Present status of different mass screening procedures for phenylketonuria. Medical Research Council Working Party on phenylketonuria.

The Phenistix screening test for phenylketonuria in newborn infants, when used routinely at the recommended age of 4 to 6 weeks, passes as normal a substantial proportion-perhaps between a quarter and a half-of children with the disease, who are then diagnosed only after brain damage has occurred. Screening procedures based on three other tests-namely, the paper chromatography test for o-hydroxyphenylacetic acid in urine, the Guthrie test for phenylalanine in blood, and a modification of the Guthrie test for phenylalanine in urine-have been compared with each other and with the Phenistix test in a special field inquiry. Each of the three tests is more efficient than the Phenistix test for the detection of phenylketonuria at an early age.A full appraisal indicates that the Guthrie test on blood would be the most satisfactory of the three tests to replace the Phenistix test as a screening procedure. Routine use of this would involve comprehensive arrangements for obtaining a single specimen of blood by heel-prick from every newborn infant at the age of 6 days or more, but preferably not later than 14 days, and for the rapid and economical processing of the specimens and skilled interpretation of the results at a few laboratory centres. These requirements have not proved difficult to meet in practice in the area where this procedure has been studied.A positive screening test result does not signify a firm diagnosis of phenylketonuria, but is an indication for fuller investigation, most suitably at one of a small number of centres specializing in the management of phenylketonuria and other inborn errors of metabolism.