Therapeutic Drug Levels: Ranges, Timing and Toxicity
What it is
Narrow therapeutic index drugs have a small gap between the effective and the toxic serum concentration. A small change in dose, kidney function, fluid status, electrolytes or an interacting drug pushes the level into the toxic range. The nurse draws levels at the correct time, recognizes toxicity before the result returns, and holds the dose and reports when toxicity is suspected. A trough is drawn just before the next dose; a peak is drawn after absorption and distribution are complete.
What the NCLEX focuses on
- Digoxin: hypokalemia, hypomagnesemia and hypercalcemia cause toxicity even at a therapeutic level; draw the level at least 6 to 8 hours after the last dose.
- Lithium: draw the level 12 hours after the last dose; dehydration, low sodium intake, vomiting, diarrhea, heavy sweating, thiazide diuretics, NSAIDs and ACE inhibitors raise the level.
- Lithium: fine hand tremor, mild thirst and polyuria are expected; vomiting, diarrhea, coarse tremor, ataxia, slurred speech and confusion signal toxicity.
- Phenytoin: nystagmus is the earliest toxicity sign, followed by ataxia, slurred speech and confusion; gingival hyperplasia is an adverse effect, not toxicity.
- Phenytoin is highly protein bound; a low albumin raises the free drug and causes toxicity at a normal total level.
- Vancomycin: draw the trough within 30 minutes before the next dose; infuse over at least 60 minutes and monitor creatinine and hearing.
Priority nursing actions
- Assess for toxicity before every dose: apical pulse and nausea for digoxin, tremor and GI symptoms for lithium, nystagmus and gait for phenytoin.
- Hold the dose when toxicity signs are present or the level is above range.
- Notify the prescriber and obtain the serum level and related labs (potassium, sodium, creatinine, albumin) as ordered.
- Draw each level at the correct time and record the time of the last dose on the requisition.
- Maintain safety: cardiac monitoring for digoxin toxicity, fall precautions for ataxia, seizure precautions for antiepileptic changes.
Numbers to know
| Drug | Therapeutic range | Toxicity signs |
|---|---|---|
| Digoxin | 0.5 to 2.0 ng/mL (0.6 to 2.6 nmol/L) | Nausea, anorexia, visual halos, bradycardia, new dysrhythmias |
| Lithium | 0.6 to 1.2 mEq/L (0.6 to 1.2 mmol/L) | Above 1.5 mEq/L: vomiting, diarrhea, coarse tremor, ataxia, confusion |
| Phenytoin | 10 to 20 mcg/mL (40 to 79 micromol/L) | Nystagmus, ataxia, slurred speech, confusion |
| Carbamazepine | 4 to 12 mcg/mL (17 to 51 micromol/L) | Ataxia, diplopia, drowsiness |
| Valproic acid | 50 to 100 mcg/mL (347 to 693 micromol/L) | Drowsiness, confusion; check ammonia and liver enzymes |
| Vancomycin trough | 10 to 20 mcg/mL | Rising creatinine, tinnitus, hearing loss |
Common exam traps
- Choosing to give the next lithium dose and then draw a level for a client with vomiting and a coarse tremor: wrong because these are toxicity signs; hold the dose first.
- Choosing to draw a vancomycin trough right after the infusion ends: wrong because that sample reflects the peak; the trough is drawn just before the next dose.
- Choosing to restrict sodium or fluids for a client on lithium: wrong because low sodium and dehydration increase renal lithium reabsorption and cause toxicity.
- Choosing to accept a normal digoxin level as safe when the potassium is 3.0 mEq/L: wrong because hypokalemia causes toxicity at a therapeutic level.
- Choosing to report gum overgrowth on phenytoin as toxicity: wrong because it is an expected adverse effect managed with oral hygiene; nystagmus and ataxia signal toxicity.
What to teach
- Keep every lab appointment; on the day of a lithium level, take the morning dose after the blood draw.
- Lithium: keep sodium intake steady, drink 2 to 3 L of fluid daily, and report vomiting, diarrhea or heavy sweating.
- Check with the prescriber before taking NSAIDs, herbal products or any new medication.
- Report early toxicity signs: nausea, visual changes, unsteady walking, new tremor or confusion.
Client handout: Taking Lithium Safely (nurse guide and printable patient handout).
Try 2 NCLEX-style questions on this topic
From the NCLEXPace question bank. Pick your answers, then check them inside NCLEXPace.
A nurse is assessing a client stabilized on lithium. For each finding, indicate whether it is an expected mild effect or a sign of lithium toxicity requiring the provider to be notified.
For each finding in a client taking lithium, indicate whether it is an expected mild effect or a sign of lithium toxicity requiring the provider to be notified.
| Expected mild effect | Sign of lithium toxicity | |
|---|---|---|
| Mild hand tremor and occasional thirst | ||
| Mild nausea during the first days of therapy | ||
| Severe vomiting, diarrhea, and slurred speech | ||
| Coarse tremor, ataxia, and confusion |
A client is receiving intravenous vancomycin every 12 hours. A trough level is ordered. When should the nurse draw the blood sample?
Select the best answer.
- A. Halfway between two scheduled vancomycin doses
- B. Just before the next scheduled dose
- C. At any convenient time during the shift as long as the exact collection time is documented
- D. Immediately after the vancomycin infusion finishes
The correct answers, the reasoning for every option and 3,200+ more NCLEX questions are inside NCLEXPace. Create your account, then choose a plan to open them.
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- Burchum JR, Rosenthal LD. Lehne's Pharmacology for Nursing Care. 11th ed. Elsevier; 2022.
- Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Heart Failure.
- Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Chronic Neurologic Problems.
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