Laboratory Evaluation

Serum potassium and digoxin levels will assist in providing information necessary to make adequate therapeutic decisions. Acute poisoning of the Na-K ATPase pump may result in markedly elevated serum potassium levels.3 A high incidence of hyperkalemia has been noted in patients with severe acute poisoning. The serum potassium may be a better indicator of end organ toxicity and a better prognostic indicator than the serum digoxin level in the acutely poisoned patient. Hyperkalemia is less common in chronically poisoned patients. Potassium status may be less predictive in patients with renal insufficiency or dehydration, or who are receiving diuretic therapy.

Accepted therapeutic digoxin levels are 0.5 to 2.0 ng/mL. In most laboratories, the serum digoxin level is not part of the routine toxicologic screen and must be specifically requested.

Serum digoxin levels in both acute and chronic toxicity should be interpreted in the overall clinical context, and not relied upon as the sole indicator of the presence or absence of toxicity. In acute exposures, digoxin is absorbed into the plasma compartment and then redistributed slowly into the tissue compartment. Hence, high digoxin levels are not always associated with clinical signs and symptoms of poisoning. Serum levels are most reliable when obtained six hours after ingestion, when distribution is complete. In patients with clinical evidence of chronic digitalis toxicity, a "therapeutic" level does not exclude toxicity, especially when predisposing factors are present. Conversely, levels above the upper limits of normal do not always cause symptoms. Given the above limitations, it is still most common that the higher the serum level, the greater the likelihood of toxicity.8

A positive serum assay is diagnostic of acute ingestion if the patient has not received digitalis glycosides therapeutically. The rare exception is in the presence of digoxin-like immunoreactive substance that has been detected in neonates and patients with renal insufficiency or hepatic dysfunction. In addition, naturally occurring digitalis glycosides from plants and animals may cross-react with the digoxin assay. The degree of cross-reactivity is unknown and no correlation has been established between serum levels of these glycosides and toxicity.

Additional laboratory evaluation includes the determination of adequate oxygenation, renal and hepatic function, and electrolyte determinations in addition to potassium. Continuous electrocardiographic monitoring is essential to detect dysrhythmias and evidence of hyperkalemia.

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