Which investigation is commonly used to diagnose carcinoid syndrome?

The diagnosis of carcinoid syndrome is primarily made through the measurement of 5-hydroxyindoleacetic acid (5-HIAA) in a 24-hour urine sample. This is because carcinoid tumors, which often arise in the gastrointestinal tract or bronchial tube, secrete serotonin, which is metabolized into 5-HIAA. High levels of 5-HIAA in the urine are indicative of the presence of these neuroendocrine tumors and are essential for confirming a diagnosis of carcinoid syndrome.

In contrast, other investigations may not provide relevant information for diagnosing carcinoid syndrome. For instance, a chest X-ray is typically used to assess lung pathology or look for metastatic disease but does not specifically identify neuroendocrine tumors. A complete blood count can reveal general health information, such as signs of anemia or infection, but it is not targeted to diagnose carcinoid syndrome. Thyroid function tests evaluate the function of the thyroid gland and do not relate to the detection of serotonin-secreting tumors. Thus, the measurement of 5-HIAA remains the gold standard for confirming carcinoid syndrome.