Relative potency of commercial calibrators for fructosamine, and their effect on measurements of fructosamine in serum.

The protein-based secondary standardsfor serum fructosamine in a commercial test kit (Roche Diagnostics) have recently been modified by the manufacturer. Under certain assay conditions, the new calibrator gave significantly greater serum fructosamine values than those obtained with the originally supplied calibrator. However, for 20-4, samples and absorbance measurements at 10 and 15 mm, hereafter called “calibrator conditions,” assay values were unaffected. Table 1 summarizes our results for comparing the activity in the new protein standard (lot K3034) with that in the old (lot C0342). The relative activity of the calibrators in reducing nitroblue tetrazolium was calculated from changes in absorbance measurements, ,..,,Jmin, over 1-15 mm with 10and 20-4, samples. We performed the evaluations with sera from non-selected patients and carried out all measurements with a Cobas Bio centrifugal analyzer and reagents supplied with the kit. The relative decrease in the activity of the new calibrator as the pre-incubation time of the assay was shortened caused an apparent shift towards higher values for serum fructosamine measured with use of the new calibratori.e., serum fructosamine concentration = k X Meample/AAcalibrator, where k is the assigned calibrator concentration. Sample volumes less than 20 /LL in the assays with the new calibrator caused a shift towards values for serum fructosanune higher than those obtained with the old calibrator. The activity of the old calibrator, like that of serum, showed a nonlinear dilution effect, and decreased less than in proportion to the dilution (2). In contrast, the new calibrator showed a linear dilution effect; i.e., at the same nomiTable 1. Relative Activity of New and Old Fructosamine Assay Calibrators Assay timIng, mln