W8CL3 - Inherited & Acquired Disorders of Coagulation Factors

Question 1

Examples of Inherited and Acquired Disorders of Coagulation Factors

Answer 1

Inherited disorders
- haemophilias
- 'rare bleeding disorders’
Acquired disorders
- liver disease
- vitamin K deficiency/antagonism
- inhibitors
- disseminated intravascular coagulation

Question 2

Coagulation Pathway

Answer 2

The most crucial feature of the coagulation pathway is the generation of thrombin
Thrombin is responsible for cleaving fibrinogen to fibrin, activating FXIII to FXIIIa (which cross-links fibrin), activating platelets, and positively feeding back into the cycle by activating upstream factors
Thrombin generation is initiated when damage to a vessel wall exposes the blood to tissue factor (TF) in the subendothelium

Question 3

Coagulation Pathway - Steps for Thrombin Formation

Answer 3

1. TF forms a complex with FVIIa and activates FX
2. Together, FVa and FXa form the prothrombinase complex, which then cleaves a small amount of prothrombin (FII) to thrombin (FIIa)
3. This small amount of thrombin activates platelets, FV, FVIII and FXI, feeding back into the cycle to increase thrombin formation
4. Factor IXa, previously activated by either TF-VIIa or by FXIa on the platelet surface, and FVIIIa in the presence of calcium, complex on the platelet surface to form the platelet tenase complex
5. Platelet tenase activates more FX, which with FVa, generates a thrombin burst
6. It is this burst of thrombin rather than the initial thrombin activation that is crucial for the formation of a stable haemostatic plug

Question 4

Screening Tests

Answer 4

aPTT; intrinsic & common pathways
PT; extrinsic & common pathways
Combined aPTT & PT
- ‘normal’ in either aPTT/PT precludes common pathway deficit
- prolongation of both aPTT & PT may indicate common pathway deficiency

Question 5

Determination of Individual Coagulation Factors

Answer 5

Principle:
- two samples containing the same coagulation factor are assayed in a specific assay system in a range of dilutions
- the clotting times are plotted against the plasma concentration on linear graph paper, curved dose–response lines are obtained
- the plot is redrawn on double-log paper, a sigmoid curve with a
straight middle section is obtained
- if the dilutions of the test and standard materials are chosen carefully, it should be possible to draw two straight parallel lines
- the dilutions of known plasma provides a standard curve which can be interpreted

Question 6

Bethesda Unit

Answer 6

1 Bethesda unit = inhibitor that will inactivate half the factor present in mixture of equal volumes of patient plasma & pooled normal plasma after 2h of incubation

Question 7

Haemophilia A

Answer 7

FVIII deficiency
X-linked recessive inheritance
Clinical manifestation is dependent on [factor]

Question 8

Haemophilia A - Clinical

Answer 8

Broadly classified clinically as mild, moderate & severe

1. Severe Haemophilia (less than 1% Factor)
   - bleeding occurs without known trauma
   - haemarthrosis frequent with developing walking & chronic haemophilic arthropathy by early adulthood
   - soft tissue haemorrhages dissect tissue planes and compromise vital organs
2. Moderate Haemophilia (1%-5% Factor)
   - occasional haemarthrosis & haematomas, usually associated with trauma
   - haemarthropathy is less disabling than in the severe patients
3. Mild Haemophilia (6%-30% Factor)
   - infrequent bleeding episodes
   - may go undiagnosed
   - excessive haemorrhage following surgery or injury

Question 9

Haemophilia A - Screening Results

Answer 9

Characteristic screening results => prolonged aPTT
Specific [FVIII] needed to confirm as haemophilia A
Molecular characterisation of F8 gene lesions

Question 10

Haemophilia B

Answer 10

FIX deficiency
X-linked recessive inheritance
Clinical manifestation is dependent on [factor]

Question 11

Haemophilia B - Clinical

Answer 11

Same as Haemophilia A with severe, moderate and mild haemophilia

Question 12

Haemophilia B - Screening Results

Answer 12

Characteristic screening results => prolonged aPTT
Specific [FIX] needed to confirm type
Molecular characterisation of F9 gene lesions

Question 13

Rare Bleeding (Coagulation) Disorders

Answer 13

Rare bleeding disorders represent 3% to 5% of all inherited coagulation deficiencies
Usually transmitted as autosomal recessive traits
They include inherited deficiencies of…
- fibrinogen (FI)
- FII
- FV
- FVII
- FX
- FXI
- FXIII
- combined FV & FVIII deficiencies

Question 14

Fibrinogen Deficiency

Answer 14

Afibrinogenemia & severe hypofibrinogenemia
Typical severe clinical manifestation
Common pathway deficit
Lab tests based on fibrin clot formation are abnormal
- prolonged PT, aPTT
- decreased fibrinogen
Definitive diagnosis
- immunologic measurement of [fibrinogen]

