Osmotic Fragility Calculator
Calculate red blood cell osmotic fragility, 50% lysis point, and compare normal vs pathological.
Why Shape Determines Fragility
A normal red cell is a biconcave disc with roughly 40% more membrane surface than the minimum needed to enclose its volume. That surplus is what allows it to swell in hypotonic solution before the membrane reaches its limit.
A spherocyte has lost surface area through membrane defects, so it is already close to spherical — the shape with the least surface per unit volume. With little reserve, it lyses at a much higher salt concentration.
| Cell type | Surface-to-volume | 50% lysis at | Fragility |
|---|---|---|---|
| Normal biconcave | High reserve | 0.40–0.50% NaCl | Normal |
| Spherocyte | Low reserve | 0.55–0.70% | Increased |
| Target cell | Excess membrane | 0.30–0.40% | Decreased |
Target cells illustrate the principle in reverse: they have excess membrane relative to volume, so they tolerate far more swelling and resist lysis. This is why thalassaemia and iron deficiency give decreased osmotic fragility.
Reading the Curve
Results are plotted as percent haemolysis against decreasing NaCl concentration, giving a sigmoid curve. The median corpuscular fragility — the concentration causing 50% lysis — is the summary statistic. A curve shifted right means increased fragility, since lysis begins at a higher, less hypotonic salt concentration.
| Finding | Associated with |
|---|---|
| Right shift | Hereditary spherocytosis, autoimmune haemolytic anaemia |
| Left shift | Thalassaemia, iron deficiency, liver disease, post-splenectomy |
| Tail on the curve | Mixed population — some fragile cells |
Incubating blood at 37°C for 24 hours before testing increases sensitivity. Spherocytes deteriorate faster than normal cells under those conditions, so a borderline result often becomes clearly abnormal — useful in mild hereditary spherocytosis.
The test has largely been superseded by eosin-5-maleimide binding by flow cytometry, which is quicker and more specific for the membrane protein defects underlying spherocytosis. Osmotic fragility remains useful where flow cytometry is unavailable.
Worked Examples
Common Mistakes
A right shift means lysis at higher salt concentration — the cells are more fragile, not less. This is a frequent misreading of the curve.
Twenty-four hours at 37°C markedly increases sensitivity for mild hereditary spherocytosis. A normal fresh result does not exclude it.
Any condition producing spherocytes raises fragility, including autoimmune haemolytic anaemia and burns. The test detects shape, not cause.
A left shift is also informative, indicating excess membrane relative to volume as seen in thalassaemia and iron deficiency.