Newborn screening for Classic Galactosemia involves a dried blood spot test, where a sample of blood is taken from the heel. The blood is tested for GALT enzyme activity and total levels of galactose. If GALT enzyme activity is very low or completely undetected, or if there are high levels of galactose, it means that the patient may have Classic Galactosemia.⁶

If the blood test shows high levels of galactose but normal GALT enzyme activity, the doctors will need additional tests to find out if the patient has a different subtype of Galactosemia, such as a genetic screening.⁵