Complete Blood Count (CBC): What This Common Blood Test Measures and Why Your Doctor May Order It

A complete blood count (CBC) is one of the most frequently ordered blood tests. It measures the cells that make up your blood, including red blood cells, white blood cells, and platelets, giving your care team a broad picture of your overall health and helping detect a wide range of conditions.

Base excess (BE) is a calculation that measures the amount of excess acid or base in your blood. It tells your doctor how much strong acid would be needed to bring your blood back to a normal pH of 7.4. A positive base excess means there is too much base (alkaline) in your blood, while a negative base excess indicates too much acid.

Your doctor may order a CBC as part of a routine checkup, to look into symptoms such as fatigue, weakness, fever, or unusual bruising, to help diagnose problems like anemia or infection, or to monitor how an ongoing illness or treatment is affecting your blood and your body over time.

This test is part of a blood gas analysis and is used to evaluate your acid-base balance. It helps doctors determine if you have metabolic acidosis (too acidic), metabolic alkalosis (too alkaline), or respiratory disorders affecting your blood pH. It is commonly ordered for patients in critical care, those with kidney disease, severe infections, or breathing difficulties.

A small blood sample is usually drawn from a vein in your arm, often at the inside of the elbow. A technician cleans the site, places a band around your upper arm, inserts a thin needle, and collects the blood into a tube. The process takes only a few minutes, and for children a fingerstick or heelstick may be used instead.

The test requires a blood sample drawn from a central venous catheter (a tube placed in a large vein, typically in the neck, chest, or groin). The blood is then analyzed in a laboratory using specialized equipment to calculate the base excess value. No fasting or special preparation is typically required for this test.

Your report will list values for red blood cells, white blood cells, platelets, hemoglobin, and hematocrit, often shown next to a reference range. Results slightly outside the range are not always cause for concern, and no single number tells the whole story. Review your results with your doctor, who will interpret them alongside your symptoms, history, and other tests.

Normal base excess values typically range from -2 to +2 mEq/L. A result outside this range may indicate an underlying health problem that requires further evaluation and treatment.