3.4 Therapeutic Drug Monitoring (TDM) and Clinical Application

3.4 Therapeutic Drug Monitoring (TDM) and Clinical Application

Therapeutic Drug Monitoring (TDM) is the clinical practice of measuring drug concentrations in biological matrices (typically serum or plasma) at designated intervals to optimize pharmacotherapy in individual patients. By integrating pharmacokinetic principles with patient-specific pathophysiological variables, the clinical pharmacist tailors dosing regimens to achieve maximum therapeutic efficacy while avoiding concentration-dependent toxicity.

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1. Principles and Criteria for Therapeutic Drug Monitoring

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|                            INDICATIONS FOR CLINICAL TDM                             |
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|  1. Narrow Therapeutic Index (NTI)  : Narrow margin between efficacy and toxicity   |
|  2. High PK Inter-Patient Variation : Unpredictable dose-concentration relationship |
|  3. Defined Target Range            : Direct correlation of level to clinical effect|
|  4. Lack of Direct Clinical Endpoint: Cannot titrate via BP, glucose, or INR        |
|  5. Suspected Toxicity / Noncompliance: Clarifying adverse events or lack of response|
|  6. Altered Pathophysiology         : Renal/hepatic failure, pregnancy, burns, sepsis|
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Rules for Blood Sample Collection Timing

• True Steady-State Sampling: Serum concentrations should generally be measured only after steady state has been attained ($\ge 4 \text{ to } 5 \text{ elimination half-lives}$ under a consistent dosing regimen), unless acute toxicity is suspected or early loading dose assessment is required.
• Trough Concentration ($C_{\text{min}}$): Collected immediately prior to the next scheduled dose (within $\le 30\text{ minutes}$ before dose administration). Troughs evaluate drug accumulation and clearance.
• Peak Concentration ($C_{\text{max}}$): Collected after the distribution phase is fully completed:
  • IV Aminoglycosides: $30\text{ minutes}$ after completion of a $30\text{-minute}$ IV infusion ($60\text{ minutes}$ post-infusion start).
  • IV Vancomycin: $1 \text{ to } 2\text{ hours}$ post-infusion completion (to avoid distribution-phase artifacts).
  • Oral Digoxin: $\ge 6 \text{ to } 8\text{ hours}$ post-dose (mandated by its extensive, slow multi-compartment tissue distribution).
  • Oral Lithium: $12\text{ hours}$ post-evening dose (standardized 12-hour trough).

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2. Aminoglycoside Antibiotics (Gentamicin, Tobramycin, Amikacin)

Aminoglycosides are bactericidal antibiotics that bind the bacterial 30S ribosomal subunit. They are renally eliminated via glomerular filtration ($CL_{\text{renal}} \approx GFR$) and distribute into extracellular body water ($V_d \approx 0.25 - 0.30\text{ L/kg}$ in non-critically ill adults; elevated to $0.4 - 0.5\text{ L/kg}$ in sepsis, ascites, or severe burns).

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|                 AMINOGLYCOSIDE PK/PD AND DOSING PARADIGMS                           |
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|  PK/PD Target: Concentration-Dependent Killing + Post-Antibiotic Effect (PAE)       |
|                Target Peak / MIC ratio >= 8 to 10                                   |
|                                                                                     |
|  Extended-Interval Dosing (Once-Daily):                                             |
|  - Dose: 5 to 7 mg/kg IV q24h (Gentamicin/Tobramycin); 15 mg/kg IV q24h (Amikacin)  |
|  - High Cmax -> Optimal bacterial killing; Prolonged Trough <0.5 mg/L -> Low tox   |
|  - Monitored via Hartford Nomogram (random level drawn 6-14 hours post-dose)        |
|                                                                                     |
|  Conventional Multi-Dose Dosing (q8h):                                              |
|  - Peak Target: 5 - 10 mg/L (Gram-negative sepsis/pneumonia); 3 - 5 mg/L (Synergy)  |
|  - Trough Target: < 1 - 2 mg/L (< 0.5 - 1.0 mg/L preferred)                         |
|                                                                                     |
|  Toxicities:                                                                        |
|  - Nephrotoxicity: Non-oliguric ATN; saturable proximal tubule uptake; reversible.  |
|  - Ototoxicity   : Vestibular/cochlear hair cell apoptosis; AUC-driven; irreversible|
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Hartford Nomogram for Extended-Interval Aminoglycoside Dosing