Question 15

FVII Deficiency (Hypoproconvertinaemia)

Answer 15

Rare, autosomal recessive disorder
Typically severe clinical manifestation
Extrinsic pathway deficit
Levels <15% may result in clinical haemorrhage
Shortest factor half life ~ 5 hours
PT prolonged and aPTT and Fibrinogen normal

Question 16

Liver Disease

Answer 16

Liver function affects all hemostatic functions
Liver synthesizes most
- pro-coagulant proteins e.g. coagulation factors
- physiological haemostatic inhibitors (‘anticoagulants’) e.g. tissue factor pathway inhibitor, antithrombin
- fibrinolytic proteins
Relationship (‘balance’) between pro-coagulant & anticoagulant can become ‘rebalanced’ with concomitant deficiency of both pro- and anti-coagulant proteins in cirrhotic liver disease

Question 17

Liver Disease - Clinical

Answer 17

Haemorrhage is usually the dominant clinical issue
Thrombosis may also occur in some circumstances
Excess fibrinolysis in a minority of cases (~10%)

Question 18

Liver Disease - Laboratory Assays

Answer 18

PT (& INR) are often prolonged however then is a large amount of variation
[Platelet] may be decreased in some patients
[vWF] may be increased (but HMW multimers, ADAMTS13 decreased)
[Fibrinogen] commonly used to assess fibrin forming ability
Assays for fibrinolysis (e.g. FDPs, D-dimer)

Question 19

Vitamin K Deficiency Bleeding (VKBD)

Answer 19

Precursor proteins synthesized by hepatocytes
- not γ-carboxylated
- Ca2+ binding sites are nonfunctional
- induced functional deficiencies of all vitamin-K dependent proteins
Causes of vitamin K deficiency in adults
- malabsorptive syndromes
- biliary tract obstruction
- prolonged broad-spectrum antibiotics
- ‘superwarfarin’ poisoning
Most often seen in newborns
- classical VKDB occurs in the first week of life, is related to delayed or inadequate feeding & is readily prevented by small doses of vitamin K at birth
- delayed VKDB peaks at 3–8 weeks, typically presents with intracranial haemorrhage often due to undiagnosed cholestasis with resultant malabsorption of vitamin K

Question 20

Superwarfarin Poisoning

Answer 20

Severe deficiency of vitamin K dependent proteins in patients not maintained on vitamin K antagonists
Most commonly associated with poisoning resulting from accidental or deliberate ingestion of warfarin, warfarin-like anticoagulants, or potent rodenticides (‘superwarfarins’), such as brodifacoum
Superwarfarins are ~100x more potent than warfarin and have a half-life measured in weeks
Serious bleeding manifestations are common

Question 21

Acquired Pathologic Inhibitors - Circulating Anticoagulants

Answer 21

Usually IgG or IgM immunoglobulins
Inhibitors of single factors
- patients with inherited factor deficiencies
- associated with other conditions (diseases, drugs)
- occasionally seen in otherwise healthy individuals
- interfere with or neutralize clotting factor activity
- prolonged screening test not corrected by 1:1 mixture with normal plasma
Can be measured using Bethesda assay

Question 22

Acquired Pathologic Inhibitors - Lupus Anticoagulant

Answer 22

Develop in 6-16% of patients with SLE
Also seen in other autoimmune diseases, neoplasias, certain infections, certain drugs, & some apparently normal individuals
“Antiphospholipid antibodies” (APLA), “anticardiolipin antibodies” (ACA)
It’s a laboratory phenomenon – interferes with assays and not typically associated with clinical bleeding disorder

Question 23

Lupus Anticoagulant Tests

Answer 23

Dilute Russell Viper Venom Time (RVVT) should be used
Another test will usually be an APTT using a reagent with proven LA sensitivity
A confirmatory test using a high phospholipid concentration

Question 24

Disseminated Intravascular Coagulation

Answer 24

Coagulation factors are consumed faster than they are synthesized
Acquired deficiency of multiple hemostatic components
Fibrinolysis follows fibrin formation
Patient generally bleeds at same time that disseminated clotting is occurring

Question 25

DIC - Aetiology

Answer 25

Group of haematological abnormalities secondary to a primary condition
Many disorders can trigger DIC
Initiating event =>
- generalized/systemic formation of thrombin & activation of coagulation
- unregulated thrombin => consumption of its substrates (fibrinogen, FV, FVIII, FXIII)
- depletion of prothrombin
- activation & aggregation of platelets
- release of t-PA & subsequent activation of plasminogen → plasmin & fibrinolysis

Question 26

DIC - Scoring System

Answer 26

For overt DIC, a cumulative score of five or more from prolonged PT, reduced platelets and fibrinogen, and elevated fibrin related markers (e.g. D-dimer or FDP)
A strong correlation between an increasing DIC score and mortality has been demonstrated by several studies