This is a critical, high-precision test used to find the root cause of serious or unexplained bleeding. The Factor IX test precisely measures the level and activity of a specific blood clotting protein, Factor IX, in your bloodstream. This protein is a vital part of your body's complex 'clotting cascade' that stops you from bleeding. This test is the gold-standard, definitive diagnostic tool for Hemophilia B (Christmas Disease), a genetic condition where this specific factor is low or absent. It is essential for differentiating it from other, similar-sounding bleeding disorders. A simple blood draw is required. This conclusive test provides the specific, actionable answer your doctor needs to create a personalized, effective treatment plan for your long-term health.