Ever wonder why Hem B patients sometimes bleed with relatively high trough levels and conversely don’t always bleed with lower trough levels? To answer this, it is important to understand that Factor VIII and Factor IX work differently in the body. Hence the differences between Hemophilia A vs. Hemophilia B. Factor VIII stays inside the bloodstream, while Factor IX also goes outside the bloodstream by traveling through the walls of blood vessels. Trough levels will only identify factor that is in the bloodstream. That’s why for Hemophilia B, just measuring trough may not tell the full story of what your factor replacement is doing, and you may need more information.