Cold sores have very recognizable features. They are most often diagnosed by physical exam and by your medical history. If your doctor is not sure, a sample of the fluid or tissue from the blister may be taken. A blood sample may also be taken for testing.

Testing may include any of the following:

• Viral culture—A sample of the fluid from the cold sore blisters is taken with a cotton swab. This is done as soon as possible after an outbreak begins. The virus is then grown in the laboratory. This test is very accurate if the sample is taken while there are still clear blisters.
• Tzanck test—The cold sore is lightly scraped to collect cells onto a glass slide. These cells are then examined under a microscope. This test is quick, but not as accurate.
• Tests to detect the presence of antibodies in your blood. These particular antibodies are created by your body to fight herpes simplex virus (HSV) infections. This test may be done when no sores are present.