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Letter to the Editor

Sir, I would like to comment on a particular point of interest in the article by Deuschl and Johansson which appeared in the March issue of Clinical Allergy entitled 'Specific IgE antibodies in nasal secretion from patients with allergic rhinitis and with a negative or weakly positive RAST on the serum'. This report was based on studies of eighteen patients in whom a diagnosis of allergic rhinitis had been made by history and confirmed by intradermal skin testing and nasal provocation study. None ofthese patients was positive by the RAST test according to the cut-off value for positive result as determined by the Phadebas RAST curve (Pharmacia Diagnostics, Uppsala, Sweden). Twenty-two of thirty-five patients were positive, however, when this reference system was modified by the authors. This point deserves further emphasis. The current Phadebas RAST reference system is composed of four dilutions of a pooled serum from patients highly allergic to birch antigen. Reference A, the concentrated serum in this system, has been assigned the arbitrary value of 50 Phadebas RAST units (PRU). Reference D, a l/50th dilution of A, has been assigned 1 Phadebas RAST unit. Serum with scores below Reference D have been recorded as negative. Deuschl and Johansson observed that a five-fold dilution of Reference D gave scores twice those of negative controls and established 0-2 PRU as a new lower limit for a positive score. We have performed RAST tests in our laboratory in over 300 patients and have also found that the use of Reference D as a cut-off point leads to a very high percentage of false negatives. In a typical RAST reference curve run in our laboratory, RAST Reference A gave 32,000 counts and RAST Reference D 3400 counts. A five-fold and a ten-fold dilution of RAST Reference D gave 1600 and 1000 counts respectively. Fresh pooled cord serum obtained from the hospital's newborn nursery served as a negative control and gave only 500 counts with the appropriate birch reference disc. This allowed us to extend the range of positive activity down to a ten-fold dilution of Reference D (0-1 PRU) and still have scores twice those of negative controls. In each RAST reference curve we have also included a positive control from serum of patients who score above Reference A and have assigned matching sera a value of 100 PRU. This modification of the Phadebas RAST reference system extends its readability to a range of 1000 (0-1-100 PRU) and is in keeping with the clinical observation of many investigators that the range of sensitivity among patients differs a thousand-fold. We have utilized this titre range to determine initial doses of immunotherapy. Those patients with the highest titres (100 PRU) are started on immunotherapy with the appropriate antigen 1-500,000 w/v and those in the lowest range (0-1 PRU) are started at 1-500 w/v. More details of this scoring system will be submitted for publication at a later date.