Diagnostic Testing

Patch Testing

The patch test is a valuable clinical tool to establish the diagnosis of allergic, not irritant, contact dermatitis. Approximately 80% to 90% of all cases of contact dermatitis are irritant, not allergic. Because irritant contact dermatitis is influenced by the chemical nature, quantity of substance (concentration, frequency, duration), and the nature of the contact with the skin (inflammation, skin temperature), positive patch tests to irritant may be produced in most individuals. Irritant patch test reactions only indicate that a particular substance, under conditions of occlusion against a skin surface for 24 hours, is capable of causing skin inflammation. For example, oil may be used in a patch test and may produce inflammation within 24 hours, but in actual working conditions the oil contact may be dilute and transitory. Conversely, a weakly irritant substance such as alcohol may produce no inflammation under testing conditions but in an agricultural setting may cause skin drying and eventual inflammation. Therefore, the diagnosis of irritant dermatitis is based on exclusion, a reasonable index of suspicion, knowledge of the physical properties of the chemical, and an understanding of the agricultural workplace (20).

Patch testing is usually performed for one or more of the following reasons:

1. Precise identification of an allergen

2. Facilitation of management

3. Guidance in rehabilitation and return to work (20,21)

Routine patch test screen kits are aimed at the identification of the most common cutaneous allergens. These kits have been standardized so that only allergic individuals react to patch testing. The TRUE Testâ„¢ panels, which are the only patch testing devices approved by the U.S. Food and Drug Administration, consist of 24 patches, one of which is a negative control. The remaining 23 patches contain 42 unique allergens and four complex mixtures (21,22).

Nonstandardized substances taken directly from the workplace present a special problem and should not be tested unless the physician has had a great deal of experience in testing (8,9,20).

Patch testing is especially useful in documenting contact dermatitis from cosmetics, fragrances, and botanicals. Because workers in agriculture can be employed in the growing, harvesting, and processing of natural fragrances and botanicals, a patch test has the potential of confirming or ruling out the potential source of a worker's rash as industrial or nonindustrial in origin (23).

Theoretically, the positive allergic test will develop only in exposed individuals and not in unexposed controls. When testing with nonstandardized workplace substances, as many as 20 or more controls may need to be used before a positive reaction indicates allergy and not a false-positive irritant reaction. A positive reaction is interpreted as an area of inflammation (erythema and induration) on the skin where the controls have none (20).

The material to be tested can be in either solid or liquid form. When liquids are tested, they should be placed in a relatively inert vehicle such as petroleum, water, or mineral oil. The concentration should be sufficient to elicit allergy but not to cause irritation. Some industrial chemicals are not appropriate for testing because they are too irritating; others must be diluted to concentrations of 1:100 or 1:1,000 for testing. The material is placed in chambers and taped to the patient's upper back or upper-outer arm. They remain in place for 48 hours and are read at 48, 72, or 96 hours after application (20-23).

The International Contact Dermatitis Groups has suggested the following terminology for reporting patch-test results: NT, not tested; ?+, doubtful reaction; +, weak reaction (nonvesicular); ++, strong reaction (edematous or vesicular); and +++, extreme reaction. "IR" represents an irritant reaction and "ph" placed before any of the above indicates a photoreaction (20). False-positive patch tests can occur when:

1. The test concentration is too high.

2. There is a failure to run controls.

3. Testing is done on inflamed skin.

4. There is generalized, widespread eczema.

5. There are multiple strongly positive reactions, the allergen is contaminated, and an irritant vehicle was used.

6. There is the incorrect assumption that the allergen is actually present in the work environment (8,9,21).

False-negative reactions occur when:

1. Test concentrations are too low.

2. There are deviations from the standard testing technique.

3. There is failure to test all potential environmental exposures.

4. The wrong vehicle is used.

5. The substance is a photosensitizer.

6. There is an incorrect assumption that the allergen is not in the work environment (8,9,20,21).

Inappropriate testing may sensitize a worker to a substance to which he was not previously allergic. There may also be localized complications of the site, including pigment changes, keloid formation, scarring, infections, and a flare of generalized eczema. The strip patch test is useful in testing for substances with poor percutaneous penetration. Penetration of the substances is enhanced by repeated applications of adhesive tape prior to their application to the skin (20-22).


The surgical removal of skin can be diagnostic as with punch biopsies or inci-sional biopsies or curative as with an excisional biopsy. Written consent must be obtained, and sterile conditions maintained. Proper wound management is essential. The specimen needs to be evaluated by a pathologist or competent dermatologist. Biopsies are especially helpful in situations where neoplasms are suspected (24).

Special stains, including immunological studies, can aid in diagnosis. These special stains are useful in diagnosing rare skin cancers and deciding between irritant and allergic contact dermatitis in questionable clinical presentations (24).


Cultures are important in confirming or disproving viral, bacterial, or mycotic skin infections. They are only as good as the techniques used to collect, transport, and evaluate them. Sensitivity reports are helpful in ensuring the correct antibiotic has been used.


Scrapings are useful in two ways: (1) diagnosing scabies and (2) confirming mycotic infections. Farm workers often confuse scabies with pesticide rashes. By scraping the scabietic areas and demonstrating the parts of Sarcoptes sca-biei on a slide with KOH, the worker can see that the rash is really an infestation and not due to a chemical (1,8,9).

When cutaneous mycotic infections are inspected, a useful technique is to scrape some of the top layer of the skin and place it on a microscope slide. The specimen is then treated with KOH and examined under the microscope for hyphae and conidia (8,9).

Ultraviolet (UV) Light (Wood's Light)

The actual use of the Wood's light requires minimal skill. The lamp should be allowed to warm for 1 minute and be used in a dark room. It is important the user be dark adapted to see the contrasts clearly. Wood's light is unreliable in darker skin types, and it is possible to obtain fluorescence from topical medications, lint, and soap residue. It is useful in diagnosing pigmentary disorders, cutaneous infections, and the porphyrias. A Wood's light is useful in diagnosing the following cutaneous infections:

1. Pseudomonas where the bacteria fluoresce green

2. Erythrasma caused by Corynebacterium minutissimum, which shows coral-red fluorescence

3. Propionibacterium acne, which shows an orange-red fluorescence and is useful in distinguishing the chloracne of organochloride exposure from adolescent acne

4. Dermatophytes

5. Tinea versicolor caused by Malassezia furfur, which shows a yellowish-white or copper-orange fluorescence and is common in agricultural workers who work in damp areas or water (25)

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