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 typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site 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 blood sample is sent to a laboratory to be tested. When a low-salt solution containing sucrose (sugar) is added to certain fragile cells, a part of the body's defense mechanism called complement is activated, binds to the cells, and bursts them.

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.

Your doctor may order this test if you have signs or symptoms of paroxysmal nocturnal hemoglobinuria (PNH) or hemolytic anemia of unknown cause. PNH red blood cells are very likely to be harmed by the body's complement system.

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

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

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

There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)

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

Elghetany M, Banki K. Erythrocytic disorders. In: McPherson R, Pincus M, eds. Henry’s Clinical Diagnosis and Management by Laboratory Methods. 21st ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 31.

Schwartz R. Autoimmune and intravascular hemolytic anemias. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 164.