Investigation of the hereditary haemolytic anaemias: membrane and enzyme abnormalities

Chapter 12 Investigation of the hereditary haemolytic anaemias


membrane and enzyme abnormalities




The various initial steps to be taken in the investigation of a patient suspected of having a haemolytic anaemia are outlined in Chapter 11 and the changes in red cell morphology that may be found in haemolytic anaemias are illustrated in Chapter 5. This chapter describes procedures useful in investigating haemolytic anaemias suspected to result from defects within the red cell membrane or deficiency of enzymes important in red cell metabolism.


The precise identification of an enzyme defect is beyond the scope of most haematology laboratories; it may require the isolation and purification of the enzyme and the determination of its kinetic and structural properties. In a service laboratory, it is sufficient to identify the general nature of the defect, whether it be in the membrane or the metabolic pathways of the red cell. In the case of putative metabolic defects, an attempt should be made, where possible, to pinpoint the enzyme involved. The first part of this chapter describes screening tests for spherocytosis, including hereditary spherocytosis (HS) and for glucose-6-phosphate dehydrogenase (G6PD) deficiency. The later sections of the chapter describe specific enzyme assays and the measurement of 2,3-diphosphoglycerate (2,3-DPG) and reduced glutathione (GSH).


Most of the enzyme assays have been standardized by the International Council for Standardization in Haematology (ICSH). Commercial kits are also available for some quantitative assays and screening tests. These are noted in the relevant sections.




Osmotic fragility as measured by lysis in hypotonic saline



Principle


The method to be described is based on that of Parpart et al.3 Small volumes of blood are mixed with a large excess of buffered saline solutions of varying concentration. The fraction of red cells lysed at each saline concentration is determined colorimetrically. The test is normally carried out at room temperature (15–25°C).




Method


Heparinized venous blood or defibrinated blood may be used; oxalated or citrated blood is not suitable because of the additional salts added to it. The test should be carried out within 2 h of collection with blood stored at room temperature or within 6 h if the blood has been kept at 4°C.










Notes




2. The blood must be delivered into the 12 tubes with great care. The critical point is not that the amount be exactly 50 μl, but rather that the amount added to each tube must be the same. Two methods are recommended:






The sigmoid shape of the normal osmotic fragility curve indicates that normal red cells vary in their resistance to hypotonic solutions. Indeed, this resistance varies gradually (osmotically) as a function of red cell age, with the youngest cells being the most resistant and the oldest cells being the most fragile. The reason for this is that old cells have a higher sodium content and a decreased capacity to pump out sodium.



Osmotic Fragility after Incubating the Blood at 37°C for 24 Hours




Factors Affecting Osmotic Fragility Tests


In carrying out osmotic fragility tests by any method, three variables capable of markedly affecting the results must be controlled, quite apart from the accuracy with which the saline solutions have been made up. These are as follows:





A proportion of 1 volume of blood to 100 volumes of saline is chosen because the concentration of blood is so small that the effect of the plasma on the final tonicity of the suspension is negligible. When weak suspensions of blood in saline are used, it is necessary to control the pH of the hypotonic solutions and it is for this reason that phosphate buffer is added to the saline. Even so, small differences will be found between the fragility of venous blood and maximally aerated (i.e. oxygenated) blood. For the most accurate results, it is recommended that the blood should be mixed until bright red. Finally, it is ideal for tests to be carried out always at the same temperature, although for most purposes room temperature is sufficiently constant.


The extent of the effect of pH and temperature on osmotic fragility was well illustrated in the paper of Parpart et al.3 The effect of pH is more important: a shift of 0.1 of a pH unit is equivalent to altering the saline concentration by 0.1 g/l, the fragility of the red cells being increased by a decrease in pH. An increase in temperature decreases the fragility, an increase of 5°C being equivalent to an increase in saline concentration of about 0.1 g/l.


Lysis is virtually complete at the end of 30 min at 20°C and the hypotonic solutions may be centrifuged at the end of this time.


Further details of the factors that affect and control haemolysis of red cells in hypotonic solutions were given by Murphy.4



Recording the Results of Osmotic Fragility Tests


In the past, osmotic fragility most often has been expressed in terms of the highest concentration of saline at which lysis is just detectable (initial lysis or minimum resistance) and the highest concentration of saline in which lysis appears to be complete (complete lysis or maximum resistance). It is, however, useful also to record the concentration of saline causing 50% lysis (i.e. the median corpuscular fragility, MCF) and to inspect the entire fragility curve (Fig. 12.1). The findings in health are summarized in Table 12.1.


Table 12.1 Osmotic fragility in health (at 20°C and pH 7.4)



















  Fresh blood (g/l NaCl) Blood incubated 24 h, 37°C (g/l NaCl)
Initial lysis 5.0 7.0
Complete lysis 3.0 2.0
MCF (50% lysis) 4.0–4.45 4.65–5.9

MCF, median corpuscular fragility.




