Therapeutic Drug Levels

Pharmacology Basics

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Therapeutic drug monitoring (TDM) keeps serum levels in the safe, effective window. Essential for drugs with a NARROW therapeutic index.

Therapeutic Ranges

Digoxin

Therapeutic: 0.5–2 ng/mLToxic: >2 ng/mL

Toxicity signs: Nausea, vomiting, visual halos (yellow-green), bradycardia, arrhythmias

Lithium

Therapeutic: 0.6–1.2 mEq/LToxic: >1.5 mEq/L

Toxicity signs: Tremor, confusion, ataxia, slurred speech, seizures

Phenytoin (Dilantin)

Therapeutic: 10–20 mcg/mLToxic: >20 mcg/mL

Toxicity signs: Nystagmus, ataxia, slurred speech, gingival hyperplasia (chronic)

Valproic acid

Therapeutic: 50–100 mcg/mLToxic: >100 mcg/mL

Toxicity signs: Sedation, tremor, hepatotoxicity, thrombocytopenia

Carbamazepine

Therapeutic: 4–12 mcg/mLToxic: >12 mcg/mL

Toxicity signs: Drowsiness, ataxia, blood dyscrasias

Theophylline

Therapeutic: 10–20 mcg/mLToxic: >20 mcg/mL

Toxicity signs: Tachycardia, nausea, restlessness, seizures

Vancomycin (trough)

Therapeutic: 10–20 mcg/mLToxic: >20 mcg/mL

Toxicity signs: Nephrotoxicity, ototoxicity, 'red man syndrome' (rate-related)

Gentamicin / Tobramycin

Therapeutic: Peak 5–10, Trough <2 mcg/mLToxic: Peak >12, Trough >2

Toxicity signs: Nephrotoxicity, ototoxicity (irreversible)

Warfarin (INR)

Therapeutic: INR 2–3 (2.5–3.5 mech valve)Toxic: INR >4

Toxicity signs: Bleeding (gums, urine, stool, bruising)

Heparin (aPTT)

Therapeutic: 1.5–2.5× control (60–80 sec)Toxic: >2.5× control

Toxicity signs: Bleeding, HIT (heparin-induced thrombocytopenia)

Acetaminophen (4 hr level)

Therapeutic: <150 mcg/mL at 4 hrToxic: >150 mcg/mL at 4 hr

Toxicity signs: Initial: asymptomatic; 24–72 hr: hepatotoxicity (↑ AST/ALT)

Peak vs Trough

Peak

Drawn when serum concentration is HIGHEST (usually 30 min after IV infusion ends, or 1 hr after IM/oral). Tells you about TOXICITY risk.

Trough

Drawn JUST BEFORE the next scheduled dose. Tells you whether the level stays therapeutic between doses.

Key Pearls

Always document the EXACT time the dose was given AND the time the level was drawn
Hold the next dose until the level is reviewed (per facility policy)
Steady state is reached after ~4–5 half-lives — check levels then
Notify the provider promptly for sub- or supratherapeutic levels
INR is the standard for warfarin; PT/aPTT for heparin

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Disclaimer: PharmPal Nurse is for education only and is not medical advice. Do not use it to diagnose, treat, prescribe, or make patient-care decisions; always verify with current drug references, your instructor, facility policy, and a licensed provider. In emergencies, call local emergency services.