Complete Blood Count (CBC) Test: What to Expect and What Your Results Mean

A complete blood count, or CBC, is a common blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets. It gives your provider a broad picture of your overall health and can help detect many conditions early.

The plasma ammonia test measures the amount of ammonia (also called ammonium) in the liquid portion of your blood (plasma). Ammonia is a waste product that is normally processed by your liver and excreted by your kidneys. When the liver cannot properly remove ammonia, it builds up in the blood and can cause health problems.

Your provider may order a CBC as part of a routine checkup, to look into symptoms such as fatigue, weakness, fever, or easy bruising, to monitor an ongoing condition like anemia or infection, or to see how a treatment is affecting your blood cells.

Your doctor may order this test if you have symptoms of liver disease (such as yellowing of skin/eyes, confusion, or unusual drowsiness), have a known liver condition, or are being treated for conditions that affect ammonia metabolism. It is also used to monitor patients with liver cirrhosis, evaluate unexplained mental status changes, and screen for inherited metabolic disorders.

Most people do not need any special preparation for a CBC, and you can usually eat and drink normally beforehand. If your sample will also be used for other tests, your provider may ask you to fast for a period of time. Tell your provider about all medicines, vitamins, and supplements you take.

Follow these guidelines before your test: Fast (do not eat) for 8-12 hours before the blood draw, as food can affect ammonia levels. Avoid strenuous exercise for 24 hours before the test, as exercise can increase ammonia levels. Tell your doctor about all medications you are taking, as some drugs can affect test results. Do not smoke before the test, as smoking can increase ammonia levels.

A health care professional will draw a small amount of blood, usually from a vein on the inside of your elbow or the back of your hand. The site is cleaned with an antiseptic, an elastic band is placed around your upper arm, and the blood is collected into a tube in a process that usually takes less than five minutes.

A healthcare professional will draw blood from a vein in your arm using a small needle. The sample will be placed in a special tube and sent to a laboratory for analysis. The collection process usually takes only a few minutes.

Your results will show whether your red blood cells, white blood cells, and platelets fall within the normal reference ranges. Values that are too high or too low may point to conditions such as anemia, infection, inflammation, or bleeding problems, but your provider will interpret them alongside your symptoms, medications, and medical history before making any diagnosis.

Normal ammonia levels vary by laboratory, but typically range from 15-45 micromoles per liter (μmol/L). High ammonia levels (hyperammonemia) may indicate liver disease, liver failure, certain genetic metabolic disorders, kidney failure, or severe dehydration. Low levels are less common but can sometimes occur with certain medications. Your doctor will interpret your results based on your overall health and medical history.

The risks of a CBC are very small. You may feel a quick sting when the needle goes in, and slight soreness or bruising at the puncture site is common. Some people feel briefly lightheaded. Serious problems such as excessive bleeding or infection are rare, and the information the test provides usually far outweighs these minimal risks.

The risks of having blood drawn are minimal. You may experience slight pain, bruising, or bleeding at the puncture site. Some people feel lightheaded. These side effects usually resolve quickly.