Samples may be frozen prior to use with this kit. Freezing is actually part of the protocol for whole blood samples. However, reduced GSH is readily oxidized to form GSSG. This kit is capable of measuring total, reduced, and oxidized glutathione. In order to get a true profile of the sample it is best to run the assay through to completion following lysis and deproteinization. A similar product, CS0260, limits the storage of extracted samples to 10 days. Extracted sample storage has not been evaluated for the MAK440, however similar limitation would be expected. Should storage of extracted and/or deproteinized samples be required, samples should remain on ice during the assay procedure.
Please see the link below to review the protocol:
https://www.sigmaaldrich.com/deepweb/assets/sigmaaldrich/product/documents/179/194/mak440pis-mk.pdf