Original article Long-term repeatability of the skin prick test is high when supported by history or allergen-sensitivity tests: a prospective clinical study Skin prick test (SPT) is considered a valid, safe, low-cost and easy-to-perform method to demonstrate IgE-sensiti- zation towards aeroallergens (1, 2). It is recommended as the allergological Ôtest of choiceÕ in clinical practice as well as in clinical and epidemiological research (3, 4). Short- term reproducibility is high and adequately examined whereas long-term reproducibility using unchanged batch of allergen extracts is lower and less established (5–11). Reproducibility data are rarely available, but data on repeatability are present in – or can be calculated from – a few studies (6–12). Repeatability, or persistency, is a measure of the reproducibility of the initial qualitative outcome, i.e. the positive or negative SPT (7). The median repeatabilityofapositiveSPTis87.0%(range62.1–95.8%), and 86.8% (range 77.4–93.6%) for a negative SPT (6–11). This suggests that both losses and onsets of skin sensitization are expectable in long-term follow-up stud- ies. In epidemiological studies, the incidence of new skin sensitization is an established measure for the increase of atopy in a population (8–10, 13). Onset of new sensiti- zation has been found to be more frequent in subjects with pre-existing sensitization than in nonsensitized subjects (7, 14). Loss of SPT positivity has attracted little attention, mostly considered a technical problem related to extract, performer, and/or patient (6). Increasing the cut-off limit from the recommended ‡3 to ‡4 mm was reported to exclude loss of positive SPT reactions (9). Loss of SPT positivity has been associated with lower total IgE levels than in persistently SPT-positive subjects (10). However, no studies have investigated the clinical relevance of SPT changes by challenge tests or completion of symptom diaries. The aim of the present study was to evaluate long-term repeatability of the SPT in a commu- nity-based population during 3 years, and to relate SPT repeatability to clinical allergen sensitivity. Methods Design In a cohort of adults recruited from a community-based population, skin sensitization was annually investigated four times (Fig. 1) with 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 ‡3 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. U. Bodtger, C. R. Jacobsen, L. K. Poulsen, H.-J. Malling Allergy Clinic, National University Hospital, Copenhagen, Denmark Key words: allergy; birch; history; IgE sensitisation; reproducibility; skin prick test. Uffe Bodtger MD Allergy Clinic TA 7551 National University Hospital Blegdamsvej 9 DK-2100 Copenhagen Denmark Accepted for publication 16 June 2003 Abbreviations: CPT, conjunctival challenge test; IgE, immuno- globulin E; LPR, late-phase reaction; NPT, nasal challenge test; SPT, skin prick test. Allergy 2003: 58: 1180–1186 Printed in UK. All rights reserved Copyright Ó Blackwell Munksgaard 2003 ALLERGY 1180