Osmotic Fragility Test

Written by Debra Stang | Published on July 9, 2012
Medically Reviewed by George Krucik, MD

What Is an Osmotic Fragility Test?

An osmotic fragility test is performed to diagnose two hereditary conditions: thalassemia and hereditary spherocytosis.

Thalassemia causes your body to make an abnormal form of hemoglobin. Hemoglobin is the protein that allows red blood cells to carry oxygen. If you have thalassemia, your red blood cells are more likely to be destroyed. This can lead to anemia.

Hereditary spherocytosis causes problems with the outer layer of your blood cells. This can also lead to anemia.

To perform this test, you will give a blood sample. Your red blood cells will be tested to see how easily they break. If your red blood cells are more fragile than normal, the test is considered positive.

Why Doctors Order Osmotic Fragility Tests

Doctors may order osmotic fragility tests for infants with a family history of thalassemia or hereditary spherocytosis. This is a faster way to diagnose the disease rather than waiting for genetic tests. It also allows a doctor to identify patients with symptomatic disease instead of just those with a mutation.

The test may also be used to diagnose symptoms of anemia such as:

  • fatigue
  • shortness of breath
  • dizziness
  • chronically cold hands and feet

This can help identify genetic diseases which cause anemia in people who do not know they are at risk.

How Is the Test Performed?

There are no special preparations required for the test. It is a simple blood test, also known as a venipuncture. It can be performed in either a lab or a doctor’s office.

If you are wearing a long-sleeve shirt, the technician will ask you to roll up one of your sleeves or to remove your arm from the sleeve. The technician will then tie a piece or rubber tightly around your upper arm to help the veins stand out. You may find this part of the process uncomfortable.

Once a vein has been identified, the technician will clean the spot on your skin with rubbing alcohol. A hollow needle will be inserted through the skin and into the vein. For most people, this sensation feels like a sharp pinch.

After enough blood has been collected, the needle will be removed. Pressure will be put on the puncture for a few seconds. Then the wound will be covered with a bandage.

Risks of the Test

Having blood drawn carries few risks. The greatest risk, which occurs extremely rarely, is infection. Tell your doctor if you start running a temperature above 100 degrees F. You should also seek help if the skin around the puncture becomes red, swollen, and painful to touch.

For a few days after the test, the skin around the puncture may be bruised or tender. This is normal. Applying ice to the area can reduce bruising and ease discomfort. Unless your doctor tells you otherwise, you are free to resume all normal activities after the test.

Understanding Your Results

Doctors will examine the blood sample under a microscope. They are looking for red blood cells that are smaller than normal or have an unusual shape. To test the fragility of your red blood cells, they will be added into solutions with different salt concentrations. Normal blood cells will be able to remain intact at lower salt concentrations than fragile blood cells.

If your cells are diagnosed as fragile, you probably have either hereditary spherocytosis or thalassemia. Both of these genetic conditions can cause hemolytic anemia. This is a form of anemia that occurs because your red blood cells are destroyed too quickly. If your osmotic fragility test is positive, the next step is to test whether you are actively anemic.


Not everyone with these diseases will have the same level of symptoms. Some people will have only mild forms with occasional symptoms. Others will have severe forms that require immediate treatment and may shorten the life span.

Once your doctor determines how severe your condition is, you will discuss the need for treatment. If your illness is mild and you have few symptoms, watchful waiting may be all that is necessary. Treatment for severe disease will depend on your specific diagnosis.

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