Long-term repeatability of the skin prick test is high when supported by history or allergen-sensitivity tests: a prospective clinical study

Background: Long-term reproducibility of the skin-prick test (SPT) has been questioned. The aim of the study was to investigate the clinical relevance of SPT changes. Methods: SPT to 10 common inhalation allergens was performed annually from 1999 to 2001 in 25 nonsensitized and 21 sensitized subjects. An SPT was positive when greater than or equal to3 mm, and repeatable if either persistently positive or negative. Clinical sensitivity to birch pollen was used as model for inhalation allergy, and was investigated at inclusion and at study termination by challenge tests, intradermal test, titrated SPT and IgE measurements. Birch pollen symptoms were confirmed in diaries. Results: The repeatability of a positive SPT was 67%, increasing significantly to 100% when supported by the history. When not supported by history, the presence of specific IgE was significantly associated with a repeatable SPT. Allergen sensitivity was significantly lower in subjects loosing SPT positivity. The repeatability of a negative test was 95%, decreasing significantly to 87% by the presence of other sensitization. Development of a positive SPT was clinically relevant. Elevation of SPT cut-off point did not enhance repeatability. Conclusion: SPT changes are clinically relevant. Further studies using other allergens are needed. Long-term repeatability of SPT is high in the presence of a supportive history.