Blood counts or CBCs are commonly ordered for babies under two months with fever. The WBC or white blood cell count has been thought to be a good test.
High or low white blood cell counts with increase in the number of neutrophils or cells which rise during infection, would lead the doctor to order other tests such as a urine probe, Chest xray and at times a spinal tap. Serious infections such as meningitis, pneumonia and sepsis can then be diagnosed and treated quickly.
Like many things in medicine and for that matter in life, what was thought to be true turns out to be not necessarily the case.
A recent study cast significant doubt on the value of the CBC test. In this study, infants younger than 60 days who turned out to have a serious infection did not always have high or low white blood counts. In fact about 20% of infants had normal counts!
Other tests which measure inflammation called inflammatory markers have been developed. These include procalcitonin and C reactive protein. These studies will undoubtedly help the doctor decide whether a baby with a fever just has a virus or something more serious.
In my mind just as valuable is experience. In other words a physician in practice for several years will usually know by looking at the baby whether or not this baby is sick and needs more testing.
So both newer tests and intuition are helpful tools to make important decisions.