Prick testing is done first, and the results are read 15 to 20 minutes later. If the prick test findings are negative, intradermal testing is performed.The skin test is positive if the major or minor determinant tests have a wheal that is larger than 3 mm. the wheal produced by the negative saline control. Both prick and intradermal tests are done using diluted penicillin G at a concentration of 10,000 u/ml, Pre-pen at full strenght and minor determinant mixture if available. If skin test for penicillin using major determinants (benzylpenicilloyl), a mixture of minor determinants and pencillin G is negative, up to 99% of patients will tolerate penicillin. If skin testing using benzylpenicilloyl and penicillin G (as the sole source of minor determinants) is negative, approximately 97% of patients with a negative skin test will tolerate penicillin. However, a few patients who are at risk for anaphylactic reaction will be missed with this testing method because penicillin G does not contain all the minor determinants