The invention provides a method for determining the level of VEGF-A121 isoform in a sample by selectively removing the VEGF-A165 isoform from the sample using a neuropilin-1 pull-down procedure, then determining the total amount of VEGF-A remaining afterward. The invention provides methods of treating a patient suffering from a disease which may benefit from the administration of a VEGF antagonist by determining the level of VEGF-A121 in the patient's circulation. Methods of diagnosis, prognosis, monitoring, and patient stratification are also provided.