Andrew Blann

SUMMARY

This chapter will outline:

• the basis of the main haematological investigations

• what these blood tests can tell us about the patient

• red blood cells and white blood cells in surgery

• the role of coagulation and anti-coagulation in surgery.

INTRODUCTION

Haematology involves the study of the form and function of the following blood components: red blood cells, white blood cells, platelets and coagulation proteins, the latter two components being important to form a clot (or thrombus). The haematology laboratory commonly provides support for preoperative assessment in the form of haematology investigations of which the most frequently requested is the full blood count and the coagulation screen. These tests will allow the preoperative assessor a view of the ability of the patient’s blood to deliver oxygen to the tissues, and also an opportunity to pick up any abnormality that may affect the patient during the perioperative period, especially with respect to the patient’s coagulation status.

The results of these investigations may require the practitioner to instigate changes in the patient’s on-going care in an effort to avoid the potential delay or cancellation of this surgery. Common issues identified relate to either anaemia or a coagulopathy but may identify previously unidentified disease states such as leukaemia or aplastic anaemia. The haematology laboratory will therefore offer additional support to the practitioner in providing the expertise to advise and to agree to these changes in consultation with other members of the multi-disciplinary team such as the surgeon and the anaesthetist as well as providing information for the patient. These changes should be instigated well in advance of planned surgery but may need expediting if required in the emergency setting, such as reversal of anti-coagulation prior to emergency surgery.

The origin of blood cells

There are three types of cells found in blood – red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). These are produced in the bone marrow from differentiation of stem cells in response to stimuli such as hormones, whose production can be increased in response to pathological processes such as infection and sepsis. An outline diagram (Figure 10.1) shows the differentiation of the stem cells into the various categories of blood cells.

Figure 10.1 The development of blood cell types from the undifferentiated stem cells in the bone marrow. The white blood cells (leucocytes) can be divided into granulocytes and agranulocytes, the former consisting of basophils, eosinophils and neutrophils and the latter consisting of lymphocytes and monocytes.

Further information about the differentiation of the blood cell types and the pathological diseases such as leukaemia can be found in specific haematology textbooks.

The production of these blood cells can be affected by pathological processes and this may lead to failure to produce sufficient red blood cells, which will lead to anaemia, failure to produce sufficient white blood cells (leukopenia), which will lead to infections, whilst insufficient platelets (thrombocytopenia) will lead to bruising and bleeding. Each of these low levels may be present by themselves, though low levels of all three types occur simultaneously – this is known as pancytopenia. This is often seen in patients undergoing chemotherapy and radiotherapy associated with cancer, as the bone marrow is very sensitive to these toxic drugs. Routine monitoring is required to evaluate the impact of these drugs on the bone marrow of such patients.

The most common serious bone marrow disease is leukaemia, which is caused by a failure to correctly regulate the number of white blood cells being produced. Not only are there increased numbers of white blood cells generated, but also those that are produced are immature and cannot fulfil their function of defending us from infections. As the tumour of immature white blood cells grows inside the bone marrow, the production of the two other types of blood cells, red blood cells and platelets, is also affected, and levels fall, leading to anaemia and thrombocytopenia.

Haematological investigations

The two commonest requested haematological investigations from the preoperative assessment clinic are the full blood count (FBC) and the coagulation screen. Historically an allied test – the ESR or erythrocyte sedimentation rate – was requested as a measure of the physical property of the blood. Its use has reduced significantly over the last two decades as it shares common ground with the ‘gold standard’ serum marker of inflammation – C reactive protein (CRP). It is now rarely requested within the preoperative assessment environment.

Full blood count and coagulation screen require blood to be collected into specific blood collection bottles, each requiring a specific anticoagulant prior to processing. FBC requires the anticoagulant ethylene diamine tetra-acetic acid (EDTA), while the coagulation screen requires the anticoagulant sodium citrate. Practitioners must check with the laboratory of their specific healthcare organisation as to the particular specifications of their organisation’s blood collection tubes as this may vary with geography and supplier. Collection into the incorrect tubes will require duplication of the investigation, much to the annoyance of the patient, and may delay the eventual surgical procedure.

Full blood count (FBC)

The FBC provides numerous results: those of red blood cells, of white blood cells, and of the number of platelets. An example of the results of a full blood count is displayed in Figure 10.2.

Figure 10.2 Full blood count result of a middle-aged healthy male patient, showing information relating to the three cellular components, red blood cells, white blood cells and platelets

Red blood cells

The important red blood cell measurements are haemoglobin (Hb), the red blood cell count (RBC in Figure 10.2) and haematocrit (Hct). There are also three other red cell indices: mean cell volume (MCV), mean cell haemoglobin (MCH) and mean cell haemoglobin concentration (MCHC) – the MCV being the most useful.

