If one digoxin result is 2.65 nmol/L at 3 hours 52 minutes after a dose and another is 1.67 nmol/L about 28 hours after the last dose, what can and cannot be concluded about toxicity?

A digoxin level drawn 3 hours 52 minutes after a dose is a distribution-phase snapshot, not a steady-state truth. The second level at 28 hours is a true trough. Together they tell you something, but not everything.

The first level, 2.65 nmol/L (about 2.07 ng/mL), sits right at the upper edge of the conventional therapeutic range of 0.5 to 2.0 ng/mL (0.64 to 2.56 nmol/L) [2]. But because it was drawn during the distribution phase, it is artificially high. The drug was still leaving the bloodstream for the tissues. That number cannot tell you whether the heart is being poisoned. It tells you the dose is at the high end, nothing more.

The second level, 1.67 nmol/L (about 1.30 ng/mL) at 28 hours, is a true trough. That is a comfortable number, well within the therapeutic range. If the patient is on a steady daily dose, this level suggests the drug is not accumulating to toxic concentrations. But there is a catch. The 28-hour level is only interpretable if the patient has been on the same dose for at least 5 to 7 days, roughly 5 half-lives. If the dose was changed recently, or if the patient has worsening kidney function, that level is a snapshot of a moving target.

What you cannot conclude from these two numbers alone is whether the patient is toxic. Digoxin toxicity is a clinical diagnosis, not a lab diagnosis. The symptoms matter: nausea, vomiting, visual disturbances, arrhythmias like heart block or ventricular ectopy. The electrocardiogram matters. The serum potassium matters. A high potassium with a high digoxin level is a much more dangerous combination [2, 3]. The 1992 study of digoxin toxicity found that clinical presentation did not differ by age group, but the symptoms were common: fatigue in 95%, visual symptoms in 95%, nausea in 81% [1, 4]. A patient with none of those and a normal ECG is not toxic, even if the early level is at the upper edge.

The evidence base for the therapeutic range itself is thin. The 0.5 to 2.0 ng/mL range comes from decades-old studies in adults with heart failure and atrial fibrillation [2]. A 2015 pediatric study found little association between serum digoxin concentrations and signs of toxicity in children, and questioned routine monitoring altogether [5]. That study found only 6.9% of children had levels above 2.0 ng/mL, and even among those, symptoms were not clearly different from the rest [5]. The range is a guide, not a rule.

What you can conclude: the patient is not in an acutely dangerous range at 28 hours. The early level is not interpretable for toxicity. The next step is clinical: look at the patient, look at the ECG, check the potassium and kidney function. If the patient looks fine and the ECG is clean, these levels do not indicate toxicity, regardless of the early number.

My call: these two levels alone cannot diagnose or rule out digoxin toxicity. The 28-hour trough is reassuring, but the clinical picture is the real test. Confidence: moderate. The evidence for the therapeutic range is weak, and the 2015 pediatric study [5] directly questions the value of routine levels.