Haematology

Key Points

• Postoperative bleeding usually has a local cause

• Von Willebrand disease is the most common inherited bleeding disorder

• Anaemia is common and usually due to chronic blood loss

• Leukaemia presents with bleeding, anaemia and infections

Haemostasis

Normal haemostasis (bleeding cessation) depends on the interaction of blood vessels, platelets, fibrin coagulation and deposition, and fibrinolytic proteins (Fig. 8.1). Three reactions – primary, secondary and tertiary haemostasis – act simultaneously. Excessive blood clotting is prevented by the endothelium, which provides a physical barrier and secretes products that inhibit platelets (e.g. nitric oxide and prostaglandin I₂ [prostacyclin]). Inhibitors limit haemostasis to the site of injury and prevent overproduction of the clot – which could lead to pathological thrombosis.

Primary Haemostasis (Vascular and Platelet Activity)

Primary haemostasis is by vasoconstriction after injury, which retards blood loss, slows local blood flow, enhances platelet adherence to subendothelial surfaces and activates coagulation.

The endothelial lining of the vessel wall is a barrier between the circulating platelets and the prothrombotic subendothelial matrix. Upon vessel injury, the endothelial layer is disrupted and the circulating platelets interact with the subendothelium through adhesive receptors, such as glycoprotein (GP) Ib-IX-V receptors, that mediate rolling and tethering of the platelets to von Willebrand factor (vWF) at the site. Platelet collagen receptors α₂β₁ and GP VI then mediate more firm adhesion and further platelet activation, causing release of platelet dense granules contents; these contain agonists such as adenosine diphosphate (ADP), and α-granules, which contain fibrinogen, factor V and P-selectin. The platelet generates lipid mediators such as thromboxane A₂. ADP elicits its effects on the platelet through the P2Y₁ and P2Y₁₂ receptors, whereas thromboxane A₂ activates the thromboxane-prostanoid (TP) receptor on the platelet surface. The release of the granule contents also triggers the blood coagulation response as a result of the release of clotting factor V, and the inflammatory response through the exposure of P-selectin on the platelet surface. Also, tissue factor is exposed, which initiates the coagulation response that results in formation of thrombin. Thrombin activates platelets via interactions with the proteinase-activated receptor-1 (PAR1) and PAR4 receptors and also cleaves fibrinogen to form fibrin. Fibrin further stabilizes the accumulating platelet plug at the site of injury, resulting in a stable haemostatic plug.

Platelet activation in response to vessel injury is thus important for the arrest of bleeding (Fig. 8.2), but activation during disease states leads to vascular occlusion and ischaemic damage. The P2Y₁₂ receptor, activated by ADP, plays a central role in platelet activation and is the target of P2Y₁₂ receptor therapeutic antagonists such as clopidogrel (antiplatelets).

Fig. 8.2 **Platelet plug formation.**
GP=glycoprotein.

Abnormalities in primary haemostasis

Abnormalities in primary haemostasis (abnormal platelet number or function, abnormal vWF or defects in the blood vessel wall) lead to haemorrhage from mucosal surfaces (epistaxis, gingival bleeding, melaena, haematuria), and into skin and mucosae (petechial or ecchymotic haemorrhages). Prolonged bleeding from wounds is usually restricted by blood coagulation. However, if the defect is severe, more pronounced bleeding can result (e.g. intracavity haemorrhage).

Primary haemostasis inhibitors are the natural inhibitors of platelet function, such as bradykinin, prostacyclin and nitric oxide, released by endothelial cells. Acquired inhibitors of platelet function are rare, whereas acquired inhibitors of vWF (platelet GP Ib) form in a variety of diseases and result in acquired von Willebrand disease (avWD).

More commonly, platelet function is inhibited intentionally by the administration of agents such as aspirin or clopidogrel for the prevention of thrombosis.

Secondary Haemostasis (Blood Coagulation or Clotting)

Secondary haemostasis is the formation of fibrin through the coagulation cascade (Table 8.1, Fig. 8.3), which is traditionally separated into three pathways (extrinsic, intrinsic and common) involving blood coagulation factors acting as enzymes, which require activation and cofactors.

