Kids often ask doctors about their blood types. (Ozgur Coskun)

No one likes to have his blood drawn, and that is especially true for kids. As soon as I greet one of my patients for a checkup, I am asked two questions. First, “Do I need any shots?” Second, “Do I need any blood tests?”

I am always honest with my patients, but it’s not a good idea to talk about needle pokes at the beginning of a visit. I deal with that by telling kids that a doctor’s visit has three parts: the talking part, the examination part and (sometimes) the getting-poked part. I remind them that I won’t know the answer to their questions until we get to Part 3.

The most common blood test doctors do is a CBC, which stands for complete blood count. The test provides information about red blood cells, white blood cells and platelets. We do CBCs for different reasons, but the two most common are to check for anemia (by looking at red blood cells) and infection (by looking at white blood cells). Platelets are the element in blood that helps it clot if you become injured.

Lots of kids ask me what their blood type is. When I tell them that information isn’t important on a day-to-day basis, they look confused. So here’s the skinny on blood types.

The basic blood groups everyone knows about are A, B, AB and O. In addition, blood can be “positive” or “negative.” O positive is the most common blood type. AB negative is the least common.

The type of blood you have refers to markers on red blood cells — like the street numbers on your house — that allow your body to recognize itself. Except for people with AB positive blood, everyone has proteins in their blood called antibodies, which alert the body to blood types that don’t match. Getting transfused with mismatched blood can cause serious reactions. (A transfusion is a procedure in which a sick or injured patient receives blood into his veins that has been donated from another person.)

What most people (even parents) don’t know is that we also have minor blood groups in our system. That basically means that not all A positive blood is the same. Minor blood group mismatches don’t usually cause serious problems, but they can cause mild transfusion reactions, which should be avoided.

If a person needs a transfusion on an emergency basis, he will always get O negative blood even if his blood type is A positive. That’s because O negative blood has no markers that warn the body that the blood doesn’t match.

You may have heard doctors on TV shows shout the following order to the closest nurse: “Type and cross the patient, and get me six units of O negative blood, STAT!” (“STAT” is doctor talk for “immediately.”)

“Typing the patient” means finding out what type of blood he has. “Crossing the patient” means comparing his blood with similar blood types available in the blood bank. That way, the patient will get the most appropriate blood on hand, including the minor groups.

— Howard J. Bennett

Bennett is a Washington pediatrician. His Web site,, includes past KidsPost articles and other cool stuff.