Two teaspoons of wine or a mouthful of beer may be enough. In a few cases, a test called a food challenge may also be used to diagnose a food allergy. During this time, you should avoid bathing and activities that cause heavy sweating. The patches are removed when you return to your doctor’s office. If severe eczema or psoriasis affects large areas of skin on your arms and back — the usual testing sites — there may not be…