Patch false-positive reactions

In addition to the patch test for nickel sulphate, intradermal nickel tests have been used [407, 408]. The intradermal testing was recommended for confirmation of doubtful patch-test reactions, particularly to disclose false positive reactions. 

This case highlights the importance of patch testing with individual components of a suspected product in occupational dermatitis. This reduces the chance of false positive reactions and of missing a relevant allergen. 

Besides causing false-positive reactions, the uncritical use of undiluted industrial chemicals in patch... 

It is recommended that patch tests are undertaken with the resin the patients actually handle themselves, in addition to those allergens available from commercial sources. If an individual brings their own resin, we test at 1% and 5% and in petrolatum and, if positives are identified, controls will need to be tested. I have not seen any false positives, but Kanerva et al. (1997) showed that phenol-formaldehyde resin tested at 1% and 5% in petrolatum could produce false-positive reactions on occasions. Hermal provides a Resol and Novalak resin at 5% for patch... 

Kubo Y, et al. (1992) False positive reaction to patch testing with aqueous mercuric chloride in an aluminum Finn Chamber. Contact Dermatitis 26 136-137... 

Simple patch skin testing is naturally most useful in diagnosing contact dermatitis, but it is unreliable for other allergies. Skin prick tests are helpful in specialist hands for diagnosing IgE-dependent drug reactions, notably due to penicillin, cephalosporins, muscle relaxants, thiopental, streptokinase, cis-platin, insulin and latex. They can cause anaphylactic shock. False positive results occur. 

In a prospective study, true-positive patch tests were seen in four of six patients with known clindamycin hypersensitivity, while 22 healthy controls were negative there was one false positive and one false negative reaction (43). 

Substances that are not listed in any standard table should be patch-tested with great caution control tests are mandatory when positive reactions are obtained, so as to exclude false-positive irritant patch test reactions. 

Although generally regarded as a nontoxic and nonirritant material, lanolin and lanolin derivatives are associated with skin hypersensitivity reactions and the use of lanolin in subjects with known sensitivity should be avoided. Other reports suggest that sensitivity arises from false positives in patch testing. However, skin hypersensitivity is relatively uncommon the incidence of hypersensitivity to lanolin in the general population is estimated to be around 5 per million. ... 

Skin tests are often useful. They are commonly carried out by intradermal injection, but prick, scratch and patch tests may be used (Aldrete and Johnson 1969, 1970). Incaudo et al. (1978) obtained positive results only in 5 (8%) of the 59 patients they reported. Furthermore, the information gained from the skin test results was of little value since some of these were false positives. We had a similar experience in 18 patients with suspected acute reactions to local anaesthetics. Clearly positive results were obtained in only 2 patients, but further investigations carried out in those 18 patients cast further doubt on the validity of skin tests. These investigations are detailed in Chap. 6. 

Positive patch tests to phenolphthalein have been reported in some eczematous lesions (Bernstein 1931 Wise and Sulzberger 1933). False positive patch tests, reflecting epidermal hysteresis, have also been demonstrated after phenolphthalein administration (Shelley et al. 1972). Wyatt et al. (1972) found that serum taken during exacerbations produced a local inflammation when injected intradermally. Possible cross-reaction with erythrosin which is used as a food and drug colorant has been reported (Wile 1936). Other rare allergic manifestations after phenolphthalein administration are urticaria, Stevens-Johnson syndrome, lupus erythe-matosus-like eruptions, and encephalitis (Lindemayr 1959 Kendall 1954). 

Patch testing the patients own materials is important, but it entails many problems. Most materials are not suitable for testing as such, but have to be properly diluted, using an appropriate vehicle. The evaluation of a test reaction may be difficult. To avoid false-positive or false-negative reactions, it may be necessary to test control persons with the adopted test substance. Separate testing of the ingredients, and even chemical analyses, may be needed. Active sensitization is rare, but must be taken into consideration when the patients own materials are tested. 

A review of the MSDS of the industrial chemicals is important because this often reveals enough information for patch testing with the chemical. If the MSDS or product declaration is not available, the name and address of the manufacturer or the supplier is necessary for further contacts. The MSDS often does not give detailed information regarding the exact chemicals present, and most chemicals present in a concentration under 1%, such as preservatives, are not required to be listed (Adams 1995). More detailed information on the composition may therefore be needed from the manufacturer or the importer, especially for the evaluation of the positive patch-test reaction produced by the chemical. It is also important to determine the degree of acidity or alkalinity in a product in order to avoid false-positive, i.e., irritant, test reactions, especially if... 

Fischer T, Rystedt I (1985) False positive follicular and irritant patch test reactions to metal salts. Contact Dermatitis 12 93-98... 

Patients may also be patch tested using an absorbent disk such as a Finn Chamber (Epitest, Finland) to which (a) an extract obtained from the suspected fabric has been added or (b) a dye powder taken from a TLC plate has been added. A positive reaction in this case must be carefully studied, as the reaction might be irritant rather than allergic contact because there is no control over the concentration of the suspected compound. The advantage is that the chemical comes from the fabric. Formaldehyde must not be tested at a concentration exceeding 1% in petrolatum, as a false positive is likely. 

Patch testing with a standard series of allergens, oil batteries (Foulds and Koh 1989) and ingredients of the oil at appropriate dilutions as well as other potential contact factors have to be performed. Facilities for control testing are vital in order to exclude false positive irritant reactions. 

The allergen, alpha-methylene-gamma-butyrolactone, can be obtained in patch-test concentration from Chemotechnique as a various allergen . Testing has also been done both with Alstroemeria and Tulipa. However, because of the high concentration, especially in the petals of Alstroemeria there is some risk of both false positive irritant reactions and active sensitization. 

Benomyl is a fungicide, derived from benzimidazole. Cases of sensitization were reported in horticulturists and florists. It is however, at most, a weak sensitizer, with possible false-positive patch reactions, or with... 

For other chemicals and products, the investigator must decide how to apply them as a patch test. Chemicals usually need to be diluted, and it is of the utmost importance to use an appropriate patch test concentration and vehicle to avoid both false-negative and false-positive (irritant) reactions. The most useful reference source for documented test concentrations and vehicles of chemicals, groups of chemicals and products is the book Patch Testing, Other useful lists are provided in recent textbooks on contact dermatitis ". Guidelines for testing patients own materials are provided in chapter 47. 

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