Interpretation of Results


The osmotic fragility of freshly taken red cells reflects their ability to take up a certain amount of water before lysing. This is determined by their volume-to-surface area ratio. The ability of the normal red cell to withstand hypotonicity results from its biconcave shape, which allows the cell to increase its volume by about 70% before the surface membrane is stretched; once this limit is reached lysis occurs.6 Spherocytes have an increased volume-to-surface area ratio; their ability to take in water before stretching the surface membrane is thus more limited than normal and they are therefore particularly susceptible to osmotic lysis. The increase in osmotic fragility is a property of the spheroidal shape of the cell and is independent of the cause of the spherocytosis. Characteristically, osmotic fragility curves from patients with HS who have not been splenectomized show a ‘tail’ of very fragile cells (Fig. 12.3). When plotted on probability paper, the graph indicates two populations of cells: the very fragile and the normal or slightly fragile. After splenectomy the red cells are more homogeneous, the osmotic fragility curve indicating a more continuous spectrum of cells, from fragile to normal.



Decreased osmotic fragility indicates the presence of unusually flattened red cells (leptocytes) in which the volume-to-surface area ratio is decreased. Such a change occurs in iron deficiency anaemia and thalassaemia in which the red cells with a low mean cell haemoglobin (MCH) and mean cell volume (MCV) are unusually resistant to osmotic lysis (Fig. 12.1). A simple one-tube osmotic fragility is a useful screening test for β thalassaemia and some haemoglobinopathies in countries with a high incidence of these abnormalities (p. 612). Reticulocytes and red cells from patients who have been splenectomized also tend to have a greater amount of membrane compared with normal cells and are osmotically resistant. In liver disease, target cells may be produced by passive accumulation of lipid and these cells, too, are resistant to osmotic lysis.7


The osmotic fragility of red cells after incubation for 24 h at 37°C is also a reflection of their volume-to-surface area ratio, but the factors that alter this ratio are more complicated than in fresh red cells. The increased osmotic fragility of normal red cells, which occurs after incubation (Fig. 12.2), is mainly caused by swelling of the cells associated with an accumulation of sodium that exceeds loss of potassium. Such cation exchange is determined by the membrane properties of the red cell, which control the passive flux of ions, and the metabolic competence of the cell, which determines the active pumping of cations against concentration gradients. During incubation for 24 h, the metabolism of the red cell becomes stressed and the pumping mechanisms tend to fail, one factor being a relative lack of glucose in the medium.


The osmotic fragility of red cells that have an abnormal membrane, such as those of HS and hereditary elliptocytosis (HE), increases abnormally after incubation (Fig. 12.2). Similar results occur in hereditary stomatocytosis.8 The results with red cells with a glycolytic deficiency, such as those of pyruvate kinase (PK) deficiency, are variable. In severe deficiencies, osmotic fragility may increase substantially (Fig. 12.2), but, in other cases, the fragility may decrease owing to a greater loss of potassium than gain of sodium. In thalassaemia major and minor, osmotic fragility is frequently markedly reduced after incubation, again owing to a marked loss of potassium.9 A similar, although usually less marked, change is seen in iron deficiency anaemia.


To summarize, measurement of red cell osmotic fragility provides a useful indication as to whether a patient’s red cells are normal because an abnormal result invariably indicates abnormality. The reverse is, however, not true (i.e. a result that is within the normal range does not mean that the red cells are normal). The findings in some important haemolytic anaemias are summarized in Table 12.2.


Table 12.2 Osmotic fragility in haemolytic anaemias: a summary





































Condition Notes
A. Associated with increased osmotic fragility (OF)
Hereditary spherocytosis (HS) Entire curve may be ‘shifted to the right’, or most of it may be within the normal range but with a ‘tail’ of fragile cells. Curve within normal range in 10–20% of cases. After incubation for 24 h, abnormalities usually more marked, but still some false-negative results. Splenectomy does not affect MCF but reduces the tail of fragile cells
Hereditary elliptocytosis (HE) As in HS, but in general changes less marked. Abnormal OF usually correlates with severity of haemolysis (i.e. OF is normal in non-haemolytic HE)
Hereditary stomatocytosis As in HS with large osmotically fragile cells with low MCHC
Other inherited membrane abnormalities Results variable; with milder disorders curve more likely to be abnormal after incubation for 24 h
Autoimmune haemolytic anaemia Tail of fragile cells roughly proportional to number of spherocytes; rest of curve normal (or even left-shifted on account of reticulocytosis)
B. Associated with decreased OF
Thalassaemia MCF decreased in all forms of thalassaemia, except in some α thalassaemia heterozygotes; usually the entire curve is left-shifted
Enzyme abnormalities OF usually normal (anaemia originally referred to as hereditary nonspherocytic), but tail of highly resistant cells may be seen on account of high reticulocyte court. After incubation for 24 h, there may be a tail of fragile cells
Hereditary xerocytosis Increased resistance to osmotic lysis and increased MCHC
Iron deficiency Curve shifted to left, wholly or partly, depending on proportion of hypochromic red cells

OF, osmotic fragility; HE, hereditary elliptocytosis; HS, hereditary spherocytosis; MCF, median corpuscular fragility; MCHC, mean cell haemoglobin concentration.