Table 8.1

Blood coagulation factors

Factor	Name
I	Fibrinogen
II	Prothrombin
III	Thromboplastin
IV	Calcium
V	Proaccelerin
VI	–
VII	Proconvertin
VIII	Antihaemophilic factor
IX	Christmas factor (plasma thromboplastin component)
X	Stuart–Prower factor
XI	Plasma thromboplastin antecedent
XII	Hageman factor
XIII	Fibrin-stabilizing factor
Fitzgerald factor	High-molecular-weight kininogen
Fletcher factor	Prekallikrein

Fig. 8.3 **Blood coagulation and assays.**
APTT=activated partial thromboplastin time; PT=prothrombin time; TT=thrombin time.

The extrinsic pathway is the main pathway. It is initiated by the generation/exposure of tissue factor (factor III), with expression being upregulated by cytokines (tumour necrosis factor alpha [TNFα], interleukin [IL]-6), and the tissue factor–factor VII complex, the latter activating factor X.

The intrinsic pathway amplifies coagulation, is stimulated by thrombin, through activation of factor XI, and involves high-molecular-weight kininogen, prekallikrein, factors XII, XI and IX, and factor VIII, which acts as a cofactor (with calcium and platelet phospholipid) for the factor IX-mediated activation of factor X. A minor route of stimulation of the intrinsic pathway (the alternative pathway) is the direct activation of factor IX by the tissue factor–factor VII complex.

The common pathway involves the factor X-mediated generation of thrombin from prothrombin (facilitated by factor V, calcium and platelet phospholipid [‘prothrombinase complex’]). Thrombin then activates factors XI and VIII, amplifying the coagulation cascade. Activated factor IX, together with activated factor VIII, calcium and phospholipid (‘tenase complex’), amplify the activation of factor X, generating large amounts of thrombin.

Thrombin cleaves fibrinogen to form soluble fibrin monomers, which spontaneously polymerize to soluble fibrin polymer; thrombin also activates factor XIII, which, together with calcium, cross-links and stabilizes the soluble fibrin polymer, resulting in cross-linked (insoluble) fibrin. Thrombin also promotes coagulation by promoting platelet aggregation, and activating factors V and VIII – necessary cofactors for the ‘prothrombinase’ and ‘tenase’ complexes, respectively. Thrombin also activates factor XIII, essential for the cross-linking of the fibrin polymer, and inhibits fibrinolysis by generation of a thrombin-activatable fibrinolytic inhibitor (TAFI).

Drugs that act as anticoagulants include warfarin, new oral anti-coagulants (NOACs), heparin and hirudin.

Abnormalities in the coagulation cascade

These cause more serious bleeding than do defects of primary haemostasis, resulting in bleeding into cavities (chest, joints and cranium) and subcutaneously (haematomas). Petechial haemorrhages are rare. The main coagulation inhibitor is antithrombin (AT; also called antithrombin III, or ATIII), a liver alpha₂-globulin that inhibits thrombin (factor IIa) and many activated coagulation proteins (including factors II, IX, X, XI and XII). Heparin, produced in vivo by degranulated mast cells or basophils, enhances antithrombin binding to thrombin, and this is the basis for its use as an anticoagulant.

Protein C, a vitamin K-dependent protein produced in the liver, inactivates factors V and VIII. Protein S (named after Seattle), another vitamin K-dependent protein synthesized in endothelial cells, megakaryocytes and hepatocytes, facilitates the action of protein C.

Tertiary Haemostasis (Fibrinolysis)

Tertiary haemostasis is the formation of plasmin from plasminogen via tissue-type plasminogen activators (t-PAs); other plasminogen activators include urokinase, factor XII and kallikrein. Plasmin lyses both fibrinogen and fibrin, releasing fibrin(ogen) degradation products (FDPs). Activation of fibrinolysis is triggered by fibrin and t-PA, a process regulated by haemostasis inhibitors.

