| Falsely high values |
| Tryptophan-rich foods: avocados, pineapples, bananas, kiwi fruit, plums, eggplants, walnuts, hickory nuts, pecans, tomatoes, plantains, butternut squash |
| Drugs: acetaminophen, coumaric acid, guaifenesin, mephenesin, phenobarbital, reserpine, acetanilid, ephedrine, methamphetamine, nicotine, phentolamine, phenmetrazine, caffeine, fluorouracil, melphalan, methocarbamol, phenacetin, mesalamine |
| Falsely low values |
| Drugs: corticotrophin, ethanol, imipramine, levodopa, MAO inhibitors, phenothiazines, aspirin, isoniazid, gentisic acid, methenamine, streptozotocin, heparin, methyldopa |
