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Erythrocyte fragility refers to the propensity of
erythrocytes Red blood cells (RBCs), also referred to as red cells, red blood corpuscles (in humans or other animals not having nucleus in red blood cells), haematids, erythroid cells or erythrocytes (from Greek ''erythros'' for "red" and ''kytos'' for "holl ...
(red blood cells, RBC) to hemolyse (rupture) under stress. It can be thought of as the degree or proportion of
hemolysis Hemolysis or haemolysis (), also known by several other names, is the rupturing ( lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo ...
that occurs when a sample of red blood cells are subjected to stress (typically physical stress, and most commonly osmotic and/or mechanical stress). Depending on the application as well as the kind of fragility involved, the amount of stress applied and/or the significance of the resultant hemolysis may vary. When multiple levels of stress are applied to a given population/sample of cells, a ''fragility profile'' can be obtained by measuring the relative or absolute extent of hemolysis existing at each such level, in addition to finding one or more single-number ''indexes'' (either measured directly or interpolated) associated with particular respective levels of hemolysis and/or corresponding stress. Fragility testing can be useful to assess cells' ability (or lack thereof) to withstand sustained or repeated stress. Moreover, it can be used to assess how fragility itself varies under different or changing environmental or stress conditions, during or prior to the inducement of the hemolysis. Low fragility is often termed "stability," though technically stability refers to cells' resistance to ''both'' stress-induced lysis ''and'' spontaneous auto-lysis.


Erythrocyte ''osmotic'' fragility

Osmotic fragility (OF) refers to the degree or proportion of
hemolysis Hemolysis or haemolysis (), also known by several other names, is the rupturing ( lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo ...
that occurs when a sample of red blood cells are subjected to osmotic stress by being placed in a
hypotonic In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective membrane-imp ...
solution. Osmotic fragility is affected by various factors, including membrane composition and integrity as well as the cells' sizes or surface-area-to-volume ratios. The osmotic fragility test is common in hematology, and is often performed to aid with diagnosis of diseases associated with RBC membrane abnormalities. Some diseases linked to ''increased'' OF include
hereditary spherocytosis Hereditary spherocytosis (HS) is a congenital hemolytic disorder, wherein a genetic mutation coding for a structural membrane protein phenotype leads to a spherical shaping of erythrocytic cellular morphology. As erythrocytes are sphere-shaped (s ...
and
hypernatremia Hypernatremia, also spelled hypernatraemia, is a high concentration of sodium in the blood. Early symptoms may include a strong feeling of thirst, weakness, nausea, and loss of appetite. Severe symptoms include confusion, muscle twitching, and b ...
, while some linked to ''decreased'' OF include chronic liver disease, iron deficiency anemia, thalassemia, hyponatremia, polycythemia vera, hereditary xerocytosis, and sickle cell anemia after splenectomy. New approaches to testing OF are under development to better facilitate its use in disease diagnosis and screening, such as by utilizing microfluidic devices along with cell counting.


Erythrocyte ''mechanical'' fragility

Mechanical fragility (MF) refers to the degree or proportion of
hemolysis Hemolysis or haemolysis (), also known by several other names, is the rupturing ( lysis) of red blood cells (erythrocytes) and the release of their contents (cytoplasm) into surrounding fluid (e.g. blood plasma). Hemolysis may occur in vivo ...
that occurs when a sample of red blood cells are subjected to mechanical stress, such as (typically) some kind of shear stress. Yet unlike with osmotic fragility, no single approach for testing mechanical fragility has yet gained sufficient acceptance to enable standardization.ASAIO J. 2005 May-Jun;51(3):194-201. Mechanical fragility calibration of red blood cells. This has led to some insurance companies not currently covering the test. Uses of erythrocyte mechanical fragility can include diagnostic testing, calibrations to aid comparisons of hemolysis caused by blood-handling devices, or assessment of sublethal (i.e., non-hemolysing) damage caused to cells from devices that manipulate blood (such as for dialysis or intraoperative autotransfusion). It can also help in assessing damage of stored RBC product (so-called "storage lesion"), leading to applications in
blood transfusion Blood transfusion is the process of transferring blood products into a person's circulation intravenously. Transfusions are used for various medical conditions to replace lost components of the blood. Early transfusions used whole blood, but mo ...
and
blood bank A blood bank is a center where blood gathered as a result of blood donation is stored and preserved for later use in blood transfusion. The term "blood bank" typically refers to a department of a hospital usually within a Clinical Pathology laborat ...
ing. It's also notable that there can be a ''qualitative'' difference between a mechanical fragility test involving a comparatively lower energy stress, such as by agitating one or more beads in the presence of the sample (a common approach), versus a higher energy stress, such as by applying ultrasound to the sample.Hemolysis by ultrasound: a comparative study of the osmotic and ultrasonic fragility tests. Scandinavian journal of haematology, Supplementum, no. 29, 1976. The difference is that the lower-energy category of stress can more prominently reflect cell membrane properties, whereas the higher-energy category largely reflects other properties like hemoglobin viscosity and cell size. Viscous or fluidic-mechanical stresses can be of either sort.


Related erythrocyte properties

Susceptibility to hemolysis from causes ''other than'' osmotic or mechanical forces are not as common, but may sometimes be referred to in terms of fragility or stability. For example, photons or radicals can induce hemolysis.Erythrocytes/RBC may also be tested for related membrane properties aside from fragility, including erythrocyte deformability and cell morphology. Morphology can be measured by indexes which characterize shape changes of differences among cells. Deformability testing involves measuring the degree or ease of cells' contortion or shape change under a given level of applied force - or some indirect inference of the like. Other related red blood cell properties can include adhesion and aggregation, which along with deformability are often classed as RBC "flow properties."


References

{{Myeloid blood tests Blood tests