Prick technique

The most common allergy test involves pricking the skin to introduce small amounts of allergen extracts; if your child is allergic to any of the substances, a hive will appear. Results are available almost immediately, and they're moderately reliable (negative results are usually accurate, but positive ones can be inconclusive). On the other hand, sticking a baby with a sharp implement is no fun for anyone.

Blood test

In this, also called the radio-allergosorbent (RAST) test, a blood sample is analyzed for the IgE antibodies that indicate an allergy. For young children, this is usually the easiest option, but results can take weeks. Plus, false positives are common: Foods with similar proteins (say, shrimp and lobster) may cross-react, meaning your child will test positive to both even if she's allergic to only one. Kids with eczema may also have high overall levels of IgE, creating "background noise" that can lead to lots of low-level false positives.

Food challenge

The most reliable way to test for food allergies is to feed suspect foods to your child in a controlled medical setting, then watch to see what happens. But this method is time-consuming, expensive, and hard with very young children.

Navigating the Medical Maze

If your kid's main symptom is a rash, see a pediatric dermatologist for a first opinion. Allergists always suspect allergies; a dermatologist may have other ideas. Second, when you do see an allergist, require that she explain, in detail, what test she wants to use and why. Once the results are in, ask that each outcome be explained separately. Take notes. Look for a diagnosis that admits shades of gray. All tests are fallible, so the results should be understood as approximations, not absolutes. Finally, ask for advice that factors in the psychosocial aspects of eating, since allergies have social consequences, not just medical ones.