The 5-HIAA 1st day test is advised in suspected carcinoid tumors with excess serotonin production. It is useful in evaluating symptomatic carcinoid syndrome with diarrhea, flushing, wheezing, etc. Additionally, diagnose gastrointestinal tract and bronchial carcinoid tumors. The test is performed by measuring 5-HIAA levels in a 24-hour urine sample. Elevated levels indicate increased serotonin production, aiding healthcare providers in the diagnosis and management of carcinoid tumors.