Nursing care
Therapeutic Ranges: reading the number and acting on it
Written and reviewed by Dana Whitfield, RN, MSN · 7 min read · Updated September 2026
Short answer
A therapeutic range is the blood concentration window at which a drug is expected to work without becoming toxic; a level outside it means the dose, timing, or the patient's own clearance needs review, not automatic withholding. Digoxin, lithium, phenytoin, theophylline, and vancomycin account for most of the level questions a nurse will interpret in practice and on exam.
Why this value is ordered
A drug level is ordered when the gap between an effective dose and a toxic dose is narrow, when the drug's effect cannot be judged reliably from symptoms alone, or when a change in the patient, renal function declining, a new interacting drug started, a dose adjustment made, means the old level no longer applies. These are called narrow therapeutic index drugs, and the label matters because it changes how closely the level needs to be followed.
Digoxin, lithium, phenytoin, theophylline, and vancomycin are ordered for slightly different reasons but share this narrow margin. Digoxin and lithium are ordered because toxicity looks similar to under-treatment early on, so the level is the only reliable way to tell them apart. Phenytoin and theophylline are ordered because seizure or bronchospasm control depends on staying inside a specific window. Vancomycin is ordered because it is both the drug used to treat serious infection and a drug that damages kidneys at high sustained levels, so the level protects the treatment and the patient at the same time.
The timing of the draw is part of the order, not a separate decision. A trough drawn just before the next dose, or a peak drawn at a specified interval after infusion, is only meaningful if it is drawn at the correct point in the dosing cycle. A level drawn at the wrong time cannot be interpreted against the reference range at all, and re-ordering it wastes both the result and the patient's blood draw.
Interpreting the number in context
A number inside the therapeutic range is reassuring but is not the whole picture. Digoxin's therapeutic range is generally cited around 0.5 to 2.0 ng/mL, but a patient with hypokalemia can show signs of digoxin toxicity, nausea, visual disturbance, arrhythmia, at a level that sits within this range, because low potassium increases myocardial sensitivity to digoxin independent of the drug concentration itself.
Lithium's therapeutic range, commonly 0.6 to 1.2 mEq/L for maintenance, is highly sensitive to sodium and fluid status. A patient who becomes dehydrated, from vomiting, diarrhoea, or reduced intake, can develop lithium toxicity without any change in dose, because the kidneys reabsorb lithium alongside sodium and a fluid deficit concentrates it. The level has to be read alongside the patient's hydration, not in isolation.
Phenytoin's range, generally 10 to 20 mcg/mL total level, needs adjustment for albumin. A patient with low albumin can have a falsely low total phenytoin level while the free, active fraction is still therapeutic or even high, so a low result in a hypoalbuminemic patient does not automatically mean the dose needs increasing. Theophylline's narrow range, roughly 10 to 20 mcg/mL, is affected by smoking status and by interacting drugs that alter liver metabolism, so a level taken without knowing recent changes to either can mislead.
Critical values and what to do
A critical or panic value is reported outside the normal laboratory pathway specifically because it demands immediate action, and the nurse receiving it is expected to notify the provider without delay and document the time of notification. For digoxin, a level above roughly 2.0 to 2.4 ng/mL combined with symptoms, bradycardia, visual halos, gastrointestinal upset, warrants holding the next dose and reporting before it is given.
For vancomycin, a trough above roughly 20 mcg/mL, or the higher AUC-based target now used in many facilities, raises the risk of nephrotoxicity and warrants review of the dosing interval alongside a current creatinine, since impaired renal clearance is the most common reason a trough climbs unexpectedly. For lithium, a level above roughly 1.5 mEq/L is treated as early toxicity and above 2.0 mEq/L as severe, with the response including holding doses, correcting fluid status, and in severe cases involving nephrology for possible dialysis.
In every case the nurse's first action is the same regardless of which of the five drugs is involved: confirm the result against the correct draw time, assess the patient for signs consistent with toxicity rather than treating the number alone, and report the critical value promptly with the assessment findings attached, not the number in isolation.
