What test would be useful in diagnosing carcinoid syndrome?

The urine test for 5-HIAA (5-hydroxyindoleacetic acid) is particularly useful in diagnosing carcinoid syndrome because this test measures the levels of a serotonin metabolite in the urine. Carcinoid tumors, which are neuroendocrine tumors, can produce serotonin and its metabolites. In patients with carcinoid syndrome, excessive serotonin secretion leads to elevated levels of 5-HIAA in the urine. Therefore, measuring 5-HIAA is a reliable way to diagnose the syndrome and monitor disease activity.

The other tests mentioned do not specifically target the underlying pathology of carcinoid syndrome. Serum cortisol levels might be indicative of other conditions such as Cushing's syndrome, serum glucose levels generally reflect metabolic status rather than specific tumor activity, and a complete blood count is used to assess overall health and detect conditions like anemia or infection, but it does not provide information relevant to carcinoid tumors or their secretory activity.