Haemostasis inhibitors (inhibitors of fibrinolysis) include TAFI, alpha₂-antiplasmin, histidine-rich glycoprotein and plasminogen activator inhibitors. Amplification of the coagulation cascade from thrombin-mediated activation of factor XI leads to large amounts of thrombin and subsequent activation of TAFI, which prevents the binding of plasminogen to fibrin, thus inhibiting its conversion to plasmin. Alpha₂-antiplasmin binds free plasmin and causes its removal by the monocyte–macrophage system, preventing widespread fibrin-olysis. Plasminogen activator inhibitors (PAI-1 and PAI-2) are released by endothelial cells and limit plasmin generation by binding to t-PA.

Drugs that inhibit fibrinolysis (antifibrinolytics) include epsilon aminocaproic acid and tranexamic acid.

Haemostasis Screening Tests

Screening tests are done to show if there is a bleeding disorder, but further tests needed to define the disorder (Appendix 8.1) must follow consultation with the haematologist to ensure that they are appropriate. All that is needed on the request form is to state: ‘History of prolonged bleeding’ and to ask ‘Would you please investigate haemostatic function?’, giving as much clinical detail as possible. The blood sample should be adequately labelled and sent immediately for testing, together with relevant clinical information. Tests may need to be repeated before an accurate diagnosis can be made because levels of factors vary over time as a result of stress, pregnancy and infections.

Essential tests usually include:

■ full blood count, film. A routine ‘blood count’ [full blood picture] cannot identify a clotting defect but might show platelet deficiency

■ activated partial thromboplastin time (APTT). The APTT measures the intrinsic pathway and factors V, VIII, IX, X and XI (Table 8.2). Normal APTT values range between 25±10 seconds. The APTT was sometimes known as PTT or kaolin cephalin PTT. It is prolonged in heparin treatment, liver disease, haemophilias, disseminated intravascular coagulation (DIC), massive transfusion and some autoimmune disorders

■ prothrombin time (PT) and international normalized ratio (INR). The activity of the extrinsic coagulation pathway (factors II, V, VII and X) is measured by the PT, normal values ranging between 12 and 15 seconds. The PT will vary with the type of thromboplastin (e.g. rabbit, human, bovine, etc.) used in the assay, so therefore the INR is used. The INR is the PT ratio of a test sample compared to a normal PT corrected for the sensitivity of the thromboplastin used. Normal INR values are about 1.0. An INR above 1 indicates that clotting will take longer than normal. An INR of 2–3 is the usual therapeutic range for patients on anticoagulants used to treat deep vein thrombosis, and an INR of up to 3.5 is required for patients with prosthetic heart valves. The INR (and PT) is prolonged in warfarin treatment, liver disease, vitamin K deficiency, and DIC (Table 8.3). The INR may be quite stable or erratic in an individual but, in any event, is best assayed less than 24 hours preoperatively, though less than 72 hours may suffice. A normal PT does not necessarily exclude a significant underlying coagulopathy; for example, the PT is normal in severe haemophilias A and B and in factor XI deficiency

■ thrombin time (TT) (citrated sample)

■ platelet count. In the tourniquet test (Hess test), the appearance of more than a few petechiae on the forearm when a sphygmomanometer cuff is inflated suggests a platelet or vascular defect, but the clinical test is neither sensitive nor specific. The ‘bleeding time’ is also rarely used, as it is not completely reliable. Thus to exclude platelet defects, a platelet count is essential and platelet function assays may be indicated. A normal platelet count should be 100 000–400 000 cells/mm³ (Fig. 8.4); a count of less than 100 000 cells/mm³ (thrombocytopenia) can be associated with major postoperative bleeding

■ serum for blood grouping and cross-matching. This should be done if an operation is planned.

Table 8.2

Interpretation of activated partial thromboplastin time (APTT) results

APTT	Interpretation
Isolated prolonged	Acquired clotting factor inhibitors – usually directed against factor VIII (e.g. acquired haemophilia A)
	Congenital deficiencies of factor XII, XI, IX or VIII (in general, the factor has to be lower than 20–40% of normal before APTT is prolonged)
	Lupus anticoagulant (LA) – targets phospholipid
Prolonged+prolonged PT	Combined clotting factors deficiency
	Disseminated intravascular coagulation (DIC)
	Liver disease
	Massive blood transfusion – dilutional coagulopathy
	Thrombin inhibitors (hirudin, argatroban and dabigatran)
	Thrombolytic therapy
	Vitamin K deficiency
Increased±prolonged PT	Unfractionated heparin [UFH] significantly prolongs APTT but PT usually shows little prolongation Antiphospholipid antibodies
	Acquired clotting factor inhibitors, e.g. factors V, X
Prolonged±prolonged PT	Warfarin overdose
Short	An acute phase response with high factor VIII

