What the test investigates
Patch testing investigates delayed contact allergy. Small amounts of selected substances are placed on the skin and assessed at subsequent readings by trained specialists. It differs from skin prick testing, which mainly investigates immediate reactions, and is not a general test for every skin condition. The clinical question determines which investigation is appropriate.
The occupational question
When occupational contact dermatitis is suspected, the clinician needs to know what the person actually touches, how products are used and where the rash develops. Mixtures, gloves, cleaning products and auxiliary materials all matter. A trade name alone may conceal ingredients shared by several products. Exposure away from work must also be considered, because the same ingredient may occur in both a workplace material and a personal cosmetic product.
Interpreting a positive result
A positive reaction demonstrates sensitisation to the tested substance; it does not automatically establish the cause of the current dermatitis. The specialist checks whether relevant exposure occurs and whether the distribution and timing of symptoms fit. An old allergy may be unrelated to the present job, whereas another finding may explain a current problem and require changes to materials.
Preparing useful information
Provide available safety data sheets and ingredient lists, together with a description of wet work, skin contact and protective measures. If workplace products are needed for testing, the clinical team should explain how to provide them safely. Never apply an industrial chemical directly to the skin to investigate a suspicion: its concentration or properties may cause an injury.
Understanding the limitations
A negative result does not exclude every cause of dermatitis. The relevant allergen may not have been tested, or the condition may involve irritation rather than allergy. Test series are selected from the history and may need extension. Readings distinguish allergic, irritant and doubtful reactions. Employers should not interpret photographs or tell workers to change medication before testing.
Turning findings into prevention
Where relevant exposure is identified, action may include substituting an ingredient, preventing contact, reorganising tasks or reviewing suitable gloves. Check that replacement products do not contain the same allergen. Follow-up combines clinical progress with workplace assessment. Individual medical results remain confidential, while the preventive measures required to protect the worker are communicated through the appropriate occupational health process.
