Stable isotope dilution assay for 5 beta-cholestane-3 alpha,7 alpha,12 alpha,25-tetrol and 5 beta-cholestane-3 alpha,7 alpha,12 alpha,23,25-pentol in human serum using [26,27-D6] labeled internal standards; a highly accurate approach to the serological diagnosis of cerebrotendinous xanthomatosis.