What Is a Complete Blood Count (CBC)?

December 30, 2022

A complete blood count, abbreviated CBC, shows information on red blood cells, white blood cells, and platelets, from a small sample of blood, usually taken from a vein in the arm or hand. Red blood cells carry oxygen from the lungs to cells throughout the body and bring 75% of carbon dioxide back to the lungs to be exhaled as waste. White blood cells fight infections. Platelets form blood clots. Let’s look at some normal values:

Red blood cells (erythrocytes)

3.92 to 5.65 trillion cells per liter (about a quart)

Hemoglobin 116 to 166 grams per liter

Hematocrit (size of red blood cells) 35.5 to 48.6 per cent

White blood cells (leukocytes)

3.4 to 9.6 billion cells per liter 


Neutrophils 2500 to 8000 per cubic mm 

Lymphocytes 1000 to 4000 per cubic mm

Monocytes 100 to 700 per cubic mm

Eosinophils 30 to 350 per cubic mm


150,000 to 450,000 per microliter

Interpretation of the CBC

When the red blood cell count is low, the patient is said to be anemic. This can be accompanied by fatigue, and sometimes by irregular heartbeats, chest pains, and menstrual irregularities. The cause of anemia can be sought in the hemoglobin and hematocrit levels.  For example, small blood cells with low levels of hemoglobin might be caused by iron deficiency or chronic disease. Overly large red blood cells might be caused by vitamin B12 deficiency.

When a CBC is ordered without a differential, one number is given for all white blood cells. A differential is often ordered when an infection is suspected, or when the white cell count is high, to differentiate which type of white cells are elevated. Different types of white blood cells fight different types of diseases, so the type of white cell that is elevated is a clue to the type of infection. Elevated neutrophils can point towards an infection by bacteria or fungi. Elevated lymphocytes might be producing antibodies against bacteria, viruses, fungi, parasites, or toxic chemicals. High levels of monocytes can signal viral or parasitic infection. Elevated eosinophils can signal parasitic invasion or allergic reaction. 

Where can I order my own labs?

Many patients have problems getting a timely appointment with a physician. QuickMD now lets you order the CBC online without a doctor’s visit. The blood sample can be collected at any of the LabCorp locations in the US.

One of the QuickMD doctors will contact you with the results and–if they are abnormal–discuss the next steps for you.

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