This month sees the expansion of newborn screening in England and Northern Ireland to include screening for medium chain acylcoenzyme A dehydrogenase (MCAD) deficiency within the entire newborn population. MCAD deficiency is the most common inherited disorder of mitochondrial fatty acid oxidation in people from northern Europe. This autosomal recessive metabolic disease affects about one in 10 000 people in the United Kingdom,1 and it has a common mutation (985A>G) with a carrier rate of around one in 65.2 Homozygosity for this mutation has not been found in black or Asian ethnic groups that have been screened in England, which suggests that MCAD deficiency caused by the 985A>G mutation is a disease of white ethnic origin.2

Individuals with undiagnosed MCAD deficiency typically present clinically with failure of fatty acid oxidation after fasting and an inability to generate energy during periods of increased energy demand. This may manifest as symptomatic hypoglycaemia through hepatic encephalopathy (similar to that seen in Reye’s syndrome) to sudden unexpected infant death, which may …