PK/PD principles explain why some susceptible organisms fail to respond to apparently appropriate antibiotic therapy, why some resistant organisms can still be treated with higher doses or extended infusions, and why personalised dosing (therapeutic drug monitoring) improves outcomes in critically ill patients. The framework of PK/PD target attainment is increasingly used to define clinical breakpoints, set dosing recommendations, and design optimal antibiotic regimens.
The Three PK/PD Parameters for Antibiotics
All antibiotics are classified by one of three PK/PD parameters that best predict their efficacy against an organism:
Time above MIC (fT>MIC) is the free (unbound) drug concentration remaining above the MIC for a defined percentage of the dosing interval. This parameter applies to antibiotics whose bactericidal activity depends on the duration of exposure to concentrations above the MIC, not on how high the peak concentration is. Examples: beta-lactam antibiotics (penicillins, cephalosporins, carbapenems) and aztreonam.
For beta-lactams, maintaining fT>MIC at 40 to 50 per cent of the dosing interval is generally sufficient for bacteriostatic effect. Achieving fT>MIC at 60 to 70 per cent of the dosing interval is targeted for bactericidal effect in serious infections. This is why extended infusion (infusing over 3 to 4 hours rather than 30 minutes) of beta-lactams is increasingly used in critically ill patients with organisms at higher MICs: it extends the time the free drug concentration remains above MIC, improving PK/PD target attainment without increasing total dose.
AUC/MIC ratio (area under the concentration-time curve divided by MIC) applies to antibiotics with concentration-independent killing that nonetheless benefits from sustained exposure. Examples: vancomycin, fluoroquinolones (also Cmax/MIC dependent to some degree), azithromycin, tetracyclines.
For vancomycin, the AUC/MIC target for serious S. aureus infections is 400 to 600 mgΒ·h/L (when MIC is 1 mg/L, this requires an AUC of 400 to 600 mgΒ·h/L, typically achievable with 24-hour trough-based or continuous infusion dosing). Current ASHP/IDSA/SIDP guidelines recommend AUC-guided monitoring over trough-guided monitoring for vancomycin, as AUC monitoring better predicts both efficacy and nephrotoxicity risk.
Cmax/MIC ratio (peak concentration divided by MIC) applies to antibiotics with concentration-dependent killing, where higher peak concentrations produce greater and faster bacterial killing. Examples: aminoglycosides (gentamicin, amikacin, tobramycin), daptomycin (partially), metronidazole.
For gentamicin in gram-negative sepsis, a Cmax/MIC ratio of at least 8 to 12 is targeted. This is why extended-interval aminoglycoside dosing (once daily, at higher peak doses) has largely replaced conventional three-times-daily dosing: once-daily dosing achieves a much higher Cmax (higher Cmax/MIC) while the long trough period below detectable levels reduces nephrotoxicity and ototoxicity risk.
The Volume of Distribution: Why Dose Doesn't Equal Tissue Concentration
Volume of distribution (Vd) describes how widely a drug distributes throughout the body relative to the amount administered. A drug with a small Vd (for example, 0.1 to 0.3 L/kg) remains largely in the bloodstream (plasma and extracellular fluid). A drug with a large Vd (for example, 5 to 20 L/kg) distributes extensively into tissues.
Vancomycin has a Vd of approximately 0.5 to 0.7 L/kg in normal healthy adults. In critically ill patients with sepsis, inflammation causes massive capillary leak, increasing the Vd to 1.0 to 2.0 L/kg or more. This means a standard vancomycin dose produces much lower serum concentrations in a critically ill septic patient than in a healthy volunteer, and AUC-guided dosing with early TDM is essential.
Fluoroquinolones (ciprofloxacin, levofloxacin, moxifloxacin) have large Vd values (2 to 5 L/kg) and penetrate well into most tissues, including lung parenchyma, bone, and prostate, where some other antibiotics achieve poor concentrations.
Beta-lactams generally have smaller Vd values and moderate tissue penetration. Exceptions: certain beta-lactams achieve high urinary concentrations making them effective for UTI even when serum MIC targets are not met.
Protein Binding: The Free Fraction That Matters
Only unbound (free) drug exerts pharmacological and antibacterial effects. Drug bound to plasma proteins (primarily albumin and alpha-1-acid glycoprotein) is pharmacologically inactive. Protein binding affects the effective free drug concentration in plasma and tissues.
Ceftriaxone is approximately 95 per cent protein-bound: only 5 per cent of the plasma concentration is free drug. This is clinically relevant when considering its use in organisms where the protein-adjusted free drug concentration falls below the MIC. Albumin levels significantly affect the free fraction: in hypoalbuminaemia (common in critically ill patients), a higher proportion of ceftriaxone is free, partially compensating for the reduced total concentration.
Vancomycin is approximately 50 per cent protein-bound. Unbound (free) vancomycin is the pharmacologically relevant fraction for AUC/MIC calculations.
Renal Clearance and Dose Adjustment
Most antibiotics are cleared renally, either by glomerular filtration or tubular secretion. In patients with reduced kidney function (as measured by eGFR or creatinine clearance), renally cleared antibiotics accumulate unless doses are reduced or dosing intervals are extended.
