The 5-HIAA 3rd Day Test is advised in suspected carcinoid tumors and helps in evaluating and managing symptomatic carcinoid syndrome and carcinoid tumors affecting the gastrointestinal tract as well as the respiratory tract. The test is performed by measuring 5-HIAA levels in urine collected over 72 hours. Elevated levels provide valuable insights about increased serotonin production and metabolism, aiding healthcare professionals in the management of carcinoid tumors and monitoring treatment effectiveness.