Following a $7\text{ mg/kg}$ IV infusion of gentamicin or tobramycin, a single serum concentration is measured between $6 \text{ and } 14\text{ hours}$ post-dose start. The concentration is plotted on the nomogram to determine the appropriate dosing interval:

• Below the $q24h$ line: Administer every $24\text{ hours}$.
• Between $q24h$ and $q36h$ lines: Administer every $36\text{ hours}$.
• Between $q36h$ and $q48h$ lines: Administer every $48\text{ hours}$.
• Above the $q48h$ line: Stop therapy, wait for concentration to fall $< 1\text{ mg/L}$, and monitor renal function.

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3. Vancomycin Glycopeptide Antibiotic

Vancomycin binds the D-Ala-D-Ala terminus of bacterial cell wall peptidoglycan precursors, inhibiting cell wall synthesis in Gram-positive organisms, notably methicillin-resistant Staphylococcus aureus (MRSA).

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|                     VANCOMYCIN TDM CONSENSUS GUIDELINES                             |
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|  Primary PK/PD Parameter: AUC24 / MIC ratio >= 400 to 600 mg*h/L                    |
|                           (assuming broth microdilution MIC <= 1 mg/L)              |
|                                                                                     |
|  Monitoring Methods:                                                                |
|  1. Bayesian PK Software Estimation (Preferred: based on 1 or 2 timed levels)       |
|  2. Two-Point PK Calculation: Peak (1-2h post-infusion) + Trough (pre-next dose)    |
|                                                                                     |
|  Old Paradigm (Trough-Only 15-20 mg/L) : Associated with 3-4x higher risk of AKI;   |
|                                          NO LONGER RECOMMENDED for serious MRSA.    |
|                                                                                     |
|  Toxicities:                                                                        |
|  - Nephrotoxicity : Acute tubular necrosis; risk spikes with troughs >15-20 mg/L    |
|                     and co-administration of piperacillin-tazobactam / aminoglycos. |
|  - Vancomycin Infusion Reaction ("Red Man"): Non-IgE histamine release from rapid   |
|                     infusion (>1 g/hr); managed by infusing over >=1.5-2 hours.     |
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Two-Point PK Determination of Vancomycin AUC

When Bayesian software is unavailable, two serum concentrations are drawn at steady state:

1. $C_{\text{peak}}$: Drawn $1 \text{ to } 2\text{ hours}$ after completion of the IV infusion.
2. $C_{\text{trough}}$: Drawn within $30\text{ minutes}$ prior to the next dose.

$\text{Elimination Rate Constant } (k_e) = \frac{\ln(C_{\text{peak}} / C_{\text{trough}})}{\Delta t}$

$\text{True Peak at End of Infusion } (C_{\text{max}}) = \frac{C_{\text{peak}}}{e^{-k_e \cdot t_{\text{wait}}}}$

$\text{True Trough } (C_{\text{min}}) = C_{\text{trough}} \cdot e^{-k_e \cdot t_{\text{rem}}}$

$\text{AUC}_{\text{infusion}} = \frac{\text{Dose}}{T_{\text{inf}} \cdot k_e} \cdot \left(\frac{1 - e^{-k_e \cdot T_{\text{inf}}}}{C_{\text{max}}}\right)$

$\text{Daily AUC}_{24} = \text{AUC}_{\text{interval}} \cdot \left(\frac{24}{\tau}\right)$

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4. TDM of Cardiovascular and Neurologic Agents