Lab monitoring guidelines

TDM - Narrow Window Watch

Essential for drugs with a narrow therapeutic index to prevent toxicity while ensuring efficacy. Trough levels are typically measured just before the next dose.

📌 Warning! These Drugs Can Prove Lethal

• Warfarin
• Theophylline: 10-20 mcg/mL
• Digoxin: 0.5-2 ng/mL
• Carbamazepine: 4-12 mcg/mL
• Phenytoin: 10-20 mcg/mL
• Lithium: 0.6-1.2 mEq/L

• Others:
  • Vancomycin (trough): 15-20 mg/L
  • Aminoglycosides (trough): <2 mg/L

⭐ For Digoxin, draw the sample 6-8 hours after the last dose to ensure completion of the distribution phase. Toxicity is worsened by hypokalemia.

Chronic Disease - The Long Run

• Diabetes Mellitus:
  • HbA1c: Every 3-6 months.
  • Annually: Urine microalbumin, serum creatinine, K+, lipid profile, fundoscopy.
• Hypertension:
  • Serum K+ & Creatinine: Baseline, 1-2 weeks after starting/changing ACEi/ARB/diuretics, then annually if stable.
• Hypothyroidism (on Levothyroxine):
  • TSH: 6-8 weeks post-dose change, then every 6-12 months.
• Drug-Specific Monitoring:
  • Amiodarone: LFTs, TFTs at baseline, then every 6 months.
  • Lithium: Drug levels, TSH, creatinine, Ca²⁺ every 6-12 months.
  • Statins: LFTs at baseline & if symptoms arise. CK only if myalgia present.
  • Methotrexate: CBC, LFTs every 1-3 months.

⭐ Amiodarone can cause both hypothyroidism (more common) and hyperthyroidism. Thyroid function (TSH) must be checked at baseline and every 6 months.

Electrolyte Patrol - Ion Eyes

• Sodium (Na⁺): Correct slowly to prevent Osmotic Demyelination Syndrome (ODS). Max rate: 8-10 mEq/L/24h.
• Potassium (K⁺): IV correction requires cardiac monitoring. Max rate: 10-20 mEq/hr. Always co-check Magnesium (Mg²⁺), as hypomagnesemia impairs K⁺ correction.
• Calcium (Ca²⁺): Always check albumin. Corrected Ca²⁺ = Total Ca²⁺ + 0.8 * (4.0 - Albumin).
• Anion Gap (AG): $AG = Na⁺ - (Cl⁻ + HCO₃⁻)$. Normal: 8-12 mEq/L.
  • 📌 Mnemonic: MUDPILES for high AG metabolic acidosis.

⭐ In Refeeding Syndrome, the hallmark is severe hypophosphatemia (↓PO₄³⁻) from an intracellular shift driven by the insulin surge. Monitor phosphate, potassium, and magnesium closely when starting nutrition.

High‑Yield Points - ⚡ Biggest Takeaways

• Warfarin: Regularly monitor INR; the typical target range is 2.0-3.0.
• Amiodarone: Requires baseline and periodic TFTs, LFTs, and chest X-ray/PFTs.
• Lithium: Crucial to monitor serum levels, renal function, and thyroid function.
• Statins: Check LFTs at baseline; subsequent monitoring is symptom-driven.
• Digoxin: Closely watch serum levels, potassium, and renal function.
• Valproate: Monitor LFTs and platelet counts due to risk of toxicity.