Comparison between an absolutely specific enzymatic method, such as K-SUFRG, and non-specific methods, such as HPLC, should be performed with great consideration to the limitations involved.
For instance, it is well known that HPLC methods are prone to interference from compounds eluting at the same time as the analyte of interest. Such interference can be very difficult to identify, especially if the elution times of the analyte of interest and the contaminant are very similar.
Take a situation where the D-fructose and sucrose values agree with the HPLC data, but the D-glucose one does not. This indicates that the whole kit functioned correctly during the analysis, as determination of D-fructose and sucrose fundamentally rely on the D-glucose determination reaction (see page 1 of
  K-SUFRG booklet). In this case interference of D-glucose peak integration, due to a co-eluting contaminant, most likely explains the observation.
To confirm the enzymatic kit is giving the actual and correct values for the analyte(s) of interest, simply spike the sample with a known amount of the analyte, and determine a recovery value, by comparison to an un-spiked sample. The recovery should be certainly +/- 5%, and probably better, depending on the analyst. If this is the case, then large discrepancies between enzymatic kit and HPLC values should be assigned to interference during the HPLC analysis, with the focus being on peak integration.

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