Usually PCSK9 is quantified in cell culture medium by ELISA.
Here is a reference:
Colbert, A., Umble-Romero, A., Prokop, S., Xu, R., Gibbs, J. P., & Pederson, S. (2014). Characterization of a quantitative method to measure free proprotein convertase subtilisin/kexin type 9 in human serum. mAbs, 6(4), 1103–1113.