MODEL RELEASED. Guthrie test. Leaning over a newborn baby in a cot, a midwife takes a prick of blood from the baby's heel for a Guthrie test. A Guthrie test is carried out to screen babies for a rare genetic disease called phenylketonuria (PKU), which affects about 1 in 16000 people. Victims of PKU suffer from a dangerous accumulation of phen- ylalanine in their blood, which can lead to severe brain damage if not treated. Because phenylalanine is a natural product of protein digestion, babies found to suffer from the condition must be fed a special milk substitute free of phenylalanine. After weaning they are given a very low-protein diet, which they might have to stick to for life.