How to make cfDNA a standard sample material for cancer testing?

Blood as a surrogate (a.k.a. liquid biopsy) to tissue biopsy coupled with the next-generation sequencing tools for cancer detection is revolutionary in diagnostics.
Currently, the liquid biopsy tests rely on a sampling of 3-10 ml of blood to interrogate cell-free DNA (cfDNA). The cfDNA in the blood is minute amounts and difficult to quantify.
The second challenge of liquid biopsy is that several factors influence the amounts of cfDNA, such as the storage conditions, variation between the patients as well as different time and treatment stages within a patient. For example, an early-stage cancer individual has low amounts, while a patient under chemotherapy has large amounts of cancer- and healthy-cell cfDNA into the blood.
These factors distort the signal to noise ratio, making it challenging for cancer signal detection. In other words, it results in higher than expected false-negative cases on molecular genetic testing.
It is an open question of how to normalize cfDNA for molecular testing?