Here is a great explanation of that test!
There is one test that may be useful in screening patients suspected of being infected with Bartonella. This test may also be particularly useful in the follow-up of patients with Bartonella/ BLO. This blood test is called “vascular endothelial growth factor” (VEGF). This test measures a substance that is produced by the Bartonella microbe in order to facilitate its entry into the body tissues it likes to inhabit. Elevated levels of VEGF often (but not exclusively) mean that a patient is infected with Bartonella. By monitoring VEGF levels during the course of treatment, physicians can monitor the progress of treatment (antibiotics). When VEGF levels return to normal, it generally means that the antibiotics have been successful and can be discontinued. The VEGF test is available from standard commercial laboratories.