Managing Pharmacokinetic Dosing

1. Which drug requires PK dosing? (Default: vancomycin; alternatives: gentamicin, tobramycin, amikacin, phenytoin)
2. What are the patient demographics: age, sex, height, actual body weight? (Default: request)
3. What is the current renal function (SCr, CrCl by Cockcroft-Gault)? (Default: calculate)
4. Is this an initial dose calculation or a dose adjustment from measured levels? (Default: initial)
5. If adjusting, what are the measured serum drug levels with exact draw times and dose administration times? (Default: request)
6. What is the patient's fluid status (euvolemic, fluid overloaded, dehydrated)? (Default: euvolemic)
7. What is the infection site and suspected/confirmed pathogen with MIC? (Default: assume MRSA MIC 1 mcg/mL for vancomycin)
8. Is Bayesian PK software available (e.g., DoseMeRx, InsightRX, PrecisePK)? (Default: use first-order PK equations)

Documents to Request

• Patient demographics: age, sex, height, actual body weight
• Labs: SCr (ideally 2+ values for trend), BUN, albumin (for phenytoin binding)
• Serum drug levels with exact collection times (date/time)
• Medication administration times (exact dose, route, infusion duration)
• Fluid balance and hemodynamic status
• Dialysis status and schedule if applicable
• Concomitant nephrotoxins (amphotericin B, contrast, loop diuretics, piperacillin-tazobactam)

Step 1: Calculate Baseline Pharmacokinetic Parameters

Creatinine Clearance (Cockcroft-Gault): CrCl = [(140 - age) × weight(kg)] / [72 × SCr(mg/dL)] (× 0.85 for females)

Weight selection:

• Use actual body weight (ABW) if within 20% of ideal body weight (IBW)
• Use adjusted body weight if ABW > 120% IBW: AdjBW = IBW + 0.4 × (ABW - IBW)
• IBW (male) = 50 + 2.3 × (height in inches - 60)
• IBW (female) = 45.5 + 2.3 × (height in inches - 60)

Volume of distribution estimates:

Step 2: Drug-Specific Initial Dosing

Vancomycin (AUC/MIC-Guided per 2020 Guidelines)

Target: AUC₂₄ 400-600 mg·h/L (assuming MRSA MIC ≤ 1 mcg/mL)

Loading dose: 25-30 mg/kg ABW (for serious infections, round to nearest 250 mg, max 3 g)

Maintenance dosing estimation:

1. Estimate ke: ke = 0.00083 × CrCl + 0.0044
2. Estimate clearance: Cl = ke × Vd
3. Target AUC = 400-600: Dose/day = AUC_target × Cl
4. Divide daily dose into q8h, q12h, or q24h based on CrCl

Aminoglycosides (Extended-Interval Dosing)

Gentamicin/Tobramycin: 5-7 mg/kg AdjBW q24h (if CrCl ≥60 mL/min) Amikacin: 15-20 mg/kg AdjBW q24h (if CrCl ≥60 mL/min)

Use Hartford Nomogram: draw random level at 6-14 hours post-dose, plot on nomogram to determine interval (q24h, q36h, q48h).

Phenytoin

Loading dose: 15-20 mg/kg IV (max rate 50 mg/min, or 25 mg/min if cardiac risk) Maintenance: 4-6 mg/kg/day, adjusted by levels

Corrected phenytoin formulas:

• Hypoalbuminemia: Corrected level = Measured level / [(0.2 × albumin) + 0.1]
• Renal failure + hypoalbuminemia: Corrected level = Measured level / [(0.1 × albumin) + 0.1]

Step 3: Level Interpretation and Dose Adjustment

Vancomycin AUC Calculation (Two-Level Method)

1. Obtain trough (30 min pre-dose) and peak (1-2h post-infusion) at steady state (before 4th dose)
2. Calculate ke = ln(Cpeak/Ctrough) / Δt
3. Calculate t½ = 0.693 / ke
4. Calculate Vd = Dose / [Cpeak_extrapolated - Ctrough_extrapolated]
5. Calculate AUC₂₄ = Dose_daily / (ke × Vd)
6. Adjust dose proportionally: New dose = Current dose × (Target AUC / Calculated AUC)

Aminoglycoside Traditional Dosing Adjustment

1. Measure peak (30 min post-infusion) and trough (30 min pre-dose)
2. Target peak: Gentamicin 8-10 mcg/mL (serious infection), Amikacin 25-35 mcg/mL
3. Target trough: Gentamicin <1 mcg/mL (extended interval), Amikacin <5 mcg/mL
4. Adjust dose for peak, adjust interval for trough

Phenytoin (Michaelis-Menten Kinetics)

Phenytoin follows saturable metabolism. Small dose changes produce disproportionately large level changes.

• If at steady state with two different doses and their levels: apply Michaelis-Menten equation to solve for Vmax and Km
• Empiric adjustment: increase by no more than 30-50 mg/day when level is subtherapeutic
• Orbit graph method can be used as a clinical shortcut

Step 4: Special Population Adjustments

Checkpoint B: Post-Draft Alignment (Mandatory)

1. Were correct weight metrics used for each drug (ABW vs. AdjBW vs. IBW)?
2. Was CrCl calculated by Cockcroft-Gault (not CKD-EPI) and used for dosing?
3. Are serum level draw times verified as accurate (exact clock times)?
4. Has the AUC target (400-600) been used instead of trough-only for vancomycin?
5. Were special population adjustments applied where applicable?

Quality Audit

• Cockcroft-Gault CrCl calculated with appropriate weight metric
• Volume of distribution estimated using correct body weight
• Loading dose calculated for drugs requiring it (vancomycin, phenytoin)
• Maintenance dose and interval matched to renal function
• Vancomycin dosing targets AUC 400-600 mg·h/L per 2020 consensus guidelines
• Aminoglycoside interval determined by nomogram or level-based calculation
• Phenytoin level corrected for albumin and renal status before interpretation
• Serum level draw times documented and verified for accuracy
• Steady-state confirmed before interpreting levels (4-5 half-lives)
• Concomitant nephrotoxins identified and noted
• Dose rounding follows institutional convention (e.g., vancomycin to nearest 250 mg)
• Special population factors addressed (obesity, dialysis, critical illness)
• Follow-up monitoring plan specified with next level draw time
• All calculations shown with units for reproducibility

Guidelines

• Always verify serum level draw times with nursing documentation; incorrect times invalidate PK calculations
• Use Cockcroft-Gault for drug dosing (not CKD-EPI)—consistency with how dosing was studied in trials
• Vancomycin AUC/MIC-guided dosing is now standard of care; trough-only monitoring is outdated per 2020 guidelines
• Round vancomycin doses to the nearest 250 mg; do not exceed 4,500 mg/day without infectious disease consultation
• For phenytoin, always correct for albumin before making dose changes; free phenytoin level is gold standard
• Bayesian PK software (DoseMeRx, InsightRX) is preferred over manual calculations when available
• Document all PK calculations in the medical record with assumptions stated
• Reassess renal function daily in acutely ill patients; a stable CrCl today may change tomorrow