A method for isolating nucleic acid which comprises:

• (a) applying a sample comprising cells containing nucleic acid to a filter, whereby the cells are retained as a retentate and contaminants are removed;
• (b) lysing the retentate from step (a) while the retentate is retained by the filter to form a cell lysate containing the nucleic acid;
• (c) filtering the cell lysate with the filter to retain the nucleic acid and remove remaining cell lysate;
• (d) optionally washing the nucleic acid retained by the filter; and
• (e) eluting the nucleic acid, wherein the filter composition and dimensions are selected so that the filter is capable of retaining the cells and the nucleic acid.

Additionally, there is provided a substrate for lysing cells and purifying nucleic acid having a matrix and a coating and an integrity maintainer for maintaining the purified nucleic acid. Also provided is a method of purifying nucleic acid by applying a nucleic acid sample to a substrate having an anionic detergent affixed to a matrix, the substrate physically capturing the nucleic acid, bonding the nucleic acid to a substrate and generating a signal when the nucleic acid bonds to the substrate indicating the presence of the nucleic acid. A kit for purifying nucleic acid containing a coated matrix and an integrity maintenance provider for preserving the matrix and purifying nucleic acid is also provided. Further, there is provided a method for quantifying DNA, such as double-stranded or genomic DNA, isolated from cells, such as leukocytes to determine the numbers of leukocytes in a sample of leukoreduced blood.