Haemoglobin is undoubtedly the index most frequently referred to in clinical haematology. It is a protein designed to carry oxygen from the lungs to the tissues, where the oxygen is given up to participate in respiration. The normal range varies between the sexes. Lower levels in menstruating women seem obvious, but in post-menopausal women levels are still lower than age-matched men as the latter produce testosterone to stimulate red cell production. Haemoglobin is carried in the blood in red blood cells, the number of which is provided as the red blood cell count (RBC). Red cells are unusual as they lack a nucleus, which gives additional flexibility, allowing the cells to penetrate the smallest capillaries. They are the most abundant cell in the blood, are often called erythrocytes, and numbers can also vary between the sexes, being lower in women.

The haematocrit (Hct) expresses that proportion, as a decimal or as a percentage, of whole blood that is taken up by all the blood cells. Since there are approximately a thousand more red blood cells than both white blood cells and (tiny) platelets, the red cells make up the major proportion of the haematocrit. Consequently, at the practical level, it provides an idea of the proportion of red blood cells that makes up the whole blood pool.

White blood cells

The white blood cell indices reported include the white cell count (WCC) and the differential. The latter is actually five different results – it tells us of the different proportions of the various types of white cells. These are (in order of frequency) neutrophils, lymphocytes, monocytes, eosinophils and basophils (see Figure 10.2). All these cells perform different tasks in defending us from attack by micro-organisms, and additionally take part in processes such as hypersensitivity reactions, e.g. hay fever. There may also be some unusual cells called blasts or atypical cells, and in high numbers these are likely to have considerable adverse consequences.

Platelets

These tiny bodies are fragments of a much larger cell found only in the bone marrow (the megakaryocyte). With no nucleus, they consist only of cytoplasm, but this extremely potent cytoplasm is packed full of chemicals. Platelets form a clot, or thrombus, when aggregated together with the help of the blood protein fibrin, and so minimise haemorrhage.

Coagulation screen

This generally consists of a plasma protein (fibrinogen), two clotting times (PT and APTT), and two ratios (INR and PTT ratio). Fibrinogen is one of the more important blood proteins involved in clotting and is made in the liver. It is converted into fibrin by an enzyme, thrombin (itself derived from prothrombin, and also a product of the liver), and is crucial in clot formation.

Prothrombin time (PT) and partial thromboplastin time (PTT) are measures of the ability of blood plasma to form an artificial clot in the laboratory. Some labs place an ‘A’ for ‘activated’ in front of the PTT, thus APTT. The difference between PT and PTT is that they measure different parts of the clotting pathway, can investigate different bleeding disorders, and can also be used to monitor the effects of different drugs that interfere with different parts of the coagulation system.

What tests and when?

Table 10.1 Anaemia and its causes

Anaemia is a far more common diagnosis that is demonstrated with the full blood count. Anaemia may be defined as the clinical (symptomatic) consequence of the failure of the blood to deliver enough oxygen to the tissues so they can adequately perform their physiological function. In contrast, other authorities will define anaemia as a level of haemoglobin below a certain level. However, a haemoglobin level of, say, 11.5g/dL may well be perfectly adequate for an elderly woman with few physiological requirements and a relatively quiet life, while the same haemoglobin level in a younger person with a very active lifestyle, perhaps including sports, will be inadequate. Thus the medical state of the individual as a whole person should be considered, not merely an arbitrary number at which one acts. An alternative view of anaemia may be the level at which concern arises, and at which further investigations are considered. Certainly, anaemia should not be seen merely as that level that automatically requires a blood transfusion, a therapy that many consider should be reserved only for life-saving situations.

White blood cells

White blood cells, or leukocytes, are collectively responsible for defending us from attack by micro-organisms such as viruses, bacteria and parasites. Accordingly, raised levels of these cells (which is described as leukocytosis) can be expected in the face of infections with these microbes. However, increased numbers may also be present in a number of conditions such as rheumatoid arthritis and cancer, and also after surgery and in leukaemia. It follows, in reverse, that low levels of white blood cells (leucopenia) may be a sign that the patient could be susceptible to infections. However, the causes of unexplained low white blood cells are very rare, and of these the most common is aplastic anaemia, and if present, there is also a low red blood cell count. The most common reason for leucopenia is the effect of cytotoxic drugs on the bone marrow, a therapy only undertaken in severe disease such as cancer. In this case, the patient is most likely to be aware of this problem.

