A New Commercial Method for the Enzymatic Determination of Creatinine in Serum and Urine

biotimdase-deficient specimen in the autosampler. A sample cup containing phosphate buffer should be inserted between each sample suspected of having deficient or very low activity. Interfering substances such as sulfonamides, if present in a sample, can cause color development, such that a biotimdase-deficient specimen could exhibit color corresponding to normal activity and a normal specimen could exhibit color corresponding to activity exceeding the upper lixmt of normal. Therefore, samples tested by either method also should be tested in the absence of substrate for the presence of other chromogenic compounds. The applicability of the automated method to clinical, genetic, and epidemiological studies depends on the relative contributions of analytical and biological variation to total variation in activity, and on the relative contributions of intraand inter-individual variation to total biological variation. Although there is no clear definition of “acceptable” analytical variation, Tonks (5) suggested that total analytical variation should not exceed one-fourth of the total biological variation. The analytical variation of the automated assay is sufficiently low (less than one-sixth of biological variation) to assure that true differences between subjects will not be obscured and that abnormal laboratory results will be identified reliably. The differences among individuals accounted for two to four times more variation than did variation within a single individual and, therefore, the automated system is acceptable for use in genetic and epidemiological studies. This automated assay provides a reliable means for conveniently determining biotinidase activity in a large number of serum specimens, and results are more precise