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13.1 Introduction

The tropical climate is hot and humid throughout the year. The ambient temperature is usually >25 °C and humidity >70 % throughout the year. There is very little seasonal variation. Patch testing in the tropics becomes a challenge to dermatologists because the application of occlusive patch test allergens with sticky tapes can cause discomfort to the patient, and adhesive tapes used for holding the test chambers in place often get displaced because of sweating. In this chapter, pitfalls and tips on patch testing in the tropics are presented.

13.2 Conducting Patch Testing Procedure in the Tropics

13.2.1 Ideal Patch Test Season

As there is little climatic variation through the year in the tropics, there is no ideal season to perform the patch test. Patch testing is carried out throughout the year. However, patch testing can be an uncomfortable procedure for the patients in the tropics because the ambient heat and high humidity cause sweating, making it difficult for adhesive tape to remain on the skin. Patients should be counseled about the procedure before it is carried out.

13.2.2 Ideal Patch Test System

The Finn chambers or the IQ chambers appear to be the most suitable patch test chambers for tropical use. Scanpore or Micropore tapes appear to be suitable adhesive tapes in the tropics because they are strong and pliable and porous. The T.R.U.E. Test is also a suitable system for the tropics but is limited by its cost and limited range of allergens.

13.2.3 Patch Testing Procedure

The high ambient temperature and humidity in the tropics preclude some modification in our standard patch testing procedure.

  1. 1.

    Patients should be advised to cease all vigorous outdoor activities while undergoing the patch testing procedure. Those engaged in indoor duties should also be advised to remain in a cool and well-ventilated environment to minimize sweating.

  2. 2.

    Patients should be allowed to shower after application of the patch test chambers, but should be instructed to keep the patch test areas dry. The patient may be allowed to wipe the surrounding skin of patch test area with moist towels.

  3. 3.

    Patients should be advised to return to the clinic to reinforce the adhesive plaster when they are seen to be displaced.

  4. 4.

    After the removal of the chambers at 48 h, patients should be instructed to continue to keep the patch test area dry, but the surrounding skin can be cleaned with a moist towel, until the final patch test reading at 96 h.

  5. 5.

    Skin marking is preferably done with special marking inks. We found the preparation containing gentian violet 1 %, methyl alcohol 50 %, silver nitrate 20 %, and distilled water 29 % w/w to be most lasting. However, the preparation may cause skin irritation. Freshly prepared ink may be preferred, as the ink constituents become too concentrated as the solvent evaporates over time. Commercial skin markers and fluorescent markers do not stay long on the skin due to perspiration and humidity in the tropics.

  6. 6.

    For patients with dark skin, it may be necessary to examine the patch test area in a brightly lit room.

13.2.4 Occlusion Duration

In the tropics, it is preferred that the duration of the patch be as short as possible, as the adhesive tapes tend to get displaced because of sweating and thereby affect optimal occlusion. Ideally, patch testing is done on a Monday and reading done on the following Wednesday and Friday. Under such circumstances, it may be necessary to provide a medical certificate to the employer to exempt the patient from outdoor activities and/or strenuous activities for the first 3 days to ensure that the adhesive/test chamber remains in place.

A good alternative is to carry out the patch test over a weekend (e.g., on a Friday) so that the patient can remain indoors in a cooler environment at home. The patient can then be instructed to remove the patch test chambers themselves on a Sunday (after 48-h occlusion) and return on the following Monday (72-h reading) or on a Tuesday (96-h reading). This will omit the 48-h reading. This protocol suits most patients very well in the hot and humid tropical climate without severely affecting their work.

There have been a few studies to ascertain if the duration of occlusion of patch test allergens can be shortened to less than 2 days’ duration without affecting the diagnostic accuracy. However, the studies have indicated that although the overall concordance of results after 24 and 48 h of occlusion is high, clinically relevant allergens would have been missed if only the 24-h occlusion test was performed. In light of these results, it is recommended that the standard 48-h application remains appropriate for diagnostic patch testing [1].

13.2.5 Patch Test Reading

Most patients in the tropics have darker skin phototype, viz., skin types IV–VI. For patients with fair skin types II–IV, a positive patch test reaction is easy to identify. The erythema, papules, and mild edema and vesiculation are usually obvious. In the darker skin types V and VI, however, a mild positive allergic reaction may be overlooked as the erythema may not be visible. However, the edema, papules, and vesicles are usually discernible by palpation. Hence, when reading patch test reactions in dark-skinned individuals, it should be carried out in a well-lit room; palpation of the patch test site may help to detect features of an allergic patch test reaction.

