Galacto-oligosaccharides (GOS) interference is only an issue where the lactose levels of the sample are very low.
GOS are known to be prebiotics and are added to some commercial food preparations, most notably infant milk formulations, where they are present at a level of ~ 0.5% i.e. equivalent to that occuring naturally in human milk.
In non lactose-free products the lactose levels of these same formulations is usually ~ 7%. In such samples containing such an excess of lactose the sample preparation and dilution for the K-LACGAR assay kit results in little interference from the GOS levels present.
An issue may arise in "lactose-free" samples where the levels of lactose are to be < 0.01%. This is particularly the case in lactose-free infant milk formulations where the GOS levels present are much greater (mimic natural human milk) than that present in the milk of other domestic mammals.
As an alternative we have Lactose Assay kit (K-LOLAC ) that is highly specific, sensitive and sequential meaning that it is performed in a single assay instead of 2 assays like K-LACGAR.
K-LOLAC provides a much improved assay for measuring low lactose levels in samples and as the measurement is based on glucose released from the lactose hydrolysis with a pre-step which includes removal of free glucose.
In this method, these is a slow hydrolysis of GOS which will be measured but this can be adjusted for using the creep calculator which is included in the MegaCalc file available on the Megazyme website.
Lactose (K-LOLAC) Data Booklet