The sugar-water hemolysis test is a blood test to detect fragile red blood cells by testing their ability to withstand swelling in a low-salt solution.

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

A needle is inserted into the vein, and the blood is collected in an airtight 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. The puncture site is covered to stop any bleeding.

For an infant or young child, 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 bleeding continues.

The blood is tested in a laboratory.

There is no special preparation needed for this test.

How you prepare infants and children for this test depends on your child's age and past experience. For general information regarding how you can prepare your child, see the following topics:

• Infant test or procedure preparation (birth to 1 year)
• Toddler test or procedure preparation (1 to 3 years)
• Preschooler test or procedure preparation (3 to 6 years)
• School age test or procedure preparation (6 to 12 years)
• Adolescent test or procedure preparation (12 to 18 years)

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.

This test is ordered when the health care provider suspects paroxysmal nocturnal hemoglobinuria (PNH), and for any hemolytic anemia of unknown cause. PNH red blood cells are very likely to be harmed by a part of the body's defense mechanism called complement.

When a low-salt solution containing sucrose (sugar) is added to these cells, the complement is activated, binds to the cells, and bursts them.

• Negative: less than 5% red blood cell breakdown (hemolysis)
• Positive: more than 10% hemolysis

• Paroxysmal nocturnal hemoglobinuria (PNH)
• Autoimmune hemolytic anemias and leukemia may give false positive result

Risks associated with having blood drawn are slight:

• Excessive bleeding
• Fainting or feeling light-headed
• Blood collecting under the skin (hematoma)
• Infection
• Many needle sticks to find veins

A negative test does not rule out PNH. False-negative results may occur if the fluid part of blood (serum) lacks complement.

Smith LJ. Paroxysmal nocturnal hemoglobinuria. Clin Lab Sci. July 2004;17:172-177.

McPherson RA, Pincus MR. Henry's Clinical Diagnosis and Management by Laboratory Methods. Philadelphia Pa: Saunders; 2006.