When Buffy coat is used, the column tends to be clogged."

This indicates that the number of WBC (white blood cells) in the Buffy coat is too high, thus not being lysed and digested completely by Proteinase K. Buffy coat should be prepared from a lower volume of whole blood and make sure that fewer than 1x107 of WBC is used per preparation. Incubation should be done with constant mixing to disperse Proteinase K and sample. If Lysis is incomplete, add more Proteinase K and repeat incubation. The sample should not contain insoluble residues when it is completely digested. Centrifuge to remove any undigested residues and only use the supernate to continue the procedure.