MAOI poisoning is not associated with any confirmatory laboratory tests. It remains a clinical diagnosis based solely on the history of excessive MAOI ingestion. Plasma MAOI levels and drug screens cannot be relied upon to assist in making the diagnosis of MAOI toxicity for two reasons. First, all commonly used drug screens are qualitatively unable to detect MAOIs. Second, specific quantitative plasma MAOI levels are not routinely available in most hospitals, nor do they correlate with observed clinical toxicity. Selegiline is likely to produce amphetamine metabolites, which can be detected on most urine drug screens. Tranylcypromine has the potential to produce amphetamine metabolites, but these have rarely been detected. The best use of laboratory tests is to assist in the differential diagnosis of MAOI toxicity and to identify possible complications of MAOI overdose, which include hypoxia, rhabdomyolysis, renal failure, hyperkalemia, metabolic acidosis, hemolysis, and disseminated intravascular coagulation. Leukocytosis and thrombocytopenia are commonly seen with MAOI toxicity.
The differential diagnosis of a MAOI overdose includes all drugs and medical conditions capable of producing a hyperadrenergic state, altered mental status, and/or muscle rigidity. As evidenced by the extensive number of conditions listed in Table 15.4.-3., the differential diagnosis of the unknown MAOI ingestion is extremely challenging. In addition, MAOI toxicity can also be associated with a sympatholytic presentation, thus broadening the differential possibilities even further. In reality, without a history of exposure to MAOIs, it is highly unlikely that the correct diagnosis will be made in the emergency department (ED), since no confirmatory tests are available.
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