Patch testing in India: Are we missing clinically relevant allergens?

Specific challenges encountered by Indian dermatologists in the effective implementation and interpretation of patch testing

a) Static patch testing series despite the evolving allergen landscape: India’s consumer market has expanded dramatically in recent times. The proliferation of skin and nail cosmetics, hair dyes, personal care products, synthetic jewellery, herbal preparations, medical devices and occupational chemicals has altered the allergen exposure profile significantly. However, the ISS has not adapted accordingly. Originally developed by CODFI (Contact and Occupational Dermatoses Forum of India) in 1997, it has not undergone regular updates, thereby limiting its current relevance in the context of evolving exposure patterns.¹ Furthermore, while most global series encompass markers for both occupational and cosmetic exposures, the ISS lacks sufficient coverage for allergens prevalent in informal occupational sectors like construction and domestic work. Paraphenylenediamine (PPD), fragrance mix, paraben mix, thiomersal, nickel sulphate, and potassium dichromate are the common sensitisers reported in Indian studies.² But numerous emerging allergens like the isothiazolinones, formaldehyde-releasing preservatives (FRs), acrylates, hydroperoxides of linalool and limonene, sunscreen ingredients and PPD-substitutes are missing from the Indian Standard Series (ISS), leading to a mismatch with real-world exposures [See also Table 1].^3-10 A frequent clinical challenge occurs when patients with clear contact allergy histories have negative ISS test results.

b) Lack of systematically updated data on patch testing trends: Patch test series require regular revision to eliminate obsolete allergens and incorporate emerging sensitisers relevant to current exposure patterns. The ISS kit includes 20-24 allergens (depending upon the manufacturer) and has not undergone periodic revisions. In comparison, the European Baseline Series (EBS) has 32 allergens and was last updated in 2019 with more additions recommended in 2023; while the North American Contact Dermatitis Group (NACDG) Baseline Series employs 70 allergens, and every 2 years the NACDG collects and reports the changes in patch testing trends, on the basis of which series is modified.^11-14

c) Lack of standardisation of ingredients & inaccurate product labelling of regionally relevant products: Products like kumkum/vermillion/alta/henna/herbal oils/”thailams” (ayurvedic oils)/topical Ayurveda-based products are usually indigenously manufactured and frequently lack standardisation of ingredients and product labelling.^15-17 Many commercial hair dyes contain potent sensitisers like PPD and related derivatives, yet are labelled as herbal products, thereby creating a misperception of safety among consumers. Although most of these are frequent causes of ACD, these culprit allergens continue to remain under-researched and, thus, under-represented in the ISS.

d) Lack of clinical relevance of high patch test positivity for some allergens: Thiomersal is used as a preservative in vaccines, eye drops, cosmetics and tattoo inks. Although it frequently yields positive results on patch testing with the ISS, its clinical relevance remains questionable.¹⁸ It is believed that its high positivity rates (up to approximately 20%) arise due to sensitisation from thiomersal present as a preservative in vaccines administered during childhood/adolescence.¹⁹ Many countries have discontinued this preservative, but most Indian-manufactured multi-dose vials still contain thiomersal.^20-22 Thiomersal has been omitted from many standard patch test series, including the EBS and the NACDG Baseline series, recognising that positive patch test reactions to it are often not clinically relevant.¹⁹ It remains one of the least likely chemicals to cause contact allergy and was named the Non-allergen of the year 2002.²³ However, most Indian studies on ACD continue to report it as a prevalent allergen in India. Such conundrums necessitate the use of a system to determine the clinical relevance of positive patch test reactions. The NACDG grades relevance as current, past and unknown.²⁴ Current relevance is further sub-divided into definite (positive confirmatory use test or challenge with the suspected product or a positive patch test with the product), probable (if the substance identified by patch testing could be verified as present in the unknown skin contactants of the patient) and possible (patch test is positive and patient is in contact with materials that likely contain the suspected allergen).²⁴ The COADEX system is a structured framework developed to determine the clinical relevance of a positive patch test reaction [see Box 1].²⁵ These systems help clinicians move beyond just recording “positive” reactions and instead interpret whether the result is meaningful for the patient. It also aims to reduce overdiagnosis and unnecessary allergen avoidance advice.

e) Limited availability of supplemental patch testing series: In many institutions, only the ISS is accessible, while supplemental series such as the Cosmetic & Fragrance Series (CFS) or the Footwear Series are often unavailable. This restricts the ability to detect relevant allergens, particularly in cases of ACD related to cosmetics, footwear or occupational exposures. Use of supplemental patch test series based on medical, environmental and occupational history is critical for correct diagnosis, management and prevention of ACD.²⁶ In addition, the reluctance to perform and learn the interpretation of patch testing with patients’ own materials by many dermatologists further reduces the likelihood of identifying causative allergens.

Clinical implications of these shortcomings in patch testing in India

Despite a strong exposure history, patch test results may show up as false negatives due to the dependence on limited and/or outdated panels. This contributes to missed diagnoses, delays in management, repeated flares and eventually compromised quality of life. The lack of relevant occupational and cosmetic allergens limits the utility of ISS in occupational and cosmetic ACD, a challenge compounded by the inconsistent availability of the supplementary cosmetic, fragrance and footwear series. Conversely, positive results that are clinically irrelevant may complicate interpretation and disease management. In addition, there may be potential medico-legal implications of misdiagnosis or missed diagnosis, particularly in occupational CD. Several market analyses have highlighted the mislabelling and underreporting of PPD concentrations in so-called “herbal” or “natural” hair colourants, highlighting the urgent need for stricter regulatory review and increased consumer awareness.^27,28

What is the way forward?

The impetus for meaningful change has to come from dermatologists with advocacy for regulatory amendments in the manufacturing industry. To address the gaps, we propose the following solutions:

• Establishment of a national registry or patch test database to track trends and provide appropriate background information for emerging allergens

• Multicentric epidemiological studies on the identification of trends in ACD

• Periodic revision of the patch test series (3-4 yearly) based on epidemiological data to formulate a dynamic and data-driven series

• Digitisation and sharing of data among healthcare providers while ensuring patient privacy

• High-quality surveillance and reporting

• Integration of data from cosmetic/occupational exposure registries, cosmeto-vigilance studies and reports of newer/outdated/region-specific allergens

• Resource allocation towards the procurement of all available patch test series in institutional budgets

• Ensuring the availability of a supplemental patch test series in dermatology set-ups

• Raising awareness among the general population regarding mislabelling of products/use of safer alternatives to known allergens

• Regulatory amendments for the industry

• Interim measures - Until data is available to formulate a revised series, dermatologists must be encouraged to use extended series or add patient-specific allergens (e.g., cosmetic ingredients or workplace materials) when clinically indicated.

The ISS, though historically valuable, no longer reflects the evolving allergen landscape in India. Many relevant allergens from cosmetics, footwear and occupational sources are missing, while outdated ones remain. The discrepancy between test panels and real-world exposure undermines both diagnostic accuracy and patient care. There is an urgent need for national dermatology bodies, industry stakeholders and clinicians to collaborate in updating and revising the standard series, so it functions as an effective baseline rather than an outdated benchmark.