The Ham test is done to diagnose paroxysmal nocturnal hemoglobinuria (PNH). The test checks whether red blood cells become more fragile when they are placed in mild acid.

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to restrict blood flow through the vein. This causes the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band is removed to restore normal blood flow. 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, the area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a small glass tube (pipette), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

There is no special preparation needed for this test.

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

A positive test confirms the diagnosis of PNH.

The Ham test can also be used to diagnose another rare disorder called congenital dyserythropoietic anemia.

A negative test is normal.

• Paroxysmal nocturnal hemoglobinuria
• Congenital dyserythropoietic anemia (anemia associated with a hereditary problem with erythropoietin, the hormone that normally triggers red blood cell production)

• Excessive bleeding
• Fainting or feeling light-headed
• Blood pooling under the skin (hematoma)
• Infection
• Many punctures to locate veins

Veins and arteries vary in size from one patient to another, and from one side of the body to the other. Getting a blood sample from some people may be more difficult than from others.

Henry, JB. Clinical Diagnosis and Management by Laboratory Methods. 20th ed. Philadelphia, Pa: Saunders; 2001.