Complete Blood Count (CBC) Test

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. These cells carry oxygen throughout your body, fight infection, and help your blood clot properly, so this test gives your care team a broad picture of your overall health.

The serum ammonia test measures the amount of ammonia (a waste product) in your blood. Ammonia is produced when proteins are broken down in your body, and your liver normally converts it to urea, which is then excreted in urine. This test checks whether your liver is functioning properly to process ammonia.

Your doctor may order a CBC as part of a routine checkup, to help find the cause of symptoms such as fatigue, weakness, fever, easy bruising, or unusual bleeding, or to monitor a condition you already have, such as anemia or an infection, as well as to see how well a treatment is working over time.

Your doctor may order this test if you have symptoms of liver disease such as confusion, drowsiness, or trembling hands (asterixis). It is also used to monitor patients with known liver disease, diagnose conditions like hepatic encephalopathy or Reye's syndrome, and evaluate children with suspected urea cycle disorders.

A trained health care professional will clean your skin, wrap a band around your upper arm, and draw a small sample of blood from a vein, usually in your arm, using a thin needle. You may feel a quick pinch as the needle goes in. The sample is then sent to a laboratory, and results are often ready within a day or two.

A healthcare professional will draw a blood sample from a vein in your arm. The sample must be placed on ice and sent to the laboratory immediately because ammonia levels can change quickly after the blood is drawn.

A CBC usually requires no special preparation, and you can normally eat and drink before the test unless your provider tells you otherwise. Wear a shirt with sleeves that roll up easily, drink plenty of water ahead of time, and tell your care team about any medicines, vitamins, or supplements you are taking.

Fast for 8-12 hours before the test (usually overnight). Do not smoke before the test, as smoking can increase ammonia levels. Inform your doctor about all medications you are taking, as some drugs can affect test results. Avoid strenuous exercise before the test.

Your report will list the number, size, and types of blood cells in your sample next to reference ranges that show what is considered normal. Results outside those ranges may suggest conditions such as anemia, infection, inflammation, or clotting problems, but many factors can affect the numbers. Always review your results with your health care provider to understand what they mean for you.

Normal ammonia levels vary by laboratory, but generally range from 15-45 μmol/L. Elevated levels may indicate liver disease, liver failure, hepatic encephalopathy, or certain metabolic disorders. Your doctor will interpret your results based on your overall health and medical history.