Methods for preparing a sample for detection of an analyte of interest that is bound in vivo to a protein are provided. The method comprises providing a processing reagent comprising metaperiodate and contacting a sample suspected of comprising an analyte of interest associated with a protein with the processing reagent to form a mixture. The analyte of interest, if present in the sample (or mixture), is separated from its associated protein for detection of the analyte of interest. In one embodiment, the method is applied to samples for the detection of levels of vitamin D circulating in the blood. Samples are processed using the reagent disclosed herein to separate the analyte of interest, vitamin D or a metabolite thereof, from the vitamin D binding protein and/or albumin, for detection of the analyte separated from its binding protein(s).