NEED FOR A STANDARDIZED PROCEDURE IN THE PREPARATION OF PHENYLALANINECALIBRATORS

Introduction. Sensitive screening for phenylketonuria (PKU) and other disorders requires accurate measurement of analytes in blood eluted fr om filter paper. The development of new quantitative methods for pheny lalanine measurement in dried blood spots for PKU screening has introd uced an increased number of calibrators provided with the kits. We com pared the phenylalanine values of seven different sets of calibrators (six from commercial sources and one from a screening laboratory), all measured with the same technique. Methods. Two types of filter papers , S&S #903 and S&S #2992 were used. Phenylalanine was measured by high performance liquid chromatography (HPLC). Results. There was wide var iation in the phenylalanine values assigned to the seven different set s of calibrators used, with the ratios of HPLC to assigned value rangi ng from 0.68-1.62. This could not be only explained by the use of diff erent types of filter papers. However, factors in calibrator preparati on did influence the measured concentration. These included higher val ues when 100 mu l rather than 35 mu l of blood was spotted, when the r ed blood cells were intact rather than lysed, and as the hematocrit in creased. Discussion. These observations emphasize the need to establis h recommendations for preparation of dried blood calibrators in order to appropriately compare new techniques and exchange screening experie nces among centers.