How useful are lab tests in diagnosing allergies?

A general practitioner can read diagnostic lab reports showing, say, that a patient has allergic antibodies to pollen and nuts, but not to cat dander, or any number of other combinations. However, the bigger question is whether the tests conflict with or support the patient's history, or whether the expense and time involved could have been avoided just by listening to the patient with a trained ear. The science is there, but you don't have to look at all the molecules every time.