Flow cytometric (dye-binding) test




Principle


The osmotic fragility test lacks specificity and sensitivity and fails to detect atypical or mild HS. Moreover, it can be affected by factors unrelated to red cell cytoskeletal defects; for example, positive results may be obtained for red cells from patients who are pregnant or who have immune or other haemolytic anaemias or renal failure. The flow cytometric (dye-binding) test of King and colleagues10 measures the fluorescent intensity of intact red cells labelled with eosin-5-maleimide (EMA), which reacts covalently with Lys-430 on the first extracellular loop of Band 3 protein. The N-terminal cytoplasmic domain of Band 3 interacts with ankyrin and protein 4.2, which in turn crosslink with the spectrin-based cytoskeleton and stabilizes the membrane lipid bilayer.11 Deficiency or abnormality of Band 3 may result in decreased fluorescence. This is seen in HS red cells but has also been observed in cases of South-east Asian ovalocytosis, congenital dyserythropoietic anaemia Type II and the stomatocytic variant, cryohydrocytosis. Blood samples in ethylenediaminetetra-acetic acid (EDTA) may be analysed for up to 48 h after collection provided they have been stored in the refrigerator.






Glycerol lysis-time tests


The osmotic fragility test is somewhat cumbersome and requires 2 ml or more of whole blood. It is thus not suitable for use in newborn babies or as a population screening test. In 1974, Gottfried and Robertson12 introduced a glycerol lysis-time (GLT) test, a one-tube test, to measure the time taken for 50% haemolysis of a blood sample in a buffered hypotonic saline–glycerol mixture. The original method had greater sensitivity in the osmotic-resistant range, but it also could identify most patients with HS by a shorter GLT50. Better identification of HS blood from normal was obtained by 24-h incubation of samples and by modifying the glycerol reagent.13 Zanella et al modified the original test further by decreasing the pH.14 There is some loss of specificity for HS with the acidified glycerol lysis-time test (AGLT) compared with the original method, but in practice this loss is unimportant.



Acidified Glycerol Lysis-Time Test








Cryohaemolysis Test




Principle


Whereas osmotic fragility may be abnormal in any condition where spherocytes occur, it has been suggested that cryohaemolysis is specific for HS.15 This appears to result from the fact that the latter is dependent on factors that are related to molecular defects of the red cell membrane rather than to changes in the surface area-to-volume ratio. The test can be carried out on EDTA blood up to 1 day old.





Interpretation


Streichman et al.15 report the range of cryohaemolysis in normal subjects to be 3–15%, whereas in hereditary spherocytosis there is >20% lysis. However, it is recommended that individual laboratories establish their own reference values for the method. We have found that most normal samples give <3% lysis. Increased lysis is not exclusive to hereditary spherocytosis and may be observed in hereditary stomatocytosis.



Autohaemolysis: spontaneous haemolysis developing in blood incubated at 37°c for 48 hours


The autohaemolysis test is useful as an initial screen in suspected cases of haemolytic anaemia. It provides information about the metabolic competence of the red cells and helps to distinguish membrane and enzyme defects if the results of the tests are taken together with other observations such as morphology, inheritance and the presence or absence of associated clinical disorders.16






Defibrinating Blood


Defibrinate blood, as described on p. 5.


Use sterile defibrinated blood and deliver four 1 ml or 2 ml samples into sterile 5 ml capped bottles. Retain a portion of the original sample; separate and store this as the preincubation serum.


Add to two of the bottles 50 or 100 μl of sterile 100 g/l glucose solution, so as to provide a concentration of glucose in the blood of at least 30 mmol/l. Make sure that the caps of the bottles are tightly closed and place the series of bottles in the incubator at 37°C. A sample from a known normal individual should be run in parallel as a control.


After 24 h, thoroughly mix the content by gentle swirling. After incubating for 48 h, inspect the samples for signs of infection, thoroughly mix again, then from each bottle remove a sample for the estimation of the packed cell volume (PCV) (by the microhaematocrit method) and haemoglobin (Hb) concentration and centrifuge the remainder to obtain the supernatant serum.


