The factor IX assay is a blood test that measures the activity of factor IX -- one of the substances involved in blood clotting (coagulation).

This test is used to find the cause of too much bleeding (decreased blood clotting), or if a family member is known to have hemophilia B.

Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The area is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.

Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.

The health care provider may advise you to stop taking drugs that may affect the test.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.