Environmental allergies can be tested through skin tests or blood tests. Skin tests are more sensitive than blood. Correlation with the clinical history is important to assess the relevance of the test results. Food allergies can also be tested through skin or blood tests, but there are times when the allergy is unclear and a food challlenge is necessary. Food challenges use the actual food in incremental doses to look for objective signs of allergy. Contact allergies are diagnosed through Patch skin tests using materials on the skin that are worn for 2 days.