Osmotic fragility Test

Doctors order this test to detect two hereditary disorders in people with intravascular haemolysis: 

• Hereditary spherocytosis: A disorder in which RBC shape changes to spherical (instead of the normal biconcave), leading to increased osmotic fragility. 
• Thalassaemia: A disorder in which the RBC become thin and flat, leading to reduced osmotic fragility. (Read more: Thalassemia treatment)

Your doctor may order this test if you show the following signs and symptoms of spherocytosis:

• Anaemia 
  • Pale skin
  • Fast heartbeat
  • Tiredness
  • Shortness of breath
  • Growth problems
• Low folate levels as the body uses more of it to replace the broken down RBCs
• An enlarged spleen as it works harder to break down and filter RBCs
• Aplastic crisis in which very few RBCs are made (usually due to an infection)
• Yellowing of eyes and skin (jaundice)
• Gallstones

This test may also be ordered if you have the following symptoms of thalassaemia:

• Anaemia
  • Tiredness
  • Shortness of breath
  • Fluttering, pounding or irregular heartbeats
  • Jaundice
  • Pale skin
• Gallstones and inflammation of the gallbladder
• Delayed growth during childhood
• Osteoporosis (weakened bones)
• Unusual bone growth, such as enlarged cheeks or forehead
• Reduced fertility

You do not need any preparation for this test. Tell your healthcare provider if you are on any medications, e.g. dapsone (for the treatment of leprosy), as it may affect the test results.

The test requires a blood sample. A laboratory technician will use a sterile needle to draw the sample from a vein in your arm. He/she will transfer the sample into a labelled container and send it to the laboratory for testing.

You may feel mild pain when the needle is inserted. 

Blood tests have a few risks like:

• Excessive bleeding
• Difficulty in locating the vein
• Feeling lightheaded
• Infection
• Haematoma (pooling of blood under the skin)

If you experience persistent discomfort, please talk to your doctor.

Normal results:

The reference values for osmotic fragility may vary from one laboratory to another. Please speak to your doctor to know what exactly your results indicate. 

Incubation of the blood sample for 24 hours at 37°C increases the sensitivity of the test. The following is a sample of a reference range:

• 0.50 g/dL (grams per decilitre) of saline (NaCl) (unincubated): 0%-47.8% haemolysis (males), 0%-31.1% haemolysis (females)
• 0.60 g/dL NaCl (incubated): 18.7%-67.4% haemolysis (males), 10.9%-65.5% haemolysis (females)
• 0.65 g/dL NaCl (incubated): 4.4%-36.6% haemolysis (males), 0.2%-39.3% haemolysis (females)
• 0.75 g/dL NaCl (incubated): 0.8%-9.1% haemolysis (males), 0%-10.9% haemolysis (females)

Abnormal results:

Certain conditions associated with increased osmotic fragility are:

• Hereditary spherocytosis
• Autoimmune spherocytosis
• Poisoning
• Severe burns
• Malaria
• Pyruvate kinase deficiency
• Haemolytic disease of the newborn
• Acquired haemolytic anaemia

Osmotic fragility decreases in the following conditions:

• Thalassaemia
• Reticulocytosis
• Iron deficiency anaemia
• Sickle cell anaemia

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational point of view and is in no way a substitute for medical advice by a qualified doctor.