The importance of dose adjustment: aminoglycosides in renal impairment accumulate to toxic concentrations causing nephrotoxicity and ototoxicity. Vancomycin accumulates, increasing AUC and nephrotoxicity risk. Beta-lactams at very high concentrations due to accumulation in severe renal failure can cause neurotoxicity (penicillin encephalopathy, cefepime-associated neurotoxicity).
Calculating dose adjustment: creatinine clearance (CrCl) by Cockcroft-Gault formula or eGFR by CKD-EPI equation is used to estimate kidney function. Dosing guidelines specify adjusted doses for different degrees of renal impairment (mild, moderate, severe, end-stage renal disease, and renal replacement therapy). In patients on haemodialysis or continuous renal replacement therapy (CRRT), additional dosing guidance is required as dialysis partially removes many antibiotics.
Frequently Asked Questions
What is pharmacokinetics?
Pharmacokinetics (PK) is the study of what the body does to a drug: how it is absorbed, distributed to tissues, metabolised, and eliminated. Key PK parameters for antibiotics include volume of distribution (Vd), half-life (t1/2), protein binding, and clearance.
What is pharmacodynamics?
Pharmacodynamics (PD) is the study of what the drug does to the microorganism: the concentration-effect relationship. For antibiotics, PD describes how concentration above the MIC relates to bactericidal or bacteriostatic effect. Key PD parameters are time above MIC (fT>MIC), AUC/MIC ratio, and Cmax/MIC ratio.
What is the PK/PD parameter for beta-lactams?
Beta-lactams are time-dependent antibiotics: their killing effect depends on how long the free drug concentration remains above the MIC, not on how high the peak concentration is. The relevant parameter is fT>MIC (free drug time above MIC). Target attainment of 40 to 50 per cent of the dosing interval above MIC is bacteriostatic; 60 to 70 per cent is bactericidal.
Why are aminoglycosides given once daily?
Aminoglycosides are concentration-dependent antibiotics: their killing rate increases with increasing peak concentration relative to MIC (Cmax/MIC ratio). Once-daily dosing achieves a much higher peak concentration (higher Cmax/MIC) than three-times-daily dosing of the same total daily dose. The prolonged trough period (when concentrations are undetectable) also reduces the risk of nephrotoxicity and ototoxicity, which are related to sustained trough concentrations rather than peak concentrations.
What is therapeutic drug monitoring (TDM)?
TDM is the measurement of drug concentrations in plasma samples taken at defined points in the dosing cycle (trough, peak, or both) to guide dose individualisation. For vancomycin, TDM is used to calculate AUC/MIC and adjust doses to achieve the target 400 to 600 mgΒ·h/L. For aminoglycosides, TDM identifies peak concentrations (efficacy) and trough concentrations (toxicity risk).
What is extended infusion of beta-lactams?
Extended infusion administers beta-lactam antibiotics over 3 to 4 hours (or continuously) rather than the standard 30-minute infusion. By prolonging the infusion time, free drug concentration remains above the MIC for a larger proportion of the dosing interval, improving fT>MIC target attainment. This is particularly valuable for organisms with higher MICs (at or near the susceptibility breakpoint) and for critically ill patients where drug distribution is altered.
How does renal impairment affect antibiotic dosing?
Renally cleared antibiotics accumulate in patients with reduced kidney function. Without dose adjustment, accumulation leads to toxicity (aminoglycoside nephrotoxicity, vancomycin nephrotoxicity, penicillin neurotoxicity) or supra-therapeutic concentrations. Dose reductions or extended intervals are specified in prescribing guidelines for each level of renal impairment. In critically ill patients, renal function can change rapidly and regular reassessment is needed.
What is the significance of protein binding for antibiotic dosing?
Only the free (unbound) fraction of a drug crosses from plasma to tissues and exerts antibacterial effects. In hypoalbuminaemia (common in critically ill patients), drugs with high protein binding have a higher free fraction than expected from total drug concentration. This can affect both efficacy and toxicity assessments if total drug concentration (rather than free drug concentration) is monitored.
What is the post-antibiotic effect (PAE)?
The post-antibiotic effect is the suppression of bacterial growth that persists after antibiotic concentrations fall below the MIC. PAE is significant for aminoglycosides (long PAE of 2 to 7 hours against gram-negative rods) and fluoroquinolones. A long PAE allows extended dosing intervals without the same risk of regrowth that would occur with antibiotics that have no or minimal PAE (such as beta-lactams, which have minimal PAE against gram-negative bacteria).
What is the mutant prevention concentration (MPC)?
The MPC is the minimum drug concentration that prevents selection of first-step resistant mutants. It is the MIC for the least susceptible organism in a large population of susceptible organisms. Antibiotic concentrations between the MIC and MPC (the "mutant selection window") are sufficient to kill susceptible organisms but not single-step mutants, selecting for resistance. Dosing strategies that maximise time above the MPC minimise resistance selection during therapy.