The most common white blood cells are neutrophils and lymphocytes. Low levels of neutrophils and lymphocytes are, respectively, described as neutropenia and lymphopenia. Similarly, an increased number of neutrophils is called neutrophilia, or perhaps a neutrophil leukocytosis. High numbers of lymphocytes are referred to as lymphocytosis. Neutrophils and lymphocytes have different functions.

The fundamental purpose of the neutrophils is to protect the body from bacterial infections, such as may be caused by staphylococcal and streptococcal organisms. One mechanism by which neutrophils perform this function is to engulf the bacteria, and then kill and digest them with the help of intracellular enzymes and chemicals (phagocytosis). Another of the white blood cells, the monocyte, is also able to digest bacteria. Hence both neutrophils and monocytes may be described as phagocytes. Consequently, high numbers of neutrophils are often associated with bacterial infections, but may also be found in autoimmune diseases such as rheumatoid arthritis. This is because in arthritis the immune and inflammatory systems attack the body’s own tissues (such as the skin and joints) instead of bacteria. Increased levels of neutrophils in the absence of an infection, such as after surgery or severe exercise, are referred to as the ‘acute phase response’.

There are two major types of lymphocytes: B lymphocytes make immunoglobulins (antibodies – proteins designed to recognise, attack, and help destroy invading pathogens – Figure 10.1, page 188) whilst T lymphocytes cooperate in antibody production and also attack cells that are infected with viruses. Whilst many lymphocytes are present in the blood, they are also to be found in lymph nodes, the bone marrow, the liver and the spleen. These organs are collectively called ‘lymphoid tissue’. Mildly increased numbers of lymphocytes in the blood may be expected in conditions that also cause a raised neutrophil count. However, the highest levels often encountered in health are found during attack against viral infections, such as glandular fever. In such cases these ‘activated’ lymphocytes may be larger than usual, and sufficiently large to be described as blasts. However, some viruses actually kill lymphocytes, so that the number in the blood will fall, HIV being such a virus.

Platelets and coagulation proteins

Haemostasis is the technical name for the balance between an increased risk of clotting (thrombosis) and an increased risk of bleeding (haemorrhage). If this balance is disturbed then clinically important disease may result. Good haemostasis, as evidenced by ‘healthy’ clot formation, requires the correct activation and numbers of platelets with the correct assembly of coagulation proteins.

An increased risk for thrombosis follows from increased numbers of platelets (called thrombocytosis) and high levels of the coagulation proteins such as fibrinogen. However, platelets and fibrinogen do not normally come together to form a clot; the process of thrombosis generally requires a trigger. A good example of such a stimulus is orthopaedic surgery, which is why patients undergoing these operations are at risk of thrombosis and so need protection with anticoagulants. Thrombocytosis is often present in infections and in autoimmune diseases such as rheumatoid arthritis.

An increased risk of haemorrhage may result from low numbers of platelets (known as thrombocytopenia) and/or low levels of coagulation proteins. Drugs such as quinine and sulphonamides may cause a low platelet count, as well as poor production from the bone marrow, or excessive consumption. Low levels of coagulation proteins will be demonstrated by prolonged prothrombin and partial thromboplastin times. However, in the absence of established drug therapies such as warfarin and heparin, excess clotting times are hardly ever encountered in a routine setting. Perhaps the best-known such disease is haemophilia, caused by the lack of coagulation Factor VIII, but if present, it will have been present from childhood and very obvious to the patient and his family.

The whole topic of coagulation is very important in surgery and will be developed in great detail in a later section. Figure 10.3 (page 196) shows how the coagulation system comes together to form a clot (thrombus).

Figure 10.3 Simplified diagram of the coagulation system

Several factors start the coagulation ‘cascade’, such as crush injuries, severe bacterial infections or severed blood vessels that expose collagen. Among the first coagulation factors to be activated are Factor V and Factor VII.

Later, Factor X becomes activated and so is referred to as Factor Xa. The activity of this molecule is inhibited by low molecular weight heparin (LMWH), and Factor Xa is a major contributor to the complex of Factors and other molecules whose function is to convert prothrombin into thrombin. Once formed, thrombin acts on fibrinogen to turn it into fibrin. Fibrin forms a mesh that, with platelets and other blood cells, generates a thrombus (clot) to reduce blood flow.

Table 10.2 briefly summarises the value of the full blood count and coagulation screening tests in preoperative assessment.

Table 10.2
Interpretation of major haematological tests in preoperative assessment

Red blood cells and white blood cells in surgery

The haematology laboratory provides the practitioner with important information not merely about the patient’s general health, but also provides clues about how they may react to their surgery. As with all blood tests, the primary question in a screening setting would probably be ‘Is the patient fit for surgery?’

