From the Guidelines
The primary blood test for diagnosing carcinoid tumors is the 5-HIAA (5-hydroxyindoleacetic acid) test, which measures serotonin metabolites in the blood, and chromogranin A, an important tumor marker that can be elevated in patients with carcinoid tumors. When diagnosing carcinoid tumors, it is essential to consider the location of the tumor, as this can affect the choice of blood tests. For example, midgut carcinoid tumors usually have raised 24-hour urinary 5-HIAA levels, while foregut and hindgut tumors may have different test results 1. In addition to 5-HIAA and chromogranin A, other blood tests may be ordered, including serotonin levels and specific hormones like gastrin, insulin, or glucagon, depending on the suspected tumor location. Patients should avoid certain foods before testing, including bananas, avocados, plums, eggplant, tomatoes, and walnuts, as these can artificially elevate serotonin levels and cause false positive results 1. Certain medications, particularly antidepressants, can also interfere with test results, so patients should inform their doctor about all medications they're taking. These blood tests are usually combined with imaging studies like CT scans, MRIs, or specialized nuclear medicine scans such as octreotide scans for a definitive diagnosis. The tests work by detecting the biochemical products that carcinoid tumors release into the bloodstream, as these tumors often produce excessive amounts of hormones and other substances that can be measured. Some key points to consider when ordering blood tests for carcinoid tumors include:
- Avoiding certain foods and medications that can interfere with test results
- Considering the location of the tumor when choosing blood tests
- Combining blood tests with imaging studies for a definitive diagnosis
- Measuring 5-HIAA and chromogranin A levels, as well as other hormones and substances that may be elevated in carcinoid tumors.
From the Research
Blood Tests for Carcinoid Syndrome
- Blood tests can be used to diagnose and monitor carcinoid syndrome, including measurements of serotonin, 5-hydroxyindoleacetic acid (5-HIAA), and chromogranin A (CgA) 2, 3, 4.
- Plasma 5-HIAA is a specific marker for carcinoid syndrome and can be used to monitor treatment response 2, 3, 5.
- Chromogranin A (CgA) is a nonspecific biomarker for neuroendocrine tumors, but it can be used to monitor disease progression and predict outcome in patients with carcinoid syndrome 4.
- Blood serotonin levels can also be used to monitor treatment response, but they may not be as reliable as plasma 5-HIAA due to variability in platelet count 2.
Limitations and Variability of Blood Tests
- Blood serotonin levels can be affected by platelet count, which can lead to variability in test results 2.
- Chromogranin A (CgA) levels can be elevated in other conditions, such as renal impairment or proton pump inhibitor use, which can lead to false positives 4.
- Plasma 5-HIAA levels can be affected by dietary factors, such as consumption of serotonin-rich foods, which can lead to false positives 3.
Clinical Applications of Blood Tests
- Blood tests can be used to diagnose carcinoid syndrome and monitor treatment response in patients with neuroendocrine tumors 2, 3, 5.
- Plasma 5-HIAA and chromogranin A (CgA) levels can be used to predict disease progression and outcome in patients with carcinoid syndrome 4.
- Blood tests can be used in combination with other diagnostic tests, such as imaging studies and urinary 5-HIAA measurements, to diagnose and monitor carcinoid syndrome 3, 6.