Galactosemia is an inherited metabolic abnormality comprising three distinctive clinical entities of the galactose pathway defects. Classic galactosemia is due to -Dgalactose- 1-phosphate-uridyl-transferase (GALT) defect (1). The test principle is based on extraction of Galactose & Galactose-1-phosphate from dried blood spot followed by enzymatic conversion of Galactose-1-phosphate & Galactose. Two kinds of extraction methods (acidic and heat) can be used for extraction step. After extraction Galactose-1-phosphate converts to Galactose in a reaction catalyst by alkaline phophatase enzyme Then this Galactose & extracted Galactose from dried blood spot converts to Galactonolactone by Galactose Dehydrogenasis & NAD coenzyme. The resulted NADH reduces Tetrazolium salt indicator dye from yellow to pink or red color. The figure demonstrates the reaction cycles. Final color change is used for colorimetric measurement (490 nm ) spectrophotometrically.