infusion strategy (prolonged/continuous vs. intermittent) and rationale documented
• TDM results and dose adjustments documented with rationale
• RRT modality/settings and corresponding dosing rationale documented
• Handoff of any non-standard dosing strategy at care transitions documented

21. Key Guidelines

• ASHP/IDSA/PIDS/SIDP 2020 vancomycin guideline: current consensus favoring AUC-guided over trough-only monitoring
• 2026 Surviving Sepsis Campaign guideline (cross-reference Septic Shock and Sepsis Bundles & Source Control protocols): strong recommendation for prolonged/continuous beta-lactam infusion after loading dose

22. Landmark Trials

23. Controversies

• BLING III's individually non-significant primary outcome vs. the pooled evidence supporting benefit: this is a genuine interpretive nuance worth holding onto — a single large, well-conducted trial being individually non-significant does not mean the underlying hypothesis is false, particularly when the pooled evidence across multiple trials does reach significance; clinicians should be comfortable with the idea that guideline bodies (2026 SSC) can reasonably upgrade a recommendation based on the totality of evidence even when the most recent, largest individual trial was not itself definitively positive.
• Optimal AUC target and TDM accessibility: AUC-guided vancomycin dosing is the current preferred approach, but Bayesian estimation software and the workflow to support it are not universally available across institutions — trough-based approximations remain common in practice despite being a less precise proxy.
• Beta-lactam TDM: unlike vancomycin, routine beta-lactam TDM is not yet standardized or universally available; current practice relies more on PK/PD-informed dosing strategy (loading dose + prolonged/continuous infusion) than on individualized level monitoring for most patients, reserving TDM for specific high-risk or treatment-failure scenarios.
• ARC identification in routine practice: measured (timed urine) creatinine clearance is more accurate than estimated GFR for ARC identification, but is more resource- and time-intensive; many institutions rely on estimated GFR or clinical risk-factor pattern recognition alone, potentially under-identifying true ARC.

24. References

1. Dulhunty JM, Brett SJ, De Waele JJ, et al; BLING III Investigators. Continuous vs intermittent β-lactam antibiotic infusions in critically ill patients with sepsis: the BLING III randomized clinical trial. JAMA. 2024;332(8):629-637.
2. Monti G, Bradic N, Marzaroli M, et al. Continuous vs intermittent meropenem administration in critically ill patients with sepsis: the MERCY randomized clinical trial. JAMA. 2023;330(2):141-151.
3. Rhodes NJ, Liu J, O'Donnell JN, et al. Prolonged infusion piperacillin-tazobactam decreases mortality and improves outcomes in severely ill patients: results of a systematic review and meta-analysis. Crit Care Med. 2018;46(2):236-243.
4. Rybak MJ, Le J, Lodise TP, et al. Therapeutic monitoring of vancomycin for serious methicillin-resistant Staphylococcus aureus infections: a revised consensus guideline (ASHP/IDSA/PIDS/SIDP). Am J Health Syst Pharm. 2020;77(11):835-864.
5. He N, et al. A higher dose of vancomycin is needed in critically ill patients with augmented renal clearance. Transl Androl Urol. 2020.
6. Hobbs AL, Shea KM, Roberts KM, Daley MJ. Implications of augmented renal clearance on drug dosing in critically ill patients: a focus on antibiotics. Pharmacotherapy. 2015;35(11):1063-1075.
7. Prescott HC, Antonelli M, Alhazzani W, et al. Surviving Sepsis Campaign: international guidelines for management of sepsis and septic shock 2026. Intensive Care Med. 2026.
8. The Washington Manual of Critical Care, 4th ed. 2025 — antimicrobial pharmacology chapters.
9. ICU Protocols: A Step-wise Approach, 2nd ed. — antimicrobial therapy content.