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ORIGINAL ARTICLE, p 116

  1. Top of page
  2. ORIGINAL ARTICLE, p 116
  3. Funding sources
  4. Conflicts of interest
  5. References

Jadassohn’s description of the patch test in 1896 involved the application of a mercury-containing ointment, to which a patient had previously reacted, to an area of skin on the left arm, covered by a 5 cm2 piece of tape for 24 h, reproducing in miniature the original reaction.1

Subsequent modifications of the method were often empirical and were concerned with improving usability such as the type of tape and removing colophony from the adhesive. In the initial description, the allergen was applied to the skin and the dose nonstandardized. Later, filter discs were attached to the tape to which the allergen was applied and various chambers developed.

The procedure is time consuming for both patient and clinician. To improve outcomes for the patient and avoid false negatives the sensitivity and specificity of the procedure should be as high as possible. Preprepared allergens are commercially available in an easy-to-use format providing a consistent dose; however, the allergens available are limited. While a ‘standard’ series has been developed that empirically detects the most frequent contact allergens, this is better termed a ‘baseline’ series as it correctly implies that other allergens should be applied dependent on the patient’s presentation and exposures, including the patient’s own samples. This may increase the yield of relevant positive reactions by ∼20%. Sensitivity is further improved by occluding the allergen for 48 h and by having multiple reading times. A single reading at day 4 has been identified as yielding the most positives if a single reading point is used. An additional reading at day 7 detects a further 10% of positive reactions above combined readings at days 2 and 4. Expertise in the reading and interpretation of the test is essential to avoid misinterpreting reactions and subsequently giving irrelevant advice. Guidelines have been developed that detail the evolving evidence base.2

The work of the team from Singapore and Malmö published in this issue3 highlights a further topic that has generated debate: a belief that allergens may be safely kept beyond their expiry date and prepared before application without affecting the results of the test. The team’s work clearly shows that unless stored in a refrigerator and prepared fresh prior to application some volatile allergens could be predicted to give a false-negative reaction. Indeed, for some highly reactive allergens such as isocyanates storage in a freezer may be required4 to avoid degradation of the allergen preparation.

In addition to patient-related factors, such as immunosuppression,2 this study emphasizes that factors related to the allergen, storage and preparation of the test are essential to take into account when trying to maximize the usefulness of the investigation.

References

  1. Top of page
  2. ORIGINAL ARTICLE, p 116
  3. Funding sources
  4. Conflicts of interest
  5. References