Complete Blood Count (CBC): What It Is, How It Is Done, and What Your Results Mean

A complete blood count (CBC) is a common blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets. Your doctor may order it during a routine checkup or to help evaluate symptoms such as fatigue, weakness, fever, or easy bruising, and to look for conditions like anemia, infection, or bleeding problems.

The 5-Hydroxyindoleacetate (5-HIAA) test measures the amount of 5-HIAA in your urine over a 24-hour period. 5-HIAA is a breakdown product (metabolite) of serotonin, a hormone and neurotransmitter. This test helps doctors check for conditions that affect serotonin production in your body, particularly carcinoid tumors.

A healthcare professional will draw a small sample of blood, usually from a vein on the inside of your elbow or the back of your hand. The area is cleaned with an antiseptic, an elastic band is placed around your upper arm, and a needle is inserted to collect the blood into a tube. The whole process takes only a few minutes, and you may feel a brief pinch or sting. The sample is then sent to a laboratory for analysis.

You will collect all your urine in a special container for exactly 24 hours. Start by urinating into the toilet when you wake up, then collect all subsequent urine in the provided container for the rest of the day and night. The next morning, collect your first morning urine and add it to the container. Keep the container refrigerated during the collection period. Return the completed collection to the laboratory as instructed.

In most cases, no special preparation is needed for a CBC, and you can eat and drink normally before the test. However, if your blood will also be used for other tests that require fasting, your doctor will give you specific instructions, typically to avoid food and drinks other than water for 8 to 12 hours beforehand. Wear a shirt with short sleeves or sleeves that are easy to roll up, and let the staff know about any medicines you take, allergies you have, or bleeding problems in your past.

For 48 hours before and during the test, avoid foods rich in serotonin such as bananas, avocados, walnuts, almonds, Brazil nuts, pineapples, tomatoes, eggplants, and certain cheeses. Also avoid medications including cough syrups, antacids, muscle relaxants, and antidepressants (especially SSRIs). Inform your doctor about all medications and supplements you take.

Your results will show the levels of red blood cells, white blood cells, hemoglobin, hematocrit, and platelets, along with reference ranges that indicate what is considered normal. Values that are too high or too low can point to conditions such as anemia, infection, inflammation, dehydration, or a clotting disorder, but abnormal results do not confirm a diagnosis by themselves. Your doctor will interpret your values together with your symptoms, medical history, and a physical exam, and may recommend additional testing or a follow-up visit to discuss the findings and next steps.

Your doctor will interpret your results based on your symptoms and medical history. Elevated 5-HIAA levels may indicate a carcinoid tumor or other neuroendocrine tumor. Normal or low levels do not necessarily rule out all conditions. Your healthcare provider will discuss what your specific results mean for your situation.