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Osmotic Fragility Test

What Is an Osmotic Fragility Test?

An osmotic fragility test is used 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 red blood cells, distorting their shape. This can also lead to anemia.

For an osmotic fragility test, you’ll need to 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 who show symptoms of disease. Some people will have the genetic mutation but not show symptoms.

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 don’t know they’re at risk.

How Is the Test Performed?

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

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

The technician will find a vein and clean the area with rubbing alcohol. They’ll insert a hollow needle into the vein. For most people, this sensation feels like a sharp pinch.

After collecting enough blood, the technician will remove the needle. You’ll need to keep pressure on the puncture for a few seconds. Then, they'll cover the wound 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, or 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’re free to resume all normal activities after the test.

Understanding Your Results

A doctor will look at your blood sample under a microscope. They’re looking for red blood cells that are smaller than normal or have an unusual shape. To test the fragility of your red blood cells, your doctor will add them into solutions with different salt concentrations. Normal blood cells can stay 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’ll discuss your treatment needs. If your illness is mild and you have few symptoms, watchful waiting may be all that’s necessary. Treatment for severe disease will depend on your specific diagnosis.

Content licensed from:

Written by: Debra Stang
Medically reviewed on: Dec 23, 2015: Steven Kim, MD

This feature is for informational purposes only and should not be used to replace the care and information received from your health care provider. Please consult a health care professional with any health concerns you may have.
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