The cat scratch disease (CSD) antigen, which consists of sterile pus derived from the lymph node of a human known to have CSD, is injected under the skin to help determine if you have been infected with the disease.

The antigen is injected just under the skin, usually on the forearm, so that a small lump pushes the skin up. The lump indicates that the antigen has been injected at the correct depth. The site is labeled, usually with a pen.

After 48 - 72 hours, a medical provider will check the site of the injection to determine whether there has been a reaction.

There is no special preparation. People with dermatitis or other skin irritations should have the test performed on an area of skin where no irritation is present.

When the antigen is injected, you may feel a stinging sensation where the needle is inserted. After the reaction begins, the area may itch or burn.

This test was used more extensively in the past, before Bartonella henselae, the bacteria that causes CSD, was identified.

There should be less than a 5 mm inflammation zone around the injected disease antigen.

An inflammation zone of greater than 5 mm may indicate that you have been infected with cat scratch disease recently or in the past.

Because the antigen is obtained from sterile pus derived from human lymph nodes, there is a small potential risk of transmission of viruses. Also, there are occasional local reactions that may occur, such as itching and (rarely) hives.

Although this test has historical value -- it was used in the past as one of the keys to diagnosis -- there are better tests available for the diagnosis of CSD. In addition, the CSD antigen has not been standardized, is not widely available, and carries the potential risk of transmitting other pathogens such as viruses.