Estimate the spontaneous lysis by means of a colorimeter or a spectrometer at 540 nm.


As a rule, it is convenient to make a 1 in 10 dilution of the incubated serum in cyanide-ferricyanide (Drabkin’s) solution (p. 25), unless there is marked haemolysis, when a 1 in 25 or 1 in 50 dilution is more suitable. A corresponding dilution of the preincubation serum is used as a blank and a 1 in 100 or 1 in 200 dilution of the whole blood in Drabkin’s solution indicates the total amount of Hb present and serves as a standard.


Calculate the percentage lysis, allowing for the change in PCV resulting from the incubation as follows:



image



where R0 = reading of diluted whole blood; Rt = reading of diluted serum at 48 h; B = reading of blank; PCVt = packed cell volume at time t; D0 = dilution of whole blood (e.g. 1 in 200 = 0.005); and Dt = dilution of serum (e.g. 1 in 10 = 0.1).


The reading at time t is multiplied by (1−PCVt) so as to give the concentration that would be found if the liberated Hb was dissolved in whole blood (i.e. in both plasma and red cell compartments), not in the plasma compartment alone.




Significance of Increased Autohaemolysis


Little or no lysis takes place when normal blood is incubated for 24 h under sterile conditions and the amount present after 48 h is small.16 If glucose is added so that it is present throughout the incubation, the development of lysis is markedly slowed. The amount of autohaemolysis that occurs after 48 h with and without glucose is determined by the properties of the membrane and the metabolic competence of the red cell. In membrane disorders such as HS, the rate of glucose consumption is increased to compensate for an increased cation leak through the membrane.8 During the 48-h incubation, glucose is therefore used up relatively rapidly so that energy production fails more quickly than normal unless glucose is added. This is one factor that contributes to the increased rate of autohaemolysis in HS. Usually, but not always, the addition of glucose to the blood decreases the rate of autohaemolysis in HS. This was referred to as Type 1 autohaemolysis.16 When the utilization of glucose via the glycolytic pathway is impaired, as in PK deficiency, the rate of autohaemolysis at 48 h is usually increased but glucose fails to correct or may even aggravate lysis (Type 2 autohaemolysis).8 Although a similar result may be seen in severe HS (Type B), in the absence of spherocytosis failure of glucose to diminish autohaemolysis is a strong indication of a glycolytic block. Blood from patients with G6PD deficiency or other disorders of the pentose phosphate pathway may undergo a slight increase in autohaemolysis (without additional glucose), which is corrected by the addition of glucose. Commonly, the result is normal, but examination of the incubated blood may show an increase in methaemoglobin (Hi) (discussed later). Not all glycolytic enzyme deficiencies give a Type 2 reaction so that a Type 1 result does not exclude the possibility of such a defect.


In the acquired haemolytic anaemias, the results of the autohaemolysis test are variable and generally not very helpful in diagnosis. In the autoimmune haemolytic anaemias, lysis may be increased in the absence of additional glucose but the effect of added glucose is unpredictable. In paroxysmal nocturnal haemoglobinuria (PNH), the autohaemolysis of aerated defibrinated blood is usually normal.


Autohaemolysis may be increased in haemolytic anaemias caused by oxidant drugs or when there are defects in the reducing power of the red cell. Heinz bodies, Hi or both will be detectable at the end of incubation. Normally, red cells produce <4% Hi after 48 h incubation and Heinz bodies are not seen. Red cells containing an unstable Hb also contain Heinz bodies at the end of the incubation period and increased amounts of Hi.


The nucleosides adenosine, guanosine and inosine, like glucose, diminish the rate of autohaemolysis when added to blood. Remarkably, adenosine triphosphate (ATP) strikingly retards haemolysis in PK deficiency, although glucose itself is ineffective.18 ATP does not pass the red cell membrane.


The autohaemolysis test lacks specificity. This has drawn much criticism on the test, including the suggestion that it has no place in the screening of blood for inherited defects.19 The best way to detect metabolic defects in red cells is undoubtedly to measure glucose consumption, lactate production and the contribution to metabolism of the pentose phosphate pathway. These measurements are, unfortunately, difficult and are likely to be undertaken only by specialized laboratories. The autohaemolysis test does provide some information about the metabolic competence of the red cells and helps to distinguish membrane defects from enzyme defects.


In summary, we feel that the autohaemolysis test is still useful in the investigation of patients who have or who may have chronic haemolytic anaemia for the following reasons:





Thus, in our experience, a combination of red cell morphology with the results of the autohaemolysis tests makes it possible to differentiate membrane abnormalities from enzyme deficiencies in the vast majority of cases.


Jun 12, 2016 | Posted by in HEMATOLOGY | Comments Off on Investigation of the hereditary haemolytic anaemias: membrane and enzyme abnormalities

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