Therapeutic ranges
| Drug | Commonly cited range | Signs of toxicity |
|---|---|---|
| Digoxin | 0.5 to 2 ng/mL (about 0.5 to 0.9 ng/mL in heart failure) | Anorexia, nausea, vision changes, bradycardia, dysrhythmias |
| Lithium | 0.6 to 1.2 mEq/L (maintenance) | Vomiting, diarrhea, coarse tremor, ataxia, confusion, seizures |
| Phenytoin | 10 to 20 mcg/mL (total) | Nystagmus, ataxia, slurred speech, confusion |
| Valproic acid | 50 to 100 mcg/mL (epilepsy) | Sedation, confusion, high ammonia |
| Carbamazepine | 4 to 12 mcg/mL | Dizziness, ataxia, double vision, nausea |
| Phenobarbital | 15 to 40 mcg/mL | Sedation, nystagmus, respiratory depression |
| Theophylline | Commonly 5 to 15 mcg/mL (older references 10 to 20) | Nausea, tachycardia, restlessness, seizures |
| Gentamicin and tobramycin (traditional dosing) | Trough below 2 mcg/mL; peak depends on the infection | Rising creatinine, hearing loss, vertigo |
| Vancomycin | AUC-guided dosing (often an AUC of 400 to 600 mg·h/L); trough-based targets in some protocols | Rising creatinine, hearing changes |
| Magnesium (preeclampsia seizure prevention) | About 4.8 to 8.4 mg/dL (4 to 7 mEq/L) | Loss of reflexes, respiratory rate below 12, low urine output |
Coagulation targets
| Drug | Test | Common target |
|---|---|---|
| Warfarin | INR | 2 to 3 for most indications; 2.5 to 3.5 for some mechanical valves |
| IV heparin | aPTT or anti-Xa | aPTT about 1.5 to 2.5 times control, or the anti-Xa range in the protocol |
Peaks and troughs
- A trough is the lowest level, drawn just before the next dose. A peak is the highest level, drawn at a set time after the dose (often 30 minutes after an IV infusion ends for aminoglycosides).
- Lithium levels are drawn about 12 hours after the last dose.
- Label the sample with the exact time and record when the last dose was given: a level drawn at the wrong time cannot be interpreted.
- Hold the dose and notify the prescriber when a level is above range or the patient shows signs of toxicity, per protocol.
Frequently asked questions
What is the therapeutic range for digoxin?
Commonly 0.5 to 2 ng/mL, with lower targets (about 0.5 to 0.9 ng/mL) often used in heart failure. Low potassium increases the risk of toxicity even within range.
What is a therapeutic lithium level?
For maintenance, commonly 0.6 to 1.2 mEq/L, drawn about 12 hours after the last dose.
When is a trough level drawn?
Just before the next dose, when the drug level is at its lowest.