What is the pathergy test, its indications, contraindications, and dermatological conditions for which it can be used as a supportive diagnostic tool?

Pathergy Test: Indications, Contraindications, and Diagnostic Applications

The pathergy test is primarily used as a diagnostic tool for Behçet's disease, where it produces a characteristic hyperreactive skin lesion in response to minor trauma, manifesting as a papule or pustule within 48 hours of needle prick. 1

What is the Pathergy Test?

• The pathergy test involves inducing a skin response by needle prick, with positive reactions appearing as papules or pustules at the site within 48 hours 1
• Pathergy refers to the development of new skin lesions or aggravation of existing ones following trivial trauma 2
• There is no standardized method for conducting the test - intradermal, intravenous, and subcutaneous applications are all used in clinical practice 1
• The test is performed by pricking the skin (typically forearm) with a sterile needle, and the site is evaluated after 24-48 hours 3
• Needle size affects results - larger needles (20G/0.9mm) produce higher positivity rates (up to 65%) compared to smaller needles (26G/0.3mm) 3

Indications

• Primary indication: Suspected Behçet's disease - the test is included in diagnostic criteria for this condition 1
• The pathergy test is considered one of the major features and diagnostic criteria for Behçet's disease 1
• The test can be useful in differentiating Behçet's disease from other conditions with similar clinical presentations 4
• May be indicated when evaluating patients with recurrent oral and genital ulcers, uveitis, and other features suggestive of Behçet's disease 4

Contraindications

• No absolute contraindications are specifically mentioned in the literature
• Relative contraindications may include:
  • Active skin infection at the test site
  • Severe immunosuppression where additional skin trauma should be avoided
  • Bleeding disorders or anticoagulant therapy

Dermatological Conditions Using Pathergy Test as Diagnostic Tool

• Behçet's disease: The primary condition where pathergy testing has established diagnostic value 1, 4
• Pyoderma gangrenosum: Pathergy reactions have been reported in this neutrophilic dermatosis 2, 1
• Sweet's syndrome: Pathergy phenomenon has been noted in this neutrophilic dermatosis 2, 1
• Other neutrophilic dermatoses: May demonstrate pathergy phenomenon, though systematic testing has not been well described 2

Diagnostic Value and Interpretation

• Positive test: Development of papule or pustule at the needle prick site within 24-48 hours 1
• The sensitivity of the pathergy test for Behçet's disease has decreased over time, from 64.2% to 35.8% in recent years 5
• However, specificity has improved from 86.6% to 98.4%, making a positive test highly indicative of Behçet's disease 5
• Positive predictive value has improved to 95.7%, meaning a positive test is nearly synonymous with Behçet's disease 5
• The pathergy reaction is associated with male gender, oral aphthosis, pseudofolliculitis, and uveitis in Behçet's disease patients 4

Important Considerations and Pitfalls

• The sensitivity of the test varies significantly by geographic region, with higher positivity rates along the "Silk Road" countries 5
• The test's sensitivity has declined over time, possibly due to the use of disposable needles which cause less trauma than non-disposable ones used previously 3
• Despite decreased sensitivity, a positive test remains highly specific for Behçet's disease 5
• No standardized method exists for performing the test, which can lead to variability in results 1
• The exact pathophysiological mechanism behind the pathergy reaction remains unclear 2, 4
• The test should be interpreted in the context of other clinical features and not used in isolation for diagnosis 4

Clinical Relevance

• A positive pathergy test is associated with certain active clinical manifestations in Behçet's disease, including oral aphthosis, pseudofolliculitis, and uveitis 4
• The test is not associated with disease severity or vascular involvement in Behçet's disease 4
• Understanding pathergy may provide insights into aberrations of skin's innate reactivity and homeostasis 2