Related tests read alongside it
A drug level is rarely interpreted alone. Digoxin levels are read alongside potassium, magnesium, and renal function, since digoxin is renally cleared and electrolyte imbalance changes toxicity risk independent of the level itself. Lithium levels are read alongside sodium, renal function, and thyroid studies, since lithium affects thyroid function over time and renal impairment slows clearance.
Phenytoin levels are read alongside albumin, for the reason above, and alongside liver function where hepatic metabolism is a concern. Theophylline levels are read alongside a history of recent smoking changes and any newly started interacting medication, since several common antibiotics and other drugs alter its clearance meaningfully within days.
Vancomycin levels are read alongside serum creatinine and, in many current protocols, alongside an AUC-based calculation rather than a trough alone, since trough monitoring by itself has been shown to underpredict nephrotoxicity risk in some patients. A nurse who reports a vancomycin level without checking the paired creatinine is giving the provider half the picture needed to decide whether the interval or the dose is the problem.
Nursing implications
Draw timing is a nursing responsibility, not a laboratory formality. Drawing a vancomycin trough thirty minutes before the next dose rather than the specified window can produce a result that looks falsely low or falsely high depending on the direction of the error, and either can lead to an incorrect dose adjustment that the level was ordered to prevent.
Assessment before and after the result matters as much as the number. A patient due for a digoxin level should be assessed for apical pulse and for symptoms of toxicity before the dose is given, and a level pending should not delay reporting a pulse below 60 or new visual disturbance regardless of what the last level showed.
Documentation should tie the level to the clinical picture: the draw time relative to the last dose, the patient's renal and electrolyte status at the time, and any symptoms present. A level charted as a bare number, without this context, is difficult for the next clinician to interpret and can lead to a repeat draw that a complete note would have made unnecessary.
What patients ask about it
Patients on long-term lithium or phenytoin often ask why they need blood drawn so often when they feel well. The answer is that toxicity and under-treatment can both develop without early symptoms, and the level is the only reliable early warning, particularly for lithium where dehydration from a stomach bug can push levels up within a day or two with no dose change at all.
Patients started on vancomycin, usually during an acute admission, ask why the timing of the blood draw matters so much when other blood tests seem more flexible. Explaining that the level is only accurate if drawn at the correct point before the next dose, and that an early or late draw can lead to an unnecessary dose change, usually resolves the frustration of a draw being repeated.
Patients on digoxin frequently ask about interactions with over-the-counter products, and the honest answer is that several common supplements and some antacids can alter absorption or add to toxicity risk, so any new over-the-counter product should be checked before starting it rather than after a level comes back abnormal.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this page covers.
Common questions
Why does a digoxin level look normal but the patient still has signs of toxicity?
Digoxin toxicity risk rises with low potassium or low magnesium, independent of the digoxin level itself. A level within the 0.5 to 2.0 ng/mL range can still coexist with toxicity if the patient is hypokalemic, so electrolytes need checking alongside the level, not instead of it.
What is the difference between a trough and a peak level?
A trough is drawn just before the next scheduled dose, when the drug concentration is lowest, and it is the most common level used for vancomycin and several other drugs. A peak is drawn at a specified time after the dose is given, when concentration is highest, and is used less often because it reflects a moment rather than the steady state the trough is meant to capture.
Why does dehydration matter so much for a patient on lithium?
The kidneys reabsorb lithium through the same pathway used for sodium, so a fluid deficit from vomiting, diarrhoea, or reduced intake concentrates lithium in the blood without any change to the prescribed dose. This is why a patient who is unwell for another reason can develop lithium toxicity even on a previously stable dose.
Why is a vancomycin trough sometimes reported alongside an AUC calculation instead of alone?
A trough alone has been shown to underpredict the risk of kidney injury in some patients, so many current protocols pair it with an AUC-based estimate of total drug exposure over 24 hours. Reporting the trough without the paired creatinine or AUC context gives the prescriber an incomplete basis for adjusting the dose.
Does a low phenytoin level always mean the dose needs increasing?
Not if the patient has low albumin. Phenytoin's standard range reflects the total level, bound and free, and a low albumin state can produce a falsely low total reading while the active free fraction is still adequate or even elevated, so albumin should be checked before assuming the dose is too low.
More on dosage calculation and lab values