A blood factor was a component of blood that aided in coagulation.
Viruses mimicked blood factors to avoid being destroyed by antibodies. (VOY: "Flashback")
Bajorans shared a number of common blood factors. In 2371, Kes analysed an injured Seska's blood chemistry to determine a suitable donor might be. In the process, she discovered the woman lacked all of them, suggesting she was genetically altered to appear as one. When confronted with this information, Seska claimed to have had Orkett's disease and received a bone marrow transplant from a sympathetic Cardassian woman. (VOY: "State of Flux")
The Allocator identified patients by reading their blood factors, a limitation The Doctor exploited in 2377 to teach Chellick about the unethical triage practices of the Dinaal. (VOY: "Critical Care")