The Phadiatop® test compared with RAST, with the CAP system; proposal for a third Phadiatop outcome: “inconclusive”

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Abstract
In 19 general practices, blood samples were obtained from 361 patients aged 12 years or older with chronic nasal symptoms. The Phadiatop test and a panel of RASTs to common inhalant allergens were performed on all sera with the recently introduced Pharmacia CAP system. The RAST panel was accepted as the standard. The sensitivity of the Phadiatop was 94% (95% confidence interval (CI): 89-97%), the specificity 98% (95% CI: 95-99%), the positive predictive value 97% (95% CI: 94-99%), and the negative predictive value 95% (95% CI: 91-98%). It is noteworthy that these values are very similar to those found in hospital outpatient departments. It was possible to reduce further the small percentage of false outcomes by replacing the cutoff point of the Phadiatop ratio of 1.00 by the two cutoff points 0.75 and 1.15. This resulted in three possible outcomes: a highly predictive positive outcome, a highly predictive negative outcome, and an "inconclusive" outcome. Alternatively, the cutoff point of 1.00 may be maintained while attaching the annotation "borderline" to all positive or negative Phadiatop outcomes where the Phadiatop ratio is between 0.75 and 1.15. By this simple method, physicians are alerted to the possibility of a false outcome; on the basis of the case history and other clinical findings, they can then decide whether further testing should be done.