How can I figure out what my baby is allergic to?

It takes some careful detective work and sometimes the help of medical tests to pinpoint the exact cause of an allergy. One clue may be when the allergy attacks occur.

Mold allergies usually develop during damp or rainy weather and can be hard to distinguish from colds. Dust mites or pet allergies often cause morning congestion throughout the year. Pollen-related allergies are more common in spring, summer, and fall.

Unfortunately, sending Fluffy away for a few days won't tell you for sure whether your baby has a pet allergy. Studies at Johns Hopkins University suggest it can take more than a year for cat dander to degrade in a room, for example.

On the other hand, if you take your baby away from the pet (on vacation, for example) and she seems better, then you have a good — but by no means conclusive — lead. (You'd also want to consider that your baby might be allergic to something else in your house.)

If your own sleuthing doesn't give you the answer, it's time to see the doctor. He'll examine your baby and ask lots of questions. If he believes the problem is allergies, he may refer you directly to an allergic specialist or he may suggest a blood test to measure levels of IgE (allergy) antibodies in your baby's blood.

Blood tests may be less accurate than skin tests, especially for infants. So if the blood test does suggest an allergy, the next step for your little sniffler is a skin test. You'll need to see an allergist for that.

During a skin test, an allergist applies small amounts of common allergens to your child's skin. If your baby is allergic to a substance, she'll have a reaction (similar to a mosquito bite) on that spot. Infants may have smaller reactions than older children, but the tests can still be very useful.

"Keep in mind that testing tells you what your child is allergic to at that point, but it may change as your child gets older," says Seattle pediatric allergist Frank S. Virant. If your baby has a negative skin test but continues to have allergy symptoms, have her reevaluated in six to 12 months.