Table 3 Groups of Chemicals and Substances

Some textbooks on contact dermatitis not only provide recommendations for test concentrations and vehicles for individual chemicals, but also for groups of chemicals or substances. Such groups may consist either of chemically related compounds (e.g. sulfonamides, quaternary ammonium compounds), or functionally related chemicals or substances (e.g. pesticides, photographic chemicals, topical antihis-tamines). Tables 3.1-3.4 provide an alphabetical listing of specific categories of such groups with test recommendations and references to their sources. With chemicals of some groups the risk of inducing patch test sensitization is greater than usual, and this is indicated by an *.

When a particular chemical to be patch tested is not included in Table 1, but does belong to one of the groups of chemicals or substances in Table 3, the investigator may thus have a first clue to determine a proper patch test concentration and vehicle for this chemical. It should of course be appreciated that these test recommendations (especially for the groups of functionally related chemicals) can never be as precise for any particular chemical as those that were advised for individual chemicals.

The abbrevations used for the vehicles are as follows:

  • acet = acetone
  • alc = alcohol 70%
  • isoprop alc = isopropyl alcohol
  • MEK = methyl ethyl ketone
  • oo = olive oil
  • pet = petrolatum

TABLE 3.1 (TOPICAL) DRUGS, ANTIMICROBIALS AND COSMETIC INGREDIENTS

See for individual drugs in these groups Table 1

p-Aminobenzoic acid (PABA) esters 5% pet (1,2)
Antibiotics 10% pet (1)
Antihistamines (topical) pure (1); ADVICE: 2% pet
Antimycotics (topical) pure and 25% pet (1); ADVICE: 2% pet
Antimycotics, imidazoles 1% alc (8)
Barbiturates 10% alc or pet (8) 
Benzodiazepines 5%-10% pet or aqua (8) 
Benzotriazole derivatives 5% pet (7) 
Beta-lactam antibiotics  1%-20% pet (8) 
Carbowaxes (polyethylene glycols)  pure (1,2,6,8) 
Cephalosporins 20% pet and pure (7); 1%-5% aqua (7);10%-20% pet (8);1%-10% aqua (8)
Cinnamates 1% pet (6)
Corticosteroids 0.1% alc (809)
Essential oils 1% pet (1,2); 1% alc or 2% pet (6); 20% oo (10); ADVICE: 2%-5% pet
Gallate esters 0.1% and 1% pet or oo (5)
Halogenated salicylanilides 1% MEK (1,2)
Heparins pure (8) (probably the commercial heparin products are meant)
Oils of spices 5% alc (1)
Parabens 5% pet (1,2,6)
Penicillins 1%-20% pet (8)
Perfume oil 10% oo (5)
Phenothiazines 1% aqua (1); 1% pet (1,2); 1%-10% pet or 10% alc (8)
Phytotherapeutics pure and 10% aqua (8)
Polyethylene glycols (carbowaxes) pure (1,2,6,8)
Quaternary ammonium salts 0.01% aqua (1); 0.1% and 0.01% aqua (2); 0.1% aqua (6,10)
4-Quinolines 5% pet (1,2); 10% pet (8)
Salicylates 2% oo (6)
Sulfonamides 5% pet (1,2,5); 10% alc or pet (8)
Tetracyclines 5% pet (1,2)
Zirconium salts 1% pet (9)

 

Patch testing of systemic drugs in cases of suspected drug eruptions

General

  • Drug skin tests should be performed 6 weeks to 6 months after complete healing of the adverse drug reaction, and at least 1 month after discontinuation of systemic corticosteroid or immunosuppressive therapy.
  • Skin testing should be performed with the commercialized drug and, whenever possible, also with the pure active products and the excipients.
  • In investigating a photosensitivity reaction, both drug patch tests and photopatch tests need to be performed. Irradiation for drug photopatch tests should be performed on D1, or for practical reasons on D2, with 5J/cm2 UVA irradiation.
  • In fixed drug eruptions, patch tests should be performed both on the normal skin of the back and on the residual pigmented site of the fixed drug eruption.

Concentrations and vehicles

Patch testing with the commercialized drugs

Remove the coating from pills and then ground them to a very fine powder. This material, or powder in capsules, can usually be tested 20-30% in petrolatum and water. The gel jacket portion of the capsule is moistened and tested 'as is'. Other authors (964) advise a dilution to obtain a 10% concentration of the active drug. If the concentration of the active drug is too low, it is recommended to use a 30% concentration of the commercial drug preparation in petrolatum, water or other vehicles (30% is the highest concentration to obtain a homogenous preparation) (964,965). Non-irritating patch test concentrations for 89 different commercial drugs, along with the exact concentration of the active ingredient in each patch test material, can be found in ref. 965. Another useful review article on this subject was published in 2013 (967).

Patch testing with pure substances

The pure drug, obtained from the manufacturer, is tested 10% in petrolatum and, if possible, also 10% in water or alcohol. Concentrations and vehicles previously considered as most adequate for certain drugs (as reported in literature) should also be chosen. Drugs such as carbamazepine, diclofenac sodium salt, ketoprofen and piroxicam (the latter 3 are NSAIDs) for example can be tested in a lower concentration of 1% (8). To avoid any relapse of a severe drug reaction, drugs that have possibly caused DRESS, Stevens-