See below. The diagnostic test for this condition is the measurement of blood Amino Acid concentrations. Affected patients have elevated levels of Amino Acids (leucine, isoleucine, and valine). Urine organic acid measurement will detect elevated levels of branched-chain ketoacids, lactate, and pyruvate.
Newborn screen. In the US, MSUD is part of the newborn screen all babies get before leaving the hospital, but it's usually symptomatic in the first weeks after birth when amino acids start to accumulate in the blood. Symptoms are poor feeding, irritability, seizures, and smelly urine. Definitive test is quantitative plasma amino acids, and is usually done on day of life 2 if there is a family history of MSUD.