Table 8.3

Interpretation of international normalized ratio (INR) and prothrombin time (PT) results

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PT or INR	Interpretation
Prolonged alone	Factor VII deficiency
Prolonged, along with other coagulation abnormalities	Afibrinogenaemia and dysfibrinogenaemia
	Dilutional coagulopathy, e.g. massive blood transfusion
	Direct thrombin inhibitors, e.g. lepirudin
	Liver disease
	Malabsorption (leads to vitamin K deficiency)
	Multiple clotting factor deficiencies
	Unfractionated heparin
	Vitamin K deficiency
	Vitamin K antagonists (e.g. warfarin, phenindione)
Shortened	Treatment with recombinant factor VIIa

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Fig. 8.4 **Protocol for investigation of a bleeding disorder.**
DIC=disseminated intravascular coagulation.

Once the haemostasis screening tests have indicated that there is a bleeding disorder, other, more specific tests are required, such as:

■ factor VIII clotting activity – to measure factor VIII amount

■ vWF antigen – to measure vWF amount

■ ristocetin cofactor activity – to measure vWF function

■ vWF multimers – to measure vWF structure

■ platelet aggregation tests – before assessing platelet function using in vitro tests of platelet adhesion and aggregation, aspirin, non-steroidal inflammatory drugs (NSAIDs) and antiplatelet drugs should be avoided for at least 7 days. The average lifespan of a platelet ranges from 7 to 12 days. Platelet aggregation can be measured using aggregating agents such as ristocetin but, unfortunately, such tests are difficult to standardize and do not identify all platelet disorders; the clinical features are often more important.

A practical guide to laboratory haemostasis is provided at: http://www.practical-haemostasis.com/Screening%20Tests/tt.html (accessed 30 September 2013).

Bleeding (Haemostatic) Disorders

General aspects

Bleeding disorders manifest with common bleeding features, including epistaxis and gingival bleeding, and may occur after wounds and surgery; these are often the first indications of a haemostatic defect. Defects in haemostasis can comprise abnormalities in platelet activation and function, contact activation or with clotting proteins, or may signal excess antithrombin function. The more common causes of a bleeding tendency include:

■ aspirin (one tablet impairs platelet function for almost 1 week but this rarely causes significant postoperative haemorrhage), NSAIDs and other antiplatelet drugs

■ warfarin (the most common anticoagulant, which interferes with clotting factor production via vitamin K blockage)

■ von Willebrand disease (the most common inherited bleeding disorder).

■ acquired bleeding tendencies, such as:

◆ bone marrow disease

◆ immune disorders

◆ liver disease

◆ renal disease.

Clinical features

Examination may reveal signs of purpura in the skin or mucosae (Fig. 8.5). Signs of underlying disease – such as anaemia and lymphadenopathy in leukaemia, for example – must also be sought. Joint deformities from haemarthroses, characteristic of haemophilia, should also be looked for but are infrequently seen now (Fig. 8.6).

Fig. 8.5 Purpura in a patient with myeloid leukaemia.

Fig. 8.6 **Haemarthrosis in an older haemophiliac has caused crippling arthritis.**
His five brothers all succumbed to haemophilia at a time when factor VIII was unknown.

Alternatively, purpura may be localized to the mouth (sometimes grandiloquently termed ‘angina bullosa haemorrhagica’) and not associated with any abnormal bleeding tendencies. People with senile purpura (an age-related condition) bruise easily with minimal trauma, often without any other apparent underlying cause, and generally on the forearms and tops of the hands. The resulting dark purplish-red splotches fade gradually, often leaving a yellow/brown stain, which may disappear completely or remain. Such purpura can be caused by excessive sun exposure; drugs, such as aspirin and steroids (steroid purpura); diabetes; vascular diseases; thrombocytopenia; scurvy; and connective tissue diseases. There is no treatment available.