13.2.6 Storage of Patch Test Allergen in the Tropics

Most of the patch test allergens are suspended in petrolatum as a vehicle. Petrolatum liquefies at high ambient temperature and affects the homogeneity of patch test allergen. In the tropics, where the ambient temperature is high, it is imperative that the patch test allergen tubes be stored in a cool place, preferably in a special refrigerator. In a study, it was found that nickel sulfate and potassium dichromate patch test materials become less homogeneous if stored at room temperature in a tropical country compared to storage at 4 °C. It would appear that the patch test materials should be stored in a refrigerator in between use when in the tropics [2].

13.3 Unique Contact Allergens in the Tropics

13.3.1 Topical Medicaments and Traditional Medicine

The prevalence and cause of allergic contact dermatitis vary in tropical countries. However, topical medicaments and herbal products are more commonly used in the tropics, especially in Asian countries [3]. Self-medication with over-the-counter (OTC) medicaments is a common practice in the tropics. Some of these OTC remedies can cause contact allergy and should be considered for patch testing [4–6]. However, the exact ingredients of such OTC products are often unknown, making it difficult to ascertain the exact causative allergen in the allergic contact dermatitis. Well-known allergens include proflavine, nitrofurazone, tea tree oil, and Chinese herbal medication. These substances should be included for patch testing when investigating patients with suspected allergic contact dermatitis in the tropics.

13.3.2 Plant Dermatitis in the Tropics

In tropical Asia, a group of plants referred to locally as “rengas” are a common cause of allergic phytodermatitis. “Rengas” is a name derived by the indigenous people in Southeast Asia where the plants flourish. It consists of four genera of plants, namely, Gluta, Melanochyla, Melanorrhoea, Semecarpus, and Swintonia. All belong to the Anacardiaceae family. Injured bark of these plants secretes a toxic resinous sap that blackens when exposed to the air and becomes a resin that is notorious for causing allergic contact dermatitis. The chemical nature of this resin is unknown. It is believed to be a potent skin irritant and sensitizer. Carpenters and users of furniture are known to risk sensitization [7].

Exotic woods from tropical and subtropical regions such as South America, South Asia, and Africa (e.g., Dalbergia nigra [Rio rosewood] and Machaerium scleroxylon [Pao ferro]) frequently are used occupationally and recreationally by woodworkers and hobbyists. These exotic woods more commonly provoke irritant contact dermatitis reactions, but they also can provoke allergic contact dermatitis [8].

Another plant known to cause phytodermatitis in the tropics is the mango plant (Mangifera spp.). It too belongs to the Anacardiaceae family. Outbreaks of dermatitis from the plant saps often occur during the fruiting seasons. The allergen comes from the sap of the stem. The exact allergen remains unknown.

Clinicians must be aware of the potential for allergic contact dermatitis reactions to compounds in rengas plants, exotic woods, and mango saps. Patch testing should be performed with suspected woods and plant components for diagnostic confirmation.

13.3.3 Cosmetic Dermatitis

Characteristic allergic contact dermatitis in the tropics from cosmetics is seen in Hindu females who developed pigmented contact dermatitis from a red dye (kumkum) that is painted on their forehead. One of the causative allergens is Sudan I, which was previously reported to cause outbreaks of pigmented contact dermatitis in Japan. A similar practice of pasting red sticky paper on the forehead instead of the red dye powder has been reported to cause allergic contact dermatitis from the PTBP resins [9].

Practical Tips

  • There are observed differences in the epidemiology of contact dermatitis in the tropics and temperate countries.

  • Due to cultural differences, contact dermatitis from self-medication and use of herbal preparation is more common in the tropics, necessitating inclusion of these allergens for patch testing.

  • The high ambient temperature and humidity in the tropics throughout the year exclude any ideal season for patch testing.

  • Because of the high ambient temperature and humidity, some modification in the patch test procedures is required to ensure that the occlusive effects of patch testing are maintained and that patients comply with the procedure. Special markers and allowing patients to clean themselves are necessary.

  • It is necessary to store patch test allergens in refrigerators in between use to maintain homogeneity of test allergens.