Complete Blood Count (CBC): A Patient Guide to This Common Blood Test

A complete blood count, or CBC, is a routine blood test that measures the different cells in your blood, including red blood cells, white blood cells, and platelets. It is one of the most frequently ordered laboratory tests and provides your doctor with a broad picture of your overall health.

The ammonia blood test measures the concentration of ammonia (NH3) in your blood. Ammonia is a waste product that is normally processed by the liver and excreted by the kidneys. When the liver is not functioning properly, ammonia can build up in the blood, which may cause serious health problems.

Your doctor may order this test as part of a routine health checkup, to help diagnose symptoms such as fatigue, weakness, fever, bruising, or unexplained bleeding, to monitor an existing condition like anemia or an infection, or to track how well a treatment, such as chemotherapy, is affecting your blood cells.

Your doctor may order an ammonia test to: evaluate liver function, diagnose liver disease or damage, monitor the progression of liver conditions, check for hepatic encephalopathy (a brain disorder caused by liver disease), and assess the effectiveness of treatment for liver disease. It may also be used to monitor certain metabolic disorders.

Preparation for a CBC is usually very simple. You can normally eat and drink as you usually do, and you should take your regular medications unless your doctor tells you otherwise. If the blood sample will also be used for other tests that require fasting, your care team will give you specific instructions in advance. Drink water before your appointment so your veins are easier to find.

Before the test, follow these guidelines: fast for 8-12 hours (usually overnight) as instructed by your doctor, avoid smoking before the test as smoking can increase ammonia levels, inform your doctor about all medications you are taking, as some medications can affect test results, avoid strenuous exercise before the test, and rest quietly for 20-30 minutes before having your blood drawn.

During the test, a healthcare professional will wrap an elastic band around your upper arm to make the veins more visible, clean the skin on the inside of your elbow or the back of your hand with an antiseptic, and insert a small needle to draw a sample of blood into a tube. You may feel a brief sting or pinch. The entire process usually takes less than five minutes, after which a small bandage is placed over the puncture site.

A healthcare professional will draw a blood sample from a vein in your arm using a small needle. The blood is collected into a special tube and sent to a laboratory for analysis. The test itself takes only a few minutes. After the blood is drawn, you may experience slight bruising or soreness at the needle site, but this usually resolves quickly.

Your results will show whether the levels and characteristics of your blood cells fall within the normal reference ranges listed on your report. High or low values can point to conditions such as anemia, infection, inflammation, clotting problems, or other disorders, but a single abnormal value does not confirm a diagnosis. Your doctor will interpret the numbers together with your symptoms, medical history, and other tests, so always discuss your report with your healthcare provider before drawing any conclusions.

Normal ammonia levels typically range from 15 to 45 μmol/L (micro moles per liter). However, reference ranges may vary slightly between laboratories. Your doctor will interpret your results based on your overall health, medical history, and other test results. Elevated ammonia levels may indicate liver disease, liver failure, hepatitis, cirrhosis, or certain metabolic disorders. Low levels are less common but may occur with certain conditions or use of some medications.