Blood Tests

Testing the patient's blood offers additional information that is somewhat useful in studying asthma. Blood tests are not nearly as useful as lung function testing in diagnosing and following the course of asthma. Routine counting of the blood cells is not helpful except to call attention to a serious infection that might otherwise be overlooked.

In some patients with asthma caused by allergies, one type of cell (the eosinophile) may be unusually abundant. A high number of eosinophiles in the blood might make a doctor think of allergies as a cause of asthma, but is otherwise not helpful. There is little information otherwise to be gained by a "complete blood count". Nor are the usual "blood chemistry" tests helpful except perhaps to suggest some unsuspected condition coexisting with asthma.

There is other information to be gained from the blood. The amounts and percentages of oxygen and carbon dioxide in the blood are important criteria for thinking about the severity of an episode of asthma. These results are of great significance to the doctor. For example, one test compares the percentage of oxygen carried by the blood with the percentage it could carry at best. This measurement is called oxygen saturation. Doctors show concern about the condition of a child whose oxygen saturation is less than 91%.