How Patch Testing Works

What Is Patch Testing?

Patch testing is a specialized diagnostic procedure performed by dermatologists to identify specific allergens causing allergic contact dermatitis. It is different from skin prick testing (used for hay fever and food allergies) — patch testing is designed to detect delayed hypersensitivity reactions (Type IV).

What to Expect: The Process

• Day 1 — Application: Standardized allergen panels (typically 80–120 substances) are applied in small chambers to your upper back and covered with hypoallergenic tape
• Day 3 (48 hours) — First reading: Patches are removed. Your back is examined for reactions. Some reactions may already be visible.
• Day 5 (96 hours) — Final reading: Most important read. Delayed reactions (which appear later) are recorded.
• Day 7 (optional): Some allergens (metals, corticosteroids) may require a late read at 7 days.

What's on the Panel?

The NACDG (North American Contact Dermatitis Group) standard series contains 80+ allergens representing the most common culprits in North America. These include metals, fragrances, preservatives, rubber accelerators, dyes, and more. Additional specialized panels exist for hairdressers, healthcare workers, dental workers, and other occupational groups.

Reading the Results

• + (weak positive): Faint redness and infiltration
• ++ (strong positive): Papules, edema, vesicles
• +++ (extreme positive): Bullae, coalescing vesicles
• IR (irritant reaction): Not a true allergy — must be distinguished from + reactions
• NT (not tested)

Relevance assessment is critical — your dermatologist will determine whether each positive result is currently relevant to your symptoms.

Preparation

• Avoid topical steroids on the test site for at least 7 days before testing
• Systemic steroids >10mg prednisone may suppress reactions — discuss with your doctor
• Avoid UV exposure/tanning of the back for 2 weeks before testing
• Keep the patches dry and do not engage in sweaty exercise during the 48-hour application period