In addition to the assessment of total-body retinol stores ( TBSs ) by using RID , tests included analyses of serum carotenoids , retinyl esters , and pyridoxal-@ ' - phosphate ( PLP ) by using high-pressure liquid chromatography , retinol-binding protein by using ELISA , and alanine aminotransferase ( ALT ) activity by using a colorimetric assay .