👤 By the CinnaRN Clinical Content Team🕐 Updated 2026-08-23🏷️ Pharmacological Therapies🔖 Free to read, print, and share
Also known as: drug levels · peak and trough · therapeutic range · TDM · med levels
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Use this quick-reference guide to spot, treat, and prevent Therapeutic Drug Levels on the NCLEX. Keep it handy during review and on exam day!
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📒 The 1-minute cheat sheet
📌 Cardiac
Digoxin 0.5-2.0 ng/mL
Dig toxic >2.0; check K+
📌 Psych
Lithium 0.6-1.2 mEq/L
Toxic >1.5 mEq/L
📌 Seizure
Phenytoin (Dilantin) 10-20 mcg/mL
Toxic → nystagmus, ataxia
📌 Other
Theophylline 10-20 mcg/mL
Vancomycin trough 10-20 mcg/mL
Acetaminophen <150 mcg/mL
📚 Therapeutic Drug Levels — full study notes
The cheat sheet above is your quick recall card. These notes go deeper — what it is, what to do first, what must be reported, and what to teach.
Therapeutic drug monitoring measures the blood concentration of medications with a narrow safety margin to keep them effective but not toxic. A trough is drawn just before the next dose to check the lowest level, and a peak is drawn after absorption to check the highest level. Drugs like digoxin, lithium, vancomycin, and phenytoin require close monitoring.
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Key points
Understand these first
A narrow therapeutic index means small changes in level can move from effective to toxic.
Trough levels are drawn immediately before the next scheduled dose.
Therapeutic digoxin is 0.5 to 2.0 ng/mL, and toxicity causes nausea, visual changes, and arrhythmias.
Therapeutic lithium is 0.6 to 1.2 mEq/L, and levels above 1.5 are toxic.
Vancomycin troughs guide dosing and protect against kidney and ear toxicity.
Therapeutic phenytoin is 10 to 20 mcg/mL, and toxicity causes nystagmus and ataxia.
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Nursing priorities
What to do, in order
Draw trough and peak levels at the correct times relative to the dose.
Verify the level is therapeutic before administering the next dose of a narrow-index drug.
Assess for signs of toxicity specific to the medication being monitored.
Hold the dose and notify the provider when a level is in the toxic range.
Monitor renal function for drugs cleared by the kidneys, such as digoxin and vancomycin.
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Red flags — report now
Escalate immediately
Digoxin toxicity with bradycardia or arrhythmia is an emergency; hold the drug and report it.
Lithium toxicity with confusion, tremor, or seizures requires holding the dose and urgent notification.
Never give the next dose of a monitored drug if the level is above the therapeutic range.
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Labs & values
Numbers to know
Digoxin 0.5-2.0 ng/mL
Lithium 0.6-1.2 mEq/L (toxic >1.5)
Phenytoin (Dilantin) 10-20 mcg/mL
Vancomycin trough 10-20 mcg/mL
Theophylline 10-20 mcg/mL
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Patient teaching
What patients must know
Take the medication exactly as prescribed and keep all scheduled lab draws.
Maintain consistent fluid and salt intake with lithium, and report tremor, vomiting, or confusion.
❓ Therapeutic Drug Levels: NCLEX FAQs
What are the priority nursing interventions for Therapeutic Drug Levels?
Draw trough and peak levels at the correct times relative to the dose. Verify the level is therapeutic before administering the next dose of a narrow-index drug. Assess for signs of toxicity specific to the medication being monitored. Hold the dose and notify the provider when a level is in the toxic range.
What are the warning signs of Therapeutic Drug Levels a nurse must report?
Digoxin toxicity with bradycardia or arrhythmia is an emergency; hold the drug and report it. Lithium toxicity with confusion, tremor, or seizures requires holding the dose and urgent notification. Never give the next dose of a monitored drug if the level is above the therapeutic range.
What do I need to know about Therapeutic Drug Levels for the NCLEX?
A narrow therapeutic index means small changes in level can move from effective to toxic. Trough levels are drawn immediately before the next scheduled dose. Therapeutic digoxin is 0.5 to 2.0 ng/mL, and toxicity causes nausea, visual changes, and arrhythmias. Therapeutic lithium is 0.6 to 1.2 mEq/L, and levels above 1.5 are toxic.
What patient teaching is important for Therapeutic Drug Levels?
Take the medication exactly as prescribed and keep all scheduled lab draws. Maintain consistent fluid and salt intake with lithium, and report tremor, vomiting, or confusion.
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Quick Tip
A narrow therapeutic index means small changes in level can move from effective to toxic.