Evaluation of a fully mechanized enzymatic kinetic determination of sialic acid.

The enzymatic determination of sialic acid, commercially introduced recently as an end point method, has been considerably improved by the development of initial rate and peak rate modifications of the kinetic method, respectively, and adapted to a centrifugal analyser (Cobas Bio). The assay is completed within 3 min, takes 3 microliter sample volume and is reduced to a total reaction volume of 153 microliter. The costs per test thereby are lowered to about 10% of the original procedure. Within-run and day-to-day imprecisions (means = 0.81 g/l) are 2% and 3.9%, respectively; inaccuracy is between +1% and -2%. The detection limit is 0.012 g/l, i.e. about 3 times higher than that of the thiobarbituric acid method (0.0037 g/l) for sialic acid determination. The enzymatic-kinetic assay correlates well with the latter method (r = 0.973). There was no measurable interference by EDTA (less than 3 mg/l), bilirubin (less than 160 mumol/l), haemolysis (haemoglobin less than 5.25 g/l), or triglycerides (less than 3.50 g/l). The reference range (2.5th to 97.5th percentile) for sialic acid in sera of adults is 0.51 to 0.84 g/l (median 0.63 g/l) with no significant sex-related differences. The reference range for sialic acid in spontaneous urine samples is 0.01-0.13 g/l.