General management

Patients may already be aware of having a bleeding disorder and may carry an appropriate warning card, bracelet or wristband. Otherwise, an adequate history is the single most important part of evaluation; physical examination is necessary and laboratory tests are needed to confirm the diagnosis. Table 8.4 shows comparative features of coagulation defects and platelet defects, and Table 8.5 lists typical laboratory findings in platelet disorders.

Table 8.4

Comparative features of bleeding disorders

	Platelet defects^a	Coagulation defects
Gender affected	Females mainly	Males
Family history	Rarely	Usually positive
Nature of bleeding after trauma	Immediate	Delayed
Effect when locally applied pressure removed	May stop bleeding	Bleeding recurs
Spontaneous bleeding into skin or mucosa, or from mucosa or gingivae	Common	Uncommon
Bleeding from minor superficial injuries (e.g. needle prick)	Common	Uncommon
Deep haemorrhages or haemarthroses	Rare	Common
Bleeding time	Prolonged	Normal
Tourniquet (Hess) test	Positive	Negative
Platelet count	Often low	Normal
Clotting function	Normal	Abnormal

^a Purpura is rarely vascular.

Table 8.5

Typical laboratory findings in platelet disorders

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Disorder	Bleeding time	Platelet count	Clot retraction	Platelet aggregation	Platelet factor III activity
Thrombocytopenia	↑	↓	↓	–	–
Thrombasthenia	↑	N	↓	↓	↓
Storage pool deficiency	↑	N	N	N or ↓	↓
Aspirin/von Willebrand disease	↑	N	N	↓ (only with ristocetin)	N
Thrombocythaemia	↑	↑	N or ↓	N or ↓	N or ↓

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N=normal.

A history with any suggestion of a haemorrhagic tendency must be taken seriously. Nevertheless, patients can be remarkably capricious as to the information they provide and, in any case, cannot always be expected to know when bleeding can legitimately be regarded as ‘abnormal’. Previous dental extractions provide a useful guide, but prolonged bleeding (up to 24–48 hours) as an isolated episode is usually the result of local factors, especially excessive trauma. It should be stressed again that a history of previous haemorrhagic episodes is the most important feature since screening tests of haemostasis do not always detect mild defects.

Special emphasis must be placed on the following, which are suggestive of a bleeding disorder:

■ Deep haemorrhage into muscles, joints or skin – suggests a clotting defect

■ Bleeding from and into mucosae – suggests purpura (Fig. 8.7).

Females often ‘bruise easily’ but any such bruises are usually insignificant and small. Excessive menstrual bleeding is also rarely due to a bleeding disorder.

Most significant congenital bleeding disorders become apparent in childhood and patients may carry a warning card. However, people with mild bleeding tendencies can escape recognition until adult life if they manage to avoid injury or surgery, but can then present with oozing from an extraction socket for 2–3 weeks despite local haemostatic measures such as suturing. Patients who have had tonsillectomy or dental extractions without trouble, or previous dental bleeding that was controlled by local measures, are unlikely to have a serious congenital haemorrhagic disease. On the other hand, admission to hospital and blood transfusion or comparable measures have obvious implications. Haemorrhagic disease in a blood relative is also strongly suggestive of a clotting defect. Many drugs, such as anticoagulants (Fig. 8.8), NSAIDs and antiplatelet drugs, may cause bleeding tendencies, as may hepatic, renal, human immunodeficiency (HIV) and other disease. Some herbal products may also impair platelet aggregation and prolong bleeding (Ch. 26).

Fig. 8.8 Oral anticoagulant therapy warning card.

Precise characterization of a congenital clotting defect depends on assay of the individual factors, and a wide range of other investigations may be indicated according to the type of case. For example, the assays usually used in the diagnosis of haemophilia A are the APTT and the factor VIII coagulant (FVIIIC) activity. The bleeding time and whole-blood clotting time are uninformative and obsolete.

It is important also to investigate patients with a suspected bleeding tendency for anaemia. This is because:

■ anaemia is an expected consequence of repeated haemorrhages

■ any further bleeding as a result of surgery will worsen the anaemia, or the anaemia may need to be treated before surgery can be carried out

■ anaemia may be an essential concomitant of the haemorrhagic tendency – as in leukaemia.

