Which of the following diagnostic tests is most appropriate for suspected carcinoid tumor?
A. 24-hour urinary excretion of 5-hydroxyindoleacetic acid (5-HIAA)Correct Answer: A
24-hour urinary excretion of 5-HIAA is the recommended test for suspected carcinoid tumor. 5-HIAA is the main metabolite of serotonin, and normal urinary excretion ranges from 2 to 8 mg/d, greater than 25 mg/d is diagnostic of a carcinoid tumor.
The test has high sensitivity (>90%) and high specificity (>90%) and is most useful in patients with primary midgut (jejunoileal, appendiceal, ascending colon) tumors which produce the highest levels of serotonin. Foregut (gastroduodenal, bronchus) and hindgut (transverse, descending, sigmoid, rectum, genitourinary) tumors rarely secrete serotonin and thus, 5-HIAA levels may not be elevated in the urine. False-positive results can occur with intake of acetaminophen, guaifenesin, ephedrine, methamphetamine, nicotine, caffeine, and tryptophan-/serotonin-rich foods such as spinach, eggplant, wine, and cheese. False negatives can occur with intake of ethanol, levodopa, methyldopa, monoamine oxidase inhibitors, aspirin, heparin, isoniazid, and fluoxetine.
24-hour urinary excretion of serotonin, plasma serotonin, and chromogranin assays is of interest but have not been validated in large clinical series. The sensitivities and specificities of these tests are not well established and inferior to the 24-hour urinary excretion of 5-HIAA and thus offer little diagnostic value and are not recommended.
References: