Abstract: The present disclosure relates to, among other things, methods for determining whether a subject receiving treatment with a drug has obtained an efficacious blood level of the drug. Moreover, the present disclosure also relates to methods of determining whether a subject predisposed to or suffering from a disease will benefit from treatment with a drug, and the response of a subject receiving treatment (e.g., such as for cancer) by monitoring biomarkers of angiogenesis. In particular, the disclosure relates to PlGF-1 companion diagnostic methods and products.