Patch Testing

Patch testing applies small amounts of possible allergens in chambers taped to the back for 48 hours. The skin is then read again at day 3–7 for a delayed (type IV) eczema-like reaction. It is the only way to diagnose Allergic Contact Dermatitis (ACD), and it tests a different immune pathway from Skin Prick Testing, which detects immediate IgE sensitisation. Tam & Yu (2022) call it “the gold standard in diagnosis, evaluation, and management of ACD” in patients of all ages. ACD in children has two early peaks, at 0–3 years and 6–7 years. Positive patch test rates in children are comparable to adults. ACD is probably under-recognised in infants and toddlers.

Patch Testing [defines] Allergic Contact Dermatitis Patch Testing [relates] Skin Prick Testing

When it is indicated in eczema. Tam & Yu’s indications table for children lists: suspected ACD; persistent dermatitis or an atypical distribution (head/neck, hands/feet, nappy/perineal area, shins); late-childhood or adolescent-onset AD; dermatitis that is treatment-refractory; and testing before starting systemic medicines. NICE CG57 lists suspected contact allergy as a reason for referral (When to Refer), and Türe Avcı (2026) lists patch testing among the investigations to consider when “eczema” does not respond to treatment (Differential Diagnosis of Toddler Eczema). For toddlers the typical prompts are eczema that worsens where a cream, wipe or fragrance is applied, or a rash that keeps spreading despite correct Topical Corticosteroids. Children with Atopic Dermatitis are patch-tested earlier in life and are more often positive to emollient and personal-care ingredients: Lanolin, cocamidopropyl betaine, tixocortol pivalate (a steroid marker) and parthenolide (Compositae Allergy).

Allergic Contact Dermatitis [worsens] Atopic Dermatitis Patch Testing [relates] When to Refer

Series used. The T.R.U.E. Test (35 allergens) is FDA-approved only for ages 6–17. It misses important paediatric allergens such as cocamidopropyl betaine, propylene glycol, fragrance mix II and decyl glucoside, and it can give false negatives for MCI/MI in up to 37% of cases. The Pediatric Baseline Series (2018; 38 allergens plus 2 of the clinician’s choice) detects more positives than T.R.U.E. but is designed for children over 6. No commercial panel carries an FDA indication under 6 years. Toddlers are therefore tested with a customised panel chosen from their history, plus the family’s own products (“as is” or diluted). Specialised nappy-area series also exist. Extra allergens matter for “natural” remedies: fragrance mixes miss many Essential Oils allergens, so the actual oil and oxidised tea tree oil, plus linalool and limonene hydroperoxides (Fragrance Allergy), should be added (Barbaud 2023). For suspected lanolin allergy, test lanolin alcohol 30% plus Amerchol L-101 50%, and use a ROAT (Repeat Open Application Test) with the child’s own product.

Essential Oils [causes] Allergic Contact Dermatitis Lanolin [causes] Allergic Contact Dermatitis Fragrance Allergy [part-of] Allergic Contact Dermatitis

Practicalities in a 1–4-year-old. A 2–4-year-old’s back fits about 40–45 allergens, compared with 40–60 at age 6. Tam & Yu’s group uses standard adult concentrations and contact times at all ages, because evidence for diluting or shortening tests in under-5–8s is limited. Patches are held with hypoallergenic tape (Scanpor), reinforced with Hypafix for active children, and cartoon distraction helps during application. The family has to keep the back dry and stop the child scratching or peeling the patches for 48 hours, and attend 2–3 visits. Results are blunted by phototherapy within 1–2 weeks (up to 6 weeks in conservative advice) and by high-dose systemic steroids. Active, uncontrolled AD produces more false negatives, so testing works best once the eczema is calmer. Dupilumab (licensed from 6 months) generally does not stop testing from giving reliable results. After testing, families can be given safe-product lists (ACDS CAMP) and allergen-avoidance cards (P.E.A.S.), which are estimated to prevent about a third of paediatric ACD.

Phototherapy [contradicts] Patch Testing Dupilumab [relates] Patch Testing

Connections

• Allergic Contact Dermatitis — only diagnostic test; indications and PBS, source: Tam & Yu 2022
• Essential Oils — test own oil + oxidised tea tree oil, source: Barbaud 2023
• Lanolin — test lanolin alcohol 30% + Amerchol L-101 50% + own products; ROAT, source: Cutis 2023
• Fragrance Allergy — FM I/II, balsam of Peru + linalool/limonene hydroperoxides, source: Tam & Yu 2022; Bonamonte 2022
• Compositae Allergy — SL mix + parthenolide + Compositae mix + dandelion, source: Paulsen 2013, 2020
• When to Refer — suspected contact allergy is a routine referral criterion, source: NICE CG57
• Differential Diagnosis of Toddler Eczema — used when contact allergy is suspected, source: Türe Avcı 2026
• Phototherapy — UV within 1–2 (up to 6) weeks blunts patch reactions, source: Tam & Yu 2022
• Dupilumab — patch testing generally still reliable on dupilumab, source: Tam & Yu 2022
• Skin Prick Testing — different (IgE, immediate) pathway; not interchangeable, source: Tam & Yu 2022
• Repeat Open Application Test — adjunct with the child’s own product, source: Tam & Yu 2022