Abnormal circulating carboxylic acid concentration Elevated circulating hydroxyphenyllactic acid concentration 2020-11-29T14:16:15Z An increased concentration of 4-hydroxyphenlyllactic acid in the blood circulation. Hydroxyphenyllactic acid or 4-hydroxyphenyllactate (the L-form) is a tyrosine metabolite. The level of L-hydroxyphenyllactic acid is elevated in patients with a deficiency of the enzyme p-hydroxyphenylpyruvate oxidase.