Nursing care
Therapeutic Drug Levels: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
A therapeutic drug level tells you whether the dose in the patient's system is high enough to work and low enough to stay safe. The number only means something if it was drawn at the right time relative to the dose — a trough drawn early, or a peak drawn late, gives a result that looks normal but is not.
Why this value is ordered
Drugs with a narrow therapeutic index don't give you much room between a dose that works and a dose that harms. Vancomycin, gentamicin, digoxin, lithium, phenytoin and theophylline are the ones you'll draw levels for most often, because the gap between an ineffective dose and a toxic one is small and the consequences of missing it — ototoxicity, arrhythmia, seizure, renal injury — are serious.
The order exists to answer one of two questions: is the patient actually absorbing and clearing this drug the way we expect, or has something changed. Renal function shifts, a new interacting drug, a missed dose, poor adherence — all of these show up in the level before they show up in symptoms. That's the value of ordering it in the first place.
Interpreting the number in context
A level is only as good as its timing. Peak levels are drawn to catch the drug at its highest concentration, usually 30 to 60 minutes after an IV infusion ends or 1 to 2 hours after an oral dose, and they tell you whether the dose is enough to be effective. Trough levels are drawn immediately before the next scheduled dose, at the drug's lowest point, and they tell you whether the drug is clearing fast enough to avoid accumulation.
Draw a trough two hours early and it will read higher than the true trough, making an adequate dose look toxic. Draw it late and a genuinely accumulating level can read as safe. Neither error is caught by the number itself — it looks plausible either way. That's why the draw time goes on the chart with the level, and why a level without a documented time relative to the dose is not a value you can act on.
Critical values and what to do
A critical or panic-range level triggers a hold, not an automatic report-and-move-on. Hold the next dose, notify the provider with the result and the draw time, and reassess the patient for signs that match the specific drug — tinnitus and confusion for digoxin, nystagmus and ataxia for phenytoin, tremor and GI upset progressing to confusion for lithium, hearing changes for the aminoglycosides.
Don't give the scheduled dose while you wait for the provider unless directed to. A critical trough usually means the current regimen is accumulating, and giving another dose on top of it compounds the problem. Document the level, the time it was drawn relative to the dose, your assessment findings, and the provider notification — that sequence is what protects the patient and the record if the dose is held or adjusted.
Related tests read alongside it
Drug levels rarely stand alone. Renal function — BUN, creatinine, eGFR — drives clearance for most of the drugs you'll monitor this way, so a rising creatinine explains a rising trough before you assume the dose itself is wrong. For digoxin, check serum potassium and magnesium alongside the level, because hypokalemia increases digoxin toxicity risk even at a level that looks therapeutic.
For phenytoin, check serum albumin: a low albumin means more free, active drug even when the total level reads normal, which is why some labs report a corrected level. For lithium, thyroid and renal panels matter over the longer term, since lithium affects both. Reading the level next to these values is what turns a single number into a clinical picture.
Nursing implications
Know the therapeutic range for the drug you're monitoring before you draw the sample, not after the result comes back. That's what lets you catch a mistimed draw before it goes to the lab, and what lets you flag a result that doesn't fit the clinical picture instead of charting it and moving on.
Coordinate the draw with pharmacy and the medication administration record. If a dose is late or held, the next trough won't reflect the intended dosing interval, and that needs to be noted alongside the result. Build the habit of writing the draw time and its relation to the last dose directly next to the level whenever you document it.
What patients ask about it
Patients on long-term therapy — lithium, phenytoin, warfarin monitoring by a related logic — often ask why they need blood drawn so often when they feel fine. Explain that toxicity from these drugs can build gradually and doesn't always announce itself with symptoms until it's advanced, which is exactly why the monitoring exists.
They also ask why the timing of the blood draw matters so much, especially if a nurse has ever pushed back their draw for a busy unit. It's worth explaining plainly: a level drawn at the wrong time can look fine when the dose isn't, or look dangerous when it isn't, and either error changes what the care team decides to do next.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our pharmacology practice questions are the closest set to what this page covers.
One question from the pharmacology set
A client with heart failure is started on furosemide 40 mg PO daily. Which findings should the nurse report to the provider before administering the next dose? Select all that apply.
Rationale
Furosemide is a loop diuretic, so the two things you are watching are potassium and kidney function. A potassium of 2.9 mEq/L is below the 3.5–5.0 reference range and puts the client at risk for dysrhythmia — hold and report. Muscle cramps with palpitations are the clinical face of that same hypokalemia, so they are reported together, not separately. A creatinine that doubles signals the diuresis has outrun renal perfusion. A blood pressure of 132/78 and a 1 kg loss are the expected response to the drug working, not reasons to hold it.
Answer: A, D, E
Common questions
What's the difference between a peak and a trough level?
A peak is drawn at the drug's highest concentration, shortly after the dose is given, and shows whether the dose is high enough to be effective. A trough is drawn right before the next dose, at the lowest point, and shows whether the drug is clearing before the next dose adds to it.
What happens if a trough is drawn too early?
An early trough is drawn before the drug has finished clearing, so it reads higher than the true trough. That falsely elevated result can lead to a dose being held or reduced when the actual regimen was appropriate.
Why is digoxin level monitored alongside potassium?
Hypokalemia sensitizes the heart to digoxin's effects, so a patient can show signs of digoxin toxicity even when the digoxin level itself is within the therapeutic range. Both values need to be checked together, not separately.
Do you give the scheduled dose while waiting for a critical drug level result?
Generally no — hold the dose, notify the provider, and wait for direction unless your facility's protocol says otherwise. Giving another dose on top of a potentially toxic level risks compounding the accumulation.
Which drugs commonly require therapeutic level monitoring?
Vancomycin, gentamicin and other aminoglycosides, digoxin, lithium, phenytoin and theophylline are the ones nurses monitor most often because their therapeutic and toxic ranges sit close together.
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