A complete blood count or "CBC" is a commonly ordered blood test in children.  The CBC provides information on the concentration and character of red blood cells, white blood cells and platelets.

How is a CBC drawn?

A sample of blood is obtained by a blood draw in a lab, clinic or hospital.  This blood draw can be done by a "finger stick" capillary sample (although this method provides less accurate results), a venous blood sample (the most common method), or by an arterial blood sample.  Less than 5 milliliters of blood (one teaspoon) is typically required in children.  A CBC can be performed with a smaller amount of blood in infants if necessary.

How are medical tests interpreted?

Interpreting medical tests is an art and a science.  Doctors, laboratories and lab machines are not perfect.  Doctors must look at the "big" picture... which means they must look at the patient and perform a physical exam.  There is normal variation of all medical tests and a "normal" value in an adult may be "abnormal" in a child.  By looking at all available information and examining the patient, a doctor can decide the true significance of a lab value.

What are the parts of a CBC and what do they mean?

White blood cell (WBC) count - White blood cells help the body fight infection so as you might expect, an elevated number of WBC's may indicate an infection.  To confuse things however, a low or normal WBC count may be present during an infection also.

• Causes of a high WBC count - infection, inflammation, some medicines (such as steroids), stress, some diseases (such as sickle cell disease)
• Causes of a low WBC count - illness, infection (especially viruses and severe bacterial infections), some medicines (such as Septra, Bactrim, acyclovir, and many more)
• Extremely high or low numbers - leukemia or other types of cancer

Hemoglobin and hematocrit (H & H) - This is the concentration of red blood cells (RBC's) in the blood.  Low levels of RBC's may indicate anemia or bleeding.

• Causes of a high H & H - blood was drawn as a finger-stick (capillary sample), dehydration, plethora (a newborn baby receives a large amount of blood from the placenta at childbirth)
• Causes of a low H & H - bleeding, anemia, low iron level, low folate level, low vitamin B12 level, illness, kidney dysfunction, viruses (especially Parvovirus B19), some chronic diseases, autoimmune diseases

Platelet count - Platelets help with blood clotting.

• Causes of a high platelet count - infection or inflammation
• Causes of low platelet count - bleeding (platelets are being used quickly), a large spleen (traps and destroys too many platelets), autoimmune diseases (such as ITP), infection

Mean corpuscular volume (MCV) - The MCV estimates the average size of red blood cells.

• Causes of a high MCV - rapidly produced red blood cells, low folate level, low vitamin B12 level
• Causes of a low MCV - low iron level, chronic diseases

WBC Differential - This estimates the proportion of different white blood cell types.

• Neutrophils - especially helpful for fighting bacterial infections
• Bands - "young" neutrophils.  Produced quickly to fight bacterial infections.
• Lymphocytes - especially helpful for fighting viral infections
• Monocytes - perform different functions; may be high in infectious mononucleosis
• Eosinophils - especially helpful for fighting parasite infections; also elevated with asthma and other allergic diseases
• Basophils - perform different functions

Photo - This image depicts the contents of a Vacutainer® kit used in performing a proper venipuncture by a trained phlebotomist, i.e., a technician skilled in the practice of drawing blood from a patient. This image is the first in a series of 19 images outlining the steps involved in extracting blood from the antecubital vein located at the anterior elbow region of the arm. CDC/ Jim Gathany. 2004. Used with permission.

Photo 2 - This image depicts a trained phlebotomist, i.e., a technician skilled in the practice of drawing blood from a patient, who is “swabbing” the intended venipuncture site in a patient’s right antecubital fossa in order to extract a blood sample. This image is the fifth in a series of 19 images outlining the steps involved in extracting blood during the venipuncture process. CDC/ Jim Gathany. 2004. Used with permission.

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