This test identifies patients with biotinidase deficiency, an autosomal recessive condition also known as late-onset multiple carboxylase deficiency. When untreated, biotinidase deficiency is associated with developmental delay, seizures, eczema and lactic acidosis. Treatment by biotin supplementation is very effective, particularly when implemented very early in life (ideally presymptomatically). Biotinidase activity is determined spectrophotometrically using the substrate biotin-4-amidobenzoic acid.