Preparation of the first European working standard for phenylalanine determination in dried blood spots

Objective-To develop a standard preparation for measurement of phenylalanine in blood spots in phenylketonuria against which manufacturers of kits for phenylalanine determination could calibrate their own blood spot calibrators. Methods-As most of the variation of phenylalanine concentration in dried blood spots is due to the way the blood spot calibrators are prepared, a minimum set of prerequisites for the preparation of blood spot calibrators was established by a group of scientific experts and manufacturers. A ((European working standard for phenylalanine in blood spots)) (EWS-Phe-01) was prepared and distributed to manufacturers taking part in this project. Results-The control of the homogeneity, linearity, and stability of the EWS-Phe-01 batch gave results within expected ranges. The preliminary results of the recalibration of calibrators for phenylalanine against EWS-Phe-01 by the participating manufacturers were promising; the differences between measurements were much decreased. Conclusion-Manufacturers have agreed to use EWS-Phe-01, and since November 1996 EWS-Phe calibrators have been routinely available from these companies. This pilot study may act as a model for minimising discrepancies found in other screening tests.