The red blood cell, haemoglobin, and anaemia

The major concern about red blood cells before, during and after surgery, is anaemia. Although high number of red cells can be dangerous, there are few patients with this problem. However, even the textbook definition of anaemia can vary, such as a haemoglobin result of less than 12g/dL, or less than 11g/dL, or even lower values, and perhaps varying with sex. Others define anaemia as the point at which the body may suffer, or perhaps when treatment is needed. Consequently, in the light of impending surgery, a high degree of flexibility is called for.

Before surgery

The function of the red blood cell and haemoglobin is to deliver oxygen to the tissues. However, surgery increases the demand by the body for oxygen. Thus a mild or moderate anaemia may not be a problem without surgery, but under stress a smaller number of red cells may not be able to provide the amount of oxygen required. This may have severe consequences, such as a heart attack. It may also be the case that anaemia is caused by a more dangerous and hidden disease, such as pure red cell aplasia, which means that the patient may not be able to withstand the effects of the surgery.

Nevertheless, if anaemia is present, it may be treatable in the short or moderate term. If so, then this is certainly worth considering, especially if the surgery can be delayed, as in elective orthopaedic cases. If this option is taken, the patient will need to be passed to the haematologists for treatment. The time taken for anaemia to resolve depends on the basis of the disease. For example, iron-deficient anaemia may respond to oral, depot or infused iron within a month or six weeks. However, the cause of the anaemia must be investigated, as it may be related to other serious disease such as myeloma, although most common diseases can be detected by more than one sign, symptom or laboratory result. There are many possible causes of anaemia, as indicated in Table 10.3. The practitioner will be aware that few of these conditions are associated with a single abnormal result and that a broad knowledge is required to successfully confirm a diagnosis.

Table 10.3 Potential causes of anaemia

If the need for the surgery is vital, perhaps acting on a life-threatening condition, then an alternative approach is to perform a short-lived but effective treatment – a blood transfusion. This is discussed in more detail in Chapter 11 of this book.

The effects of surgery

Clearly, some types of surgery can be more ‘bloody’ than others, especially those involving arteries. But there are several strategies to minimise blood loss, such as salvaging, followed by recycling. Despite this, surgical procedures seemingly without notable ‘on table’ blood loss are still associated with a fall in haemoglobin. In some cases of orthopaedic surgery, haemoglobin levels measured in the postoperative period have not reached pre-surgery levels by six weeks. Very accurate measures of blood loss indicate that the loss of a litre of blood should, in theory, lead to a fall in haemoglobin of 2.5g/dL. But in practice, this level of blood loss regularly leads to a fall of 3g/dL. This increased fall above and beyond the simple physical blood loss of surgery is well established but cannot be explained by unmeasurable blood loss (e.g. via the intestines). A healthy response to surgery sees the mobilisation of inflammatory cytokines as part of the acute phase response, and these may influence iron kinetics. For example, even simple surgery such as arthroscopy of the knee or correction of hallux valgus (with minimal blood loss) can cause a significant decrease in serum iron.

Recovery after surgery

This will be associated with a degree of reconstruction of cells and tissues and so there will be a high demand for oxygen. It is possible, therefore, that postoperative anaemia may impair recovery. However, the spleen is an effective reservoir of red blood cells, and the fact that the spleen shrinks after surgery suggests that some of these stored red cells have entered the circulation. As indicated in the previous paragraph, the red blood cell count and haemoglobin count may take months to recover.

However, red blood cell production can be actively promoted by the provision of the growth factor erythropoietin. Given subcutaneously in the perioperative period, this agent is effective in increasing levels of haemoglobin and in reducing the need for blood transfusions.

Is anaemia always bad?

White blood cells and microbes

Leucocytes defend us from microbial attack. So whereas high levels may indicate an infection, low levels may predispose to an infection.

Before surgery

A low white blood cell count may be found in an assessment clinic. In the absence of cytotoxic chemotherapy (possibly anti-cancer, and which the surgeon and various healthcare professionals are likely to be aware of) a low white cell count is important and needs investigating. This is not merely because it may be linked with other and/or more severe diseases, but also because it may leave the patient open to attack by pathogenic microbes. Therefore, if surgery cannot be postponed, antibiotic (if neutrophils are low) or antiviral (if lymphocytes are low) prophylaxis may be necessary. However, no drug is without a side effect, and, ironically, one of the most common such side effects is suppression of the bone marrow. A compromised white cell response after surgery may lead to life-threatening septicaemia and thus a spell in intensive care.

