Some toxins have direct nephrotoxic effects; in other cases, renal failure is due to shock or myoglobinuria. Blood urea nitrogen and creatinine levels should be measured and urinalysis performed. Elevated serum creatine kinase (CK) and myoglobin in the urine suggest muscle necrosis due to seizures or muscular rigidity. Oxalate crystals in large numbers in the urine suggest ethylene glycol poisoning.
D. Serum Osmolality
The calculated serum osmolality is dependent mainly on the serum sodium and glucose and the blood urea nitrogen and can be estimated from the following formula:
This calculated value is normally 280–290 mOsm/L. Ethanol and other alcohols may contribute significantly to the measured serum osmolality but, since they are not included in the calculation, cause an osmol gap:
Substances that are often associated with an abnormal osmol gap include acetone, ethanol, ethylene glycol, isopropyl alcohol, methanol, and propylene glycol.
E. Electrocardiogram
Widening of the QRS complex duration (to more than 100 milliseconds) is typical of overdose of tricyclic antidepressants and other drugs that block the sodium channel in cardiac conducting tissue (Figure 58–1). The QTc interval may be prolonged in many poisonings, including antidepressants and antipsychotics,
FIGURE 58–1 Changes in the electrocardiogram in tricyclic antidepressant overdosage. A: Slowed intraventricular conduction results in prolonged QRS interval (0.18 seconds; normal, 0.08 seconds). B and C: Supraventricular tachycardia with progressive widening of QRS complexes mimics ventricular tachycardia. (Reproduced, with permission, from Benowitz NL, Goldschlager N: Cardiac disturbances. In: Haddad LM, Shannon MW, Winchester JF [editors]. Clinical Management of Poisoning and Drug Overdose, 3rd ed. WB Saunders, 1998. © Elsevier.) lithium, and arsenic (see also https://www.crediblemeds.org/ everyone/composite-list-all-qtdrugs/). Variable atrioventricular (AV) block and a variety of atrial and ventricular arrhythmias are common with poisoning by digoxin and other cardiac glycosides. Hypoxemia due to carbon monoxide poisoning may result in ischemic changes on the electrocardiogram.
F. Imaging Findings
A plain film of the abdomen may be useful because some tablets, particularly iron and potassium, may be radiopaque. Chest radiographs may reveal aspiration pneumonia, hydrocarbon pneumonia, or pulmonary edema. When head trauma is suspected, a computed tomography (CT) scan is recommended.
It is a common misconception that a toxicology “screen” is the best way to diagnose and manage an acute poisoning. Unfortunately, rapid urine “drugs of abuse” screens are limited to a few classes of drugs and are subject to many false-positive and false-negative results, and more reliable comprehensive toxicology screening is time-consuming and expensive and results of tests may not be available for days. Moreover, many highly toxic drugs such as calcium channel blockers, β blockers, and isoniazid are not included in the screening process. The clinical examination of the patient and selected routine laboratory tests are usually sufficient to generate a tentative diagnosis and an appropriate treatment plan. Although screening tests (so-called “drugs of abuse” panels) may be helpful in confirming a suspected intoxication, they should not delay needed treatment. More formal, comprehensive screening may be necessary in cases of suspected brain death (to rule out drugs as a cause of coma), child abuse, or as part of a postmortem examination.
When a specific antidote or other treatment is under consideration, quantitative laboratory testing may be indicated.