One of the advantages of skin tests is that results are known quickly. Positive reactions usually appear within 15-20 minutes for both types of skin testing, although patients should expect to remain under observation for at least 30 minutes after testing. Delayed reactions can occur several hours after skin testing, sometimes causing swollen, reddened bumps at the spot where the testing was done. The delayed reaction usually disappears 24 to 48 hours later, but should be reported to the physician. However, when there has been no immediate reaction, delayed reactions do not signify the presence of allergy.