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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Oncology

Testing for Carcinoid Syndrome: Blood, Urine, and Scans

At a Glance

Carcinoid syndrome is evaluated by combining symptoms with 5-HIAA blood or urine testing, scans such as DOTATATE PET/CT and liver MRI, and tumor pathology. A normal 5-HIAA or chromogranin A result alone does not rule it out.

Getting a clear diagnosis for Carcinoid Syndrome requires a “multimodal” approach—meaning your care team must look at your symptoms, your blood and urine chemistry, and specialized imaging scans all together [1][2]. Because the symptoms often mimic other conditions, these tests provide the hard evidence needed to confirm that a neuroendocrine tumor (NET) is the cause [3].

The Biochemical Test: 5-HIAA

The most common way to help confirm Carcinoid Syndrome is by measuring 5-HIAA (5-hydroxyindoleacetic acid), which is the waste product created when your body breaks down excess serotonin [3][4].

  • 24-Hour Urine Test: Traditionally, you are asked to collect all your urine for 24 hours. While highly accurate in classic cases, its sensitivity can vary based on the assay used, collection quality, and intermittent hormone secretion [1].
  • Plasma/Serum Test: Modern laboratories can now often measure 5-HIAA through a simple blood draw. While this is easier for patients, the results must be interpreted carefully if you have kidney issues, as reduced kidney function can artificially raise these levels [4][5].
  • Crucial Preparation: To get an accurate result, you must follow strict dietary and medication rules for 24 to 48 hours before the test [6]. You must avoid foods high in tryptophan or serotonin, such as bananas, plantains, nuts, avocados, and pineapples [6]. Certain medications, including some cough syrups and acetaminophen (Tylenol), may also interfere with the test [6][7]. Do not stop prescribed medicines without asking your clinician or the lab.

It is important to know that a “normal” 5-HIAA result does not completely rule out the syndrome. Some tumors (like those in the lungs) may cause flushing without producing enough serotonin to raise your 5-HIAA levels [8][9].

Specialized Imaging: Seeing the Receptors

Standard CT or MRI scans are excellent for seeing where tumors are, but to understand their biology, doctors use Somatostatin Receptor (SSTR) Imaging [10].

  • Ga-68 DOTATATE PET/CT: This is the modern standard for NET imaging [11]. It uses a radioactive tracer that sticks to specific receptors (somatostatin receptors) on the surface of NET cells [12]. It is much more sensitive than older “OctreoScans” and can find very small tumors that other scans might miss [13]. Note: DOTATATE scans show receptor expression; they help determine if PRRT is suitable, but they do not prove that a tumor is actively releasing serotonin.
  • Liver-Specific MRI: Because Carcinoid Syndrome is often linked to liver spread, your doctor may order an MRI using a special contrast agent (like Eovist) [14]. This scan is often better than a PET scan at mapping out very small (sub-centimeter) spots in the liver, which is vital for surgical planning [15][14].

The Pathology Report: Grade and Differentiation

If you have a biopsy, the pathologist will provide a report that “grades” the tumor. This helps your team predict how the tumor will behave [16].

  • Differentiation: This describes how much the cancer cells look like healthy cells. Well-differentiated tumors look more like normal cells and usually grow more slowly [16]. Poorly-differentiated tumors (neuroendocrine carcinomas) look very abnormal and are distinct, aggressive entities [17].
  • Ki-67 Index & Mitotic Rate: The Ki-67 is a percentage that tells you how many cells are actively dividing, while the mitotic rate counts dividing cells under a microscope [18].

For gastrointestinal and pancreatic tumors, these numbers assign a Grade:

  • Grade 1 (G1): Low growth (Ki-67 less than 3%) [19].
  • Grade 2 (G2): Intermediate growth (Ki-67 between 3% and 20%) [19].
  • Grade 3 (G3): High growth (Ki-67 greater than 20%) [19].
    (Note: Grading rules depend on the primary site; pulmonary NETs are classified differently, and your primary site, differentiation, and mitotic rate all matter.)

A Note on Biomarkers

You may also see Chromogranin A (CgA) on your bloodwork. While CgA is a useful marker for tracking many neuroendocrine tumors, it is not a specific test for Carcinoid Syndrome [20]. Like 5-HIAA, a normal CgA level does not mean you do not have the syndrome; it is just one piece of the diagnostic puzzle [9]. Evaluation by a multidisciplinary team is essential to ensure all these pieces are put together correctly [2].

Common questions in this guide

What is 5-HIAA, and why is it measured for carcinoid syndrome?
5-HIAA is a waste product made when the body breaks down serotonin. Measuring it in a 24-hour urine collection or blood sample can help clinicians evaluate whether a neuroendocrine tumor is causing carcinoid syndrome.
How should I prepare for a 5-HIAA blood or urine test?
For 24 to 48 hours before testing, follow the laboratory's instructions and avoid foods high in serotonin or tryptophan, including bananas, plantains, nuts, avocados, and pineapples. Ask your clinician or laboratory about medicines such as some cough syrups or acetaminophen, and do not stop prescribed medicine on your own.
Can I have carcinoid syndrome if my 5-HIAA result is normal?
Yes. Some neuroendocrine tumors, including some lung tumors, may cause flushing without producing enough serotonin to raise 5-HIAA. Clinicians interpret the result alongside symptoms, imaging, and other findings.
What does a Ga-68 DOTATATE PET/CT scan show?
A Ga-68 DOTATATE PET/CT scan uses a tracer that attaches to somatostatin receptors on many neuroendocrine tumor cells. It can help locate small tumors and show whether receptor-targeted treatment such as PRRT may be appropriate, but it does not prove that a tumor is releasing serotonin.
Why might my doctor order an Eovist liver MRI?
An Eovist-enhanced MRI can show very small liver lesions and may map liver spread more clearly than a PET scan in some situations. This information can help with surgical planning.
What do Ki-67 and tumor grade mean on a pathology report?
Ki-67 estimates the percentage of tumor cells that are dividing, while the mitotic rate counts dividing cells under a microscope. For many gastrointestinal and pancreatic neuroendocrine tumors, Ki-67 below 3% is grade 1, 3% to 20% is grade 2, and above 20% is grade 3; grading rules can differ by tumor site.
Is chromogranin A a specific test for carcinoid syndrome?
Chromogranin A can help track some neuroendocrine tumors, but it is not specific for carcinoid syndrome. A normal result does not rule out the syndrome, so it is interpreted with symptoms and other tests.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Was my 5-HIAA test done via a 24-hour urine collection or a blood draw, and how do my kidney function levels affect that result?
  2. 2.Does my pathology report show my tumor is 'well-differentiated' or 'poorly-differentiated,' and what was my exact Ki-67 percentage?
  3. 3.On my DOTATATE PET scan, did the tumors show somatostatin receptor expression, and what does that mean for my treatment options?
  4. 4.Given my symptoms, should I also have an 'Eovist' MRI to get a clearer map of any small liver metastases?
  5. 5.How many 'hotspots' were counted to determine my Ki-67 index, and what does my grade mean for the pace of my disease?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (20)
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This page is for informational purposes only and does not constitute medical advice. Your oncology, pathology, and radiology teams should interpret your test results and recommend next steps for your situation.

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