Guthrie blood test. Hands taking a blood sample from the heel of a newborn baby girl for a Guthrie test. This screens for a rare genetic disease, phenylketonuria (PKU), which affects around 1 in 16,000 people. PKU causes the amino acid phenylalanine to accumulate in the blood, which can cause severe brain damage. The baby's blood is smeared onto absorbent filter paper and cultured with bacteria whose growth is promoted by the presence of phenylalanine. The level of bacterial growth on the paper indicates the level of phenylalanine in the baby's blood. PKU babies are fed a phenylalanine-free milk substitute and a low-protein diet after weaning.

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