Pathergy test

Pathergy test is a medical diagnostic test used primarily to detect a hypersensitivity reaction of the skin. It is often associated with certain autoimmune diseases, most notably Behçet's disease, a chronic condition that affects various parts of the body, including the mouth, genitals, eyes, and skin. The pathergy phenomenon is characterized by the development of a sterile pustule or papule at the site of a minor skin injury, such as a needle prick.

Procedure

The pathergy test is performed by introducing a minor trauma to the skin, typically on the forearm, using a sterile needle. The procedure involves lightly pricking the skin's surface without introducing any substance into the skin. After 24 to 48 hours, the site is examined for signs of a reaction. A positive test result is indicated by the presence of a pustule or papule at the site of the needle prick, suggesting an abnormal inflammatory response to minor skin trauma.

Clinical Significance

The pathergy test is most commonly used in the diagnosis of Behçet's disease, where it serves as one of the diagnostic criteria. A positive pathergy test supports the diagnosis of Behçet's disease but is not required for diagnosis, as the sensitivity of the test varies widely among different populations. In some regions, particularly in the Middle East and Japan, the test has higher sensitivity, while in other areas, such as Northern Europe and the United States, the sensitivity is significantly lower.

The pathergy reaction is considered a manifestation of neutrophilic dysfunction, which is a hallmark of Behçet's disease. However, the exact mechanism underlying the pathergy phenomenon remains unclear. It is thought to involve an abnormal immune response, possibly triggered by an external agent, leading to excessive inflammation and tissue damage.

Limitations

The pathergy test has several limitations. Its sensitivity and specificity can vary significantly among different populations and laboratories, depending on the technique used and the criteria for a positive test. Additionally, the test may yield false-positive results in other conditions characterized by skin hyperreactivity or can be influenced by factors such as the depth of the needle penetration and the individual's skin type.

Conclusion

The pathergy test remains a useful tool in the diagnostic evaluation of Behçet's disease, particularly in populations with a high prevalence of the disease. However, due to its variable sensitivity and specificity, the test should be interpreted in the context of the overall clinical picture and other diagnostic criteria for Behçet's disease.