Does Your Child Need a Blood Test?

12/26/2007

Parents often ask me whether a blood test-- specifically a complete blood cell count (or CBC)—is a good idea to help figure out what is wrong with their sick child. In most cases, I tell them, a CBC is unnecessary.

What is a CBC and what can we learn from it?

The kind of CBC usually done in a pediatric office calls for either drawing a syringe of blood from the child’s forearm or pricking the child’s finger, spinning the blood sample in a special machine to separate the various kinds of cells, and then calculating the amounts of each kind of cell present in the blood. The results of a CBC reveal the number and kind of white-blood cells present, the levels of important components of red blood cells, and levels of the blood platelets that help blood clot.

• The white blood cell count (WBC) is a good gauge of whether your child’s body is putting out enough white blood cells to fight off an infection. Generally a bacterial infection raises the WBC significantly.  A viral infection such as flu can either raise the WBC mildly or can cause a drop in the normal count.
• The white-blood count differential tells us whether the cells are primarily the kind of immune cells that respond to bacteria (neutrophils) or the kind of immune cells that respond to viruses (lymphocytes). The percentage of each that is present gives your doctor a clue as to whether an illness has a viral or bacterial cause. High amounts of another immune cell type called eosinophils can indicate allergy problems or parasitic infestations.
• The hemoglobin, hematocrit and MCV (mean corpuscular volume) are all measures of the red cells in the blood. Low numbers here might indicate conditions like anemia.
• The platelet count measures the cells in the blood responsible for controlling bleeding or clotting within the body. These cell counts often increase in response to an inflammation somewhere in the body.

When is a CBC a good idea? Most childhood illnesses are easily treatable without one. But there are situations that may call for a closer look at what’s going on in your child’s blood:

• when the pediatrician is trying to determine if the problem is caused by a virus or a bacteria—for instance in the case of pneumonia. Bacterial illnesses respond to antibiotics, but viral illnesses do not.
• when a child has no obvious source for a fever lasting longer than five days. A high WBC count will point the physician towards tests such as checking the urine for an infection, a chest x-ray for possible pneumonia, or a blood culture.
• when anemia is a possibility. I routinely check my patients for anemia between  10-12 months of age, when I also do a lead test. If a child is anemic, the hemoglobin and hematocrit will typically be lower than normal.  I also test for anemia in older children if they are persistently pale, have a poor nutritional status, or (in the case of adolescent girls) if they suffer from heavy menstrual periods. In New York City, where I practice, a CBC and lead test is required at two years old as well.

If a very sick-looking child is not responding as expected to treatment, a blood test might be warranted. Doing a CBC for routine problems is usually both unkind and unnecessary. No child likes having blood drawn, so I reserve this procedure for when it is really needed so as not to traumatize my patients.

A normal result on a CBC done solely to reassure a parent that a child does not have a serious condition like leukemia may provide a false sense of safety, as a well-known pediatric hematologist/oncologist colleague tells me that 50% of children with leukemia have perfectly normal CBC’s at the time of presentation. If your doctor has a strong suspicion that something serious like that is going on, then more detailed tests and evaluations are in order.