Design of an optimally-diagnostic skin test solution for diagnosis of sensitivity to timothy grass (Phleum pratense) pollen


Dr A. W. Wheeler, SmithKline Beecham Bencard, Great Burgh, Yew Tree Bottom Road, Epsom, Surrey, UK.


Background Although most of the common allergen extracts that are used for diagnosis of type 1 hypersensitivity are now well standardized, this gives no assurance that they are within the concentration range that gives the best chance of a true diagnosis.

Objective The objective of this study was to identify the most appropriate concentration range of timothy grass pollen Phleum pratense extract to diagnose sensitivity to this pollen correctly through skin-testing.

Methods Dilutions of a well-standardized extract were made and used to skin test ‘true’ positive and ‘true’ negative populations of subjects as identified by case history, challenge tests and radioallergosorbent test (RAST). Weal diameters were measured and the data were submitted to receiver operating characteristics (ROC) analysis. For any particular weal size cut–off, the optimal diagnostic concentration (ODC) range was thus calculated.

Results A 3 mm weal diameter cut-off was chosen as an appropriate size for routine diagnosis. Therefore the ODC range at this diameter was used to establish a product target concentration and specification for formulation of the diagnostic reagent. This method of allergen extract standardization can lead to a true-biological unitage that can be used for labelling purposes.

Conclusion The optimun concentration range at which to formulate an allergen extract, in terms of an in vitro immunologically based assay, can be determined by carrying out ROC analysis of the results of clinical studies as described in this communication. Diagnostic units (DU), are now used by us for labelling of such final formulations which conveys the information that the product is at the most appropriate concentration for diagnosis.