There are a small number of reasons for a high white blood cell count (a leukocytosis). These include infection, an autoimmune disease such as lupus, and leukaemia. The latter is relatively easy to diagnose as there is also anaemia and low platelets (thrombocytopenia). It is possible that a preoperative blood sample may suggest an autoimmune disease, but if so there will be other clinical clues and a history. However, a leukocytosis is more than likely to indicate a current, possible low-grade infection. But once again, other blood tests should be informative, and there should also be an increase in CRP, the ‘gold standard’ marker of inflammation. If the leukocytosis does indeed suggest an infection, then postponing the surgery should be considered and the patient should be placed on antimicrobial therapy.

White blood cells do not have a great part to play during surgery. However, they are likely to be primed by the operation, as in the acute phase response.

After surgery

One of the most common and potentially life-threatening consequences of surgery is infection. As part of the acute stress response associated with surgery, the body rapidly mobilises ‘dormant’ white blood cells, and places them in the blood, ready in case of an infection. This is also associated with other changes, such as a rise in the CRP. Thus a modest increase in the white blood cell count after surgery does not necessarily imply an infection. However, if a leukocytosis is more marked (perhaps 12–15 x 10⁹/L) and persistent (exceeding three to four days), then an infection will seem more and more likely.

Ideally, a leukocytosis, either as part of the acute phase response or a genuine response to an actual infection, will eventually fall. However, a persistent leukocytosis may be the consequence of an acute inflammation becoming transformed into chronic inflammation. If so, then a different class of chemotherapy may be called for.

Coagulation and surgery

The coagulation system exists to minimise blood loss and it does this by the controlled formation of a thrombus, formed from platelets and the blood protein fibrin (see Figure 10.3, page 196). Both fibrin and platelets are necessary to form a clot and an excess of either one cannot make up for a lack of the other. However, part of the normal and healthy acute phase response is an increased tendency to clot, which, if extensive, can be fatal. In such a case, prevention is essential.

Prior to surgery

It is necessary to ensure that the patient has a coagulation system that is ‘capable’ of functioning to minimise blood loss. A clear history of previous bleeding problems at the time of surgery, either personally for the patient or from the patient’s blood relatives, needs to be elucidated, coupled with any evidence that the patient has a bleeding tendency by bruising easily or has prolonged bleeding when sustaining abrasions or cuts.

Provided that the patient is not on any antithrombotic treatment such as warfarin, the key result with such a patient initially relates to the platelet count, with further investigation of the level of fibrinogen, INR and PTT ratio. The latter two will give an indication of the workings of the two different aspects of the coagulation system. Patients with deficiencies in this pathway may be unable to form a protective thrombus and so may haemorrhage. Ideally, one would not normally operate on such patients, but life-threatening emergencies may take precedence.

Thrombocytopenia slightly below the bottom of the reference range (150 x 10¹²/L) may be acceptable if the surgery is minor and blood loss is expected to be minimal. However, if the thrombocytopenia is more profound (75–125 x 10¹²/L) then a transfusion of a platelet concentrate may be appropriate, and if less than 50 x 10¹²/L, transfusion would be mandatory. However, discussion between the haematologist and the surgeon and anaesthetist is important to address the risks of proceeding to surgery. The cause of the thrombocytopenia should alsobe determined before surgery, as this may have an impact on postoperative care. For example, in the case of immune-mediated thrombocytopenia, mediated by an autoantibody to platelets, immunosuppression with an agent such as prednisolone may be advisable.

A similar situation is present in the face of low levels of fibrinogen (possibly less than 1.0 g/L) – is the surgery really necessary, and if so, how can we improve the haemostasis of the patient? Once more, one option is a transfusion of fresh frozen plasma (FFP), which will contain fibrinogen as well as other coagulation proteins.

After surgery

A major risk of surgery is haemorrhage, and the body has its own defence system ready to cope should this occur – an increased tendency to form a clot, generally in veins. This is called venous thromboembolism, and its occurrence is a veno-thromboembolic event (therefore both abbreviated to VTE). When this happens in veins of the leg it is called a deep vein thrombosis (DVT), whereas when it happens in a vessel in the lungs it is called pulmonary embolism (PE). Not only do DVT and PE lead to considerable morbidity; clots also kill!

• A DVT leads to a swollen, painful leg such that walking can be difficult, perhaps impossible. DVTs by themselves are rarely fatal but can produce considerable long-term morbidity. There is also powerful evidence that DVT leads to PE. About two-thirds of all VTEs are DVT.

• Blockage of a crucial lung vessel (be it an artery or a vein) by a PE will lead to breathlessness and pain, can lead in the long term to congestive lung disease and heart disease, and can indeed be fatal. PEs make up about a third of all VTEs.