Patients may also need to be screened for liver disease and for blood-borne infections, particularly HIV and hepatitis viruses.

Individuals with bleeding disorders may need to be treated with replacement of the missing factor. Earlier, use of blood or blood fractions sometimes resulted in the transmission of blood-borne viruses and other infections. In many countries, blood is now collected from donors under careful precautions, and screened to exclude antibodies to infections such as HIV, hepatitis B, hepatitis C, syphilis or cytomegalovirus. In developed countries, all cellular blood products are leukodepleted to limit the risk of transmission of prions and some other infections. Leukocytes in erythrocyte and platelet concentrates are now considered as a contaminant since they can cause many other adverse effects, such as the transmission of other cell-associated infectious agents (e.g. herpesviruses and Toxoplasma), febrile non-haemolytic reactions, graft-versus-host disease and immunosuppression. Recombinant factors are thus preferred for factor replacement.

Dental aspects

Prolonged post-operative bleeding is defined if any of the following appertains:

■ Bleeding continues for 12 hours.

■ Bleeding causes the patient to return for attention.

■ Bleeding causes a large haematoma/ecchymosis.

■ The patient needs a transfusion.

All dental professionals encounter patients who experience prolonged bleeding following operative procedures, often a dental extraction. Saliva contains fibrinolytic agents that may aggravate the situation. In most cases, the cause is local, and bleeding can be managed using simple local haemostatic measures such as applying pressure to the wound with sterile pads (moistened with water, normal saline or 5% tranexamic acid solution), using absorbable oxidized cellulose sponges, and suturing, as well as giving postoperative instructions verbally and in writing.

A bleeding tendency can be encountered in patients with congenital haemostatic defects such as haemophilias or some rare platelet defects; or acquired disorders such as may arise in liver disease, renal disease, bone marrow disease, immune disorders or with some drugs. In vascular or platelet disorders, postoperative problems following an extraction usually present as prolonged bleeding immediately after the event; in coagulation defects, the bleeding typically begins after some delay. Patients with a bleeding tendency may require prophylactic measures preoperatively, special precautions perioperatively and careful management postoperatively. Patients with hereditary bleeding disorders are often registered with a haemophilia reference centre. Care is as follows:

■ Those with a mild to moderate bleeding disorder might be appropriately shared between specialist hospital care and community and/or primary care dental services for different procedures.

■ Those with a severe bleeding disorder managed from home with appropriate medication may possibly be treated in primary care under the direction of the haemophilia reference centre.

■ Those with a severe bleeding condition with associated complications such as HIV, hepatitis C or inhibitors, or those requiring surgery, are often more appropriately cared for in a hospital care setting.

Some known haemophiliacs have neglected oral health because of barriers to accessing care, including travel to specialist centres, waiting times, increased treatment needs and cost, or fear of bleeding from dental procedures. They may require more complex treatment and management by the time that they present.

Surgery should ideally be planned and done where appropriate after suitable measures to correct the bleeding tendency, such as blood clotting factor or platelet replacement or, where drugs are responsible, by modifying drug doses provided that the risk of subsequent thromboembolism is not a contra-indication:

■ at the beginning of the day – this allows more time to deal with immediate rebleeding problems

■ early in the week – this allows for delayed rebleeding episodes occurring after 24–48 hours to be dealt with during the working week.

Vascular Disorders

Vascular purpura rarely causes serious bleeding; any bleeding into mucous membranes or skin starts immediately after trauma but stops within 24–48 hours. Vascular disorders are a rare cause of bleeding problems; they include Marfan, Ehlers–Danlos (Ch. 16) and Osler–Rendu–Weber syndromes.

Osler–Weber–Rendu Syndrome (Hereditary Haemorrhagic Telangiectasia [HHT/HHT1])

General aspects

Hereditary haemorrhagic telangiectasia (HHT) is an autosomal dominant condition with a high penetrance, as 97% of people affected exhibit symptoms. There is vascular dysplasia leading to telangiectasia and arteriovenous malformations in the skin, mucosa and viscera. There may be associated immunoglobulin A (IgA) deficiency or, rarely, vWD (see below).