Embodiments of the invention include determining whether an individual has autism spectrum disorder or is at risk for developing autism spectrum disorder or at risk for regression of or into autism spectrum disorder. Specific embodiments include the determination of indicative levels of carnitine or other metabolites in carnitine biosynthesis and may include assaying for mutations in TMLHE, including in exon 2, for example. In some cases one can assay for mutations in TMLHE in the absence of biochemical analysis of carnitine biosynthesis metabolites. An individual with deficiency in TMLHE and/or carnitine levels may be administered carnitine, acetylcarnitine, butyrobetaine, or a combination thereof, for example.
