It is true that with many other lactose detection assays that 100% lactose quantification can be difficult.
At Megazyme, we have invested great resource and effort into this area to ensure that such obstacles have been overcome.

This assay includes a 10 min pre-incubation step with excess beta-galactosidase in a low reaction volume which is sufficient for 100% hydrolysis of lactose within the detection limits of the kit i.e. 8ug - 160 ug lactose. The subsequent GalDH reaction with added GalM speeds up the detection assay so that incubation times of ~ 3 min are sufficient for 100% detection of lactose concentrations within the linear range.