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Osmotic fragility test

Hematology · also called OFT, Red cell osmotic fragility

What it measures

Measures how easily red cells burst in salt solutions of decreasing strength. Spherical cells burst sooner, so the test supports a diagnosis of hereditary spherocytosis.

Specimen and preparation

Specimen: Whole blood (heparin); normal control run in parallel
Tube: Green top
Preparation: Test fresh blood and also after 24 h incubation at 37 °C to increase sensitivity.

Reference range

Group	Range
Fresh blood	Hemolysis begins 0.45–0.50% NaCl; complete by 0.30–0.35% NaCl

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Increased fragility: hereditary spherocytosis
Autoimmune hemolytic anemia with spherocytes
Burns, some toxins

Causes of low values

Decreased fragility: thalassemia
Iron deficiency (hypochromic cells)
Sickle cell disease
Liver disease, post-splenectomy

Method

Graded hypotonic saline series with spectrophotometric hemolysis measurement

Good to know

Largely replaced by the EMA-binding flow cytometry test, which is more specific for hereditary spherocytosis.

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