Hello Cecilie Kongsdal Kuzon

In QIA symphony kit, plasma is separated out from the cellular fraction of the whole blood. So, plasma will not contain erythrocytes, leukocytes and platelets. Proteinase K is a serine protease that exhibits a very broad cleavage specificity. It cleaves peptide bonds adjacent to the carboxylic group of aliphatic and aromatic amino acids and is useful for general digestion of protein in biological samples.

During purification step, there may be contaminating proteins present. These contaminants must be removed. Proteinase K is added to digest the plasma proteins so that the contaminating proteins can be eliminated. Also, proteinase K degrades nucleases and protects the nucleic acids from nuclease attack.

As far as lysis of leukocytes is concerned, proteinase K can be a part of the lysis step by digesting surface proteins. It cannot lyse cells but needs the addition of denaturing agents like SDS to bring about lysis of cells.

So, proteinase K is important in the purification of ctDNA.

Best.

Thanks for the answer. I'm just wondering why some other manufacturers do not use it and say it is because the concentration of genomic DNA will increase and it will interfere with ctDNA.