Alternative Names: Anti-P antibody

How the test is performed:

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 apply pressure and restrict blood flow through the vein. This causes 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 circulation. 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 pipette (small glass tube), 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.

How to prepare for the test:

No special preparation is needed.

How the test will feel:

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.

Why the test is performed:

This test is sometimes performed when the diagnosis of paroxysmal cold hemoglobinuria is suspected.

Normal Values:

No presence of antibodies is normal.

What abnormal results mean:

Abnormal results indicate paroxysmal cold hemoglobinuria (PCH), a disorder caused by the presence of an antibody to "P-antigen complex" on red blood cells. This antibody is also called Donath-Landsteiner antibody.

Reduced temperatures somehow cause the antibody to attach to the P-antigen on red blood cells. This causes destruction of red blood cells (complement-mediated hemolysis); and hemoglobin (a protein in red blood cells) is excreted in the urine (see complement ).

PCH is an acquired syndrome. The appearance of the antibody has been associated with viral infections (for example, measles and mumps ) and with syphilis. However, in rare cases there is no apparent disease-related association.

What the risks are:

• excessive bleeding
• fainting or feeling light-headed
• hematoma (blood accumulating under the skin)
• infection (a slight risk any time the skin is broken)
• multiple punctures to locate veins

Special considerations:

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