How to detect somatic mutations from blood sample or ctDNA sequencing?

Question

[Leon]

I know it is possible to identify germline mutations from the peripheral blood sample. But, I have serious doubt distinguishing somatic mutations and germline mutaions from ctDNA sequencing. When we are analyzing a tumor tissue sample in comparison with a normal tissue sample, it is quite easy to identify somatic variants. Ideally, the alternate allele fraction should be 0% (homozygous) or 50% (heterozygous). If you see 10%, it is possible to be a somatic variant. I have doubt about how to define somatic variants in ctDNA sequencing. I would like to know how the other people define them as well.