Therapeutic Drug Monitoring

TDM: Basics - Goal Getters

• Definition: Quantitative measurement of drug concentrations in biological fluids (e.g., plasma, serum) to refine dosage.
• Core Aims:
  • Optimize therapeutic efficacy.
  • Minimize drug toxicity.
  • Individualize dosing regimens.

⭐ Primary goal of TDM is to individualize dosage regimens to optimize efficacy and minimize toxicity.

• Essential for drugs with:
  • Narrow Therapeutic Index (NTI).
  • High pharmacokinetic variability.
  • Poorly defined clinical endpoints.
  • Suspected toxicity or non-compliance.

TDM: Indications - TDM Triggers

• 📌 TARGET N for TDM:
  • Therapeutic Index Narrow (e.g., digoxin, lithium)
  • Active Metabolites (significant & variable)
  • Route critical (e.g., erratic absorption)
  • Genetic variation (affecting metabolism)
  • End-organ damage risk (serious toxicity)
  • Therapeutic failure or suspected Toxicity
  • Non-linear pharmacokinetics (e.g., phenytoin)
• Poor dose-response correlation.
• Clinical endpoints unclear.

⭐ TDM is vital for narrow therapeutic index drugs where concentration clearly links to clinical effect/toxicity.

TDM: PK Principles - Number Ninjas

• Steady State ($C_{ss}$): Reached in 4-5 half-lives.
  • $C_{ss} = (Dose \times F) / (CL \times \tau)$ (Maintenance Dose)
• Half-life ($t_{1/2}$): Time for drug concentration to reduce by 50%.
  • $t_{1/2} = (0.693 \times V_d) / CL$
• Therapeutic Window: Safe & effective drug concentration range.
• Clearance (CL): Rate of drug elimination relative to plasma concentration.
• Volume of Distribution ($V_d$): Apparent volume drug occupies.
  • Loading Dose = $(Target \ C_{p} \times V_d) / F$

⭐ Steady state concentration ($C_{ss}$) is typically achieved after approximately 4 to 5 elimination half-lives of the drug.

TDM: Sampling & Interpretation - Sample Smarts

• Sample Matrix:
  • Serum/Plasma: Default choice.
  • Whole Blood: For drugs concentrated in RBCs (e.g., cyclosporine, tacrolimus).
  • Saliva: Non-invasive; correlates well for some (e.g., phenytoin, carbamazepine, lithium).
• Critical Interpretation:
  • Ensure steady-state (usually 3-5 $t_{1/2}$).
  • Verify patient adherence & correct sampling time.
  • Consider renal/hepatic function, co-medications.

⭐ For most drugs, trough concentrations (just before the next dose) are measured to assess risk of toxicity and ensure minimum effective concentration.

TDM: Key Drugs - The Usual Suspects

📌 'Digitalis LAVA' (P, C, V for Antiepileptics). Add Theophylline.

High‑Yield Points - ⚡ Biggest Takeaways

• TDM optimizes therapy by maintaining drug concentrations within the therapeutic window.
• Essential for drugs with a narrow therapeutic index (e.g., Digoxin, Lithium, Phenytoin).
• Samples are typically drawn at trough levels, just before the next dose, at steady state.
• Steady state is achieved after 4-5 drug half-lives; crucial for accurate interpretation.
• Always correlate TDM values with clinical status and consider drug interactions.