CBC or not CBC? That is the question.

The complete blood count (CBC) is the laboratory test most frequently ordered by emergency physicians. This is often not cost effective because of the relative inaccuracy of the white blood cell (WBC) and differential cell counts. The WBC and differential counts do not reliably distinguish between bacterial and viral infections because they lack specificity in many patients, nor do they reliably correlate with the severity of disease because they lack sensitivity in some patients. This article briefly summarizes the laboratory, physiologic, and pathophysiologic causes of variations in the WBC and differential counts; reviews some of the literature on pediatric patients with fever and appendicitis and on adult patients with appendicitis and abdominal pain; and presents suggestions for future research into the reliability and cost effectiveness of the CBC.