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Therapeutic drug levels cheat sheet

Drug level questions ask for three things: the therapeutic range, the toxic signs, and the right move.

Timing matters too: a trough is drawn right before the next dose, and a level drawn at the wrong time isn't a real level.

✔ Reviewed by a licensed RN· Updated Oct 2026· Sources

DrugTherapeutic / toxicToxic sign and move
Digoxin0.5-2.0 ng/mL · toxic > 2.0
  • Halos, nausea, brady, confusion
  • Hold apical < 60; low K+ potentiates
  • Antidote digoxin immune Fab
Lithium0.6-1.2 mEq/L · toxic > 1.5
  • Coarse tremor, vomiting, ataxia
  • Keep sodium and fluids steady
  • Draw 12 h after last dose
Phenytoin10-20 mcg/mL · toxic > 20
  • Nystagmus, ataxia, slurred speech
  • IV max 50 mg/min in NS only
Carbamazepine4-12 mcg/mL · toxic > 12
  • Agranulocytosis: monitor CBC
  • Hyponatremia, SJS risk
Valproic acid50-100 mcg/mL · toxic > 100
  • Hepatotoxicity, pancreatitis
  • Thrombocytopenia, teratogenic
Theophylline10-15 (max 20) mcg/mL · toxic > 20
  • Nausea, tachycardia, tremor, seizures
  • Smoking lowers; cipro raises
GentamicinPeak 5-10 · trough < 2 mcg/mL
  • Nephro- and ototoxic
  • Trough right before dose
VancomycinAUC or trough per pharmacy protocol
  • Infuse over >= 60 min (flushing reaction)
  • Nephro/ototoxic: watch creatinine
AcetaminophenMax 4 g/day; less if older, liver dz, alcohol
  • Toxic > 150 mcg/mL at 4 h
  • Antidote N-acetylcysteine
Warfarin (INR)INR 2-3 (2.5-3.5 mech valve)
  • Antidote vitamin K
  • Keep vitamin K intake consistent
Heparin (aPTT)1.5-2.5 x control (lab-specific)
  • Antidote protamine
  • Platelets drop >= 50%: suspect HIT
Magnesium sulfate4-7 mEq/L · toxic > 8
  • DTRs go first, then RR < 12
  • Antidote calcium gluconate

RN pearl: Trough = right before the dose, peak = 30-60 min after the IV ends; a level drawn at the wrong time is not a level.

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Frequently asked questions

What is the therapeutic range for digoxin?

0.5 to 2.0 ng/mL. Toxicity shows up as nausea, visual halos, bradycardia and confusion, and low potassium makes it more likely.

What is a toxic lithium level?

Above 1.5 mEq/L. The therapeutic range is 0.6 to 1.2 mEq/L. Watch for coarse tremor, vomiting and ataxia.

When is a trough level drawn?

Right before the next dose. A peak is drawn 30 to 60 minutes after an IV dose finishes.

Keep studying

⚡ Speed MatchSort drugs, lab values and electrolyte signsAntidotes cheat sheetPharmacologyLab values cheat sheetLab values

Sources

  1. Pharmacology for Nurses, 17.6 Unclassified Antidysrhythmics, OpenStax
  2. Pharmacology for Nurses, 13.3 Mood Stabilizers, OpenStax
  3. Phenytoin Sodium Injection: prescribing information, DailyMed, U.S. National Library of Medicine
  4. Theophylline Extended-Release: prescribing information, DailyMed, U.S. National Library of Medicine

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