Because they have very recognizable features, cold sores are most often diagnosed by physical exam and by your medical history. If your doctor is not sure, she may take a sample of the fluid or tissue from the blister or a blood sample for testing.
Testing may include any of the following:
- Viral culture—With a cotton swab, a sample of the fluid from the cold sore blisters is taken as soon as possible after an outbreak begins. The virus is then grown in the laboratory and identified. 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 to identify them. This test is quick, but is accurate in only 50%-70% of cases.
- Antibody titer (ELISA)—The body creates antibodies to fight an infection. This blood test measures the level of antibodies made by the body to fight herpes simplex virus (HSV). It is about 85% accurate in detecting if you have been infected by HSV. However, it is not used routinely as the only diagnostic test.
- Polymerase chain reaction (PCR)—This test multiplies the number of pieces of the HSV in the test fluid, making it easier to detect. This test is as accurate as a culture, but with quicker results. It is expensive, however, and not commonly used.
Beers MH, Fletcher AJ, et al.
Merck Manual of Medical Information. 2nd ed. Whitehouse Station, NJ: Merck Research Laboratories; 2003.
Cold sore. Mayo Clinic website. Available at:
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Herpes simplex. American Academy of Dermatology website. Available at:
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Miller C. Oral herpes/cold sores essential facts. Australian Herpes Management Forum website. Available at:
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