Patch Testing for ACD Overview

Patch Testing for Allergic Contact Dermatitis (ACD)

• Patch testing is the gold standard for identifying ACD allergens.
• The T.R.U.E. Test, introduced in 1995, is an FDA-approved epicutaneous patch test system for adults.
• The T.R.U.E. test system initially had 23 allergens and a control; now, it has 3 panels with 35 allergens and a blank control.
• Nonstandard/expanded patch tests, including testing with patient products and pediatric patients, are common.
• The T.R.U.E. test omits some of the 30 most common positive allergens (fragrance mix II, lanolin, etc).
• More allergens tested generally increases the sensitivity of patch testing.
• Standard patch tests place allergens on the back under occlusion, removed at 48 hours, and read again at 72/96 hours.
• Preliminary readings at 48 hours are sometimes done, but allow enough time for tape irritation to subside. (20 minutes usually sufficient).
• Delayed reactions (metals, antibiotics, corticosteroids) may take longer to show.

Patch Test Protocols

• Standard closed test protocols are used for commercially available standardized allergens and patient products.
• Repeat open application protocols are used to test leave-on products and can last 1-3 weeks.
• Semiopen patch testing is for liquid products by applying to skin and allowing them to dry.
• Test substance is measured in 1 cm2.

Patch Test Reading and Interpretation

• Patch test results are recorded as positive, negative, questionable, or irritant.
• Positive reactions are graded (Wilkinson scale): + (weak, non-vesicular), ++ (strong, edematous/vesicular), +++ (extreme, bullous/ulcerative).
• Questionable reactions (faint/macular erythema): +/-.
• Irritant reactions (IR) have distinct characteristics (erythematous, sharp margins, discrete scaling). Fragrance mix, cocamidopropyl betaine, etc. are frequent irritant triggers.
• Purpuric (petechial hemorrhage) is a specific irritant reaction from cobalt chloride.
• Pustular reactions are a sign of irritant reaction associated with metals (potassium dichromate, cobalt etc).
• Some allergens (formaldehyde, benzalkonium chloride, certain fragrances) have mild irritant potential and should be evaluated cautiously.
• Strong irritant reactions appear early and disappear quickly.
• Strong allergic reactions spread, disappear more gradually, and show a more eczematous pattern.

Patch Testing in Different Skin Types

• Reactions may be harder to evaluate in darker skin types (Fitzpatrick V/VI) due to less obvious erythema; edema & papules/vesicles are still often visible.
• Palpation helps detect reactions.
• Fluorescent marking inks (yellow highlighter) and Wood light are best for marking patch tests in dark skin types.

Relevance of Positive Patch Test Results

• A positive patch test indicates sensitization, but not necessarily ACD.
• Establishing relevance (importance to the patient's dermatitis) is a challenge.
• Relevance can be preliminary, current, past, or doubtful.
• Definitive relevance occurs with a positive use test or suspected object/product patch test.
• Relevance is judged at follow-up visits after avoidance.

Patch Test Safety and Side Effects

• Patch testing is generally safe.
• Common side effects: itching at positive reaction sites, tape irritation/pruritus.
• Less common effects: post-inflammatory changes, infections, scarring, persistent reactions, flare-ups, sensitization, and rarely anaphylaxis.
• Hyperpigmentation is common in darkly pigmented persons and usually fades with time.
• Photosensitivity to allergens with direct light exposure can occur.

ACD Burden and Patch Testing Benefits

• ACD is the third most common consultation reason for dermatologists and a common reason for primary care doctor visits.
• Estimated burden is 72 million people in US with ACD.
• 16% of chronic eczema patients may benefit from patch testing.
• Patch testing is helpful in identifying causes of chronic recalcitrant eczema.
• Approximately 2.2 million US patients could benefit from patch testing annually.

Patch Testing Considerations in Patients on Immunosuppressive Drugs

• Systemic drugs that suppress T-cell responses interfere with patch testing.
• Testing should be done when patients are off these drugs (prednisone, cyclosporine).
• If this is not possible, minimize the dose.