Disorders of Platelets and Coagulation Flashcards
1
Q
What are endothelial cells?
A
Flattened cells that line blood vessels and have pro and anticoagulant properties
2
Q
What are the functions of endothelial cells?
A
Normally anticoagulant inhibiting coagulation and platelet aggregation
Act as a barrier to sub-endothelial collagen which is procoagulant
3
Q
What produced Von Willebrand’s Factor (vWF)?
A
Endothelium and platelets
4
Q
Where is vWF stored?
A
Weibel Palade bodies
5
Q
Where is vWF located?
A
On sub-endothelial surface
6
Q
When is vWF released and what is its function?
A
Early in the haemostatic process and is responsible for platelet adhesion to collagen
7
Q
What do platelets look like?
A
Small discoid anuclear cells found in the circulation, 3-5um, pale basophilic with small red granules
8
Q
Where do platelets originate from?
A
Megakaryocytes in the bone marrow during a process called thrombopoiesis
9
Q
What mediates platelet production?
A
Thrombopoietin
10
Q
How long do platelets circulate for?
A
5-9 days in most species
11
Q
What is the functional structure of platelets?
A
Outer membrane has receptors important for adhesion and aggregation
Cytoplasm has actin and myosin allowing shape change
12
Q
What are the two different types of granules in platelets?
A
Alpha granules (red) that contain vWF, fibrinogen and factors V and VII Dense granules which contain ADP and calcium
13
Q
What glycoprotein receptors are associated with platelet membrane?
A
GP Ib binds vWF
GP IIbIIIa binds fibrinogen on adjacent platelets and allows platelets to aggregate
14
Q
What happens if there are defects in receptors?
A
Abnormal platelet function and clot formation occurs
15
Q
What is primary haemostasis?
A
Formation of the primary platelet plug
16
Q
What is the first step in primary haemostasis?
A
Damage to the endothelium and exposure of sub-endothelial collagen results in vWF release
17
Q
What is the second step in primary haemostasis?
A
Platelet adhesion occurs and platelets bind to collagen via receptor GP Ib and vWF from the endothelium
18
Q
What is the third step in primary haemostasis?
A
Platelets undergo shape change and become spherical with filipodia which exposes additional GP Ib and GP IIbIIIa
19
Q
What is the fourth step in primary haemostasis?
A
Platelets bind fibrinogen via GP IIbIIIa between adjacent platelets forming a clump/aggregate of platelets
20
Q
Where is fibrinogen generated from?
A
Coagulation cascade
21
Q
What is the fifth step in primary haemostasis?
A
Aggregating platelets rapidly degranulate releasing ADP, fibrinogen and vWF as well as thromboxane A2 from the platelet membrane which increase platelet adhesion and aggregation
22
Q
What is the final step in primary haemostasis?
A
Platelets release factors V and VIII which are involved in coagulation
23
Q
What is secondary haemostasis?
A
Activation of the coagulation cascade simultaneously with platelet plug formation
24
Q
What are the different pathways the coagulation cascade split into?
A
Intrinsic, extrinsic and common pathways but only for lab testing purposes not in vivo
25
What is the function of the extrinsic pathway of coagulation cascade?
Initiates coagulation cascade and is most important in vivo
26
What is the process of extrinsic pathway of coagulation cascade?
Tissue factor is released from damaged tissue binds to and activates FVII in the presence of calcium
TFFVII complex activates FX of the common pathway and FIX of the intrinsic pathway
27
What is the function of the intrinsic pathway of the coagulation cascade?
Amplifies the coagulation cascade
28
What is the process of intrinsic pathway of the coagulation cascade?
FXII is activated by contact with a negatively charged surface (co factor HMWK)
Activated FXII cleaves and activates FXI which in turn activates FIX which requires calcium
Activated FIX in turn activates FX of the common pathway
29
What is the process of the common pathway of the coagulation cascade?
Starts with activation of FX which binds activated FV and calcium on the platelet surface which converts prothrombin (FII) to thrombin (FIIa)
Thrombin converts fibrinogen (FI) to fibrin (FIa) and fibrin is cross-linked by activated FXIII
30
What is the importance of inhibitors of coagulation?
Required for a balanced clotting reaction only where it is required
31
What is the function of antithrombin III?
Inhibits thrombin and activated FX and is increased by heparin from the endothelium
32
What is the function of protein C?
Inactivates FV and FVIII
33
What is the process of fibrinolysis?
Enzymatic breakdown of fibrin by plasmin which is derived from plasminogen found in the platelet membrane and plasma
34
What does plasmin do?
Degrades both fibrinogen and fibrin to produce fibrin degredation products (FDPs)
35
How is platelet concentration calculated?
Automated count that can be done on platelets collected into EDTA as part of a CBC
36
In which species is platelet concentration inaccurate? Why?
Cats, sheep and goats as there is an overlap between RBC and platelet size
37
What other artefact can cause platelet concentration calculation inaccuracies?
Platelet clumps
38
In which species is an estimated platelet count from blood smear recommended?
Cats and CKCS
39
How few platelets indicate thrombocytopaenia?
< 100x10^9/L
40
How low do platelet numbers need to be for spontaneous haemorrhage to occur?
25x10^9/L
41
How many platelets do patients need to be diagnosed with thrombocytosis?
> 1000x10^9/L
42
What is the risk of thrombocytosis?
Increased risk of thrombi forming
43
What is measured in buccal mucosal bleeding time?
Length of time for a platelet plug to form evaluated primary haemostasis and platelet formation
44
How is a buccal mucosal bleeding time calculated?
Using a spring loaded cassette to make a small incision in the buccal mucosa and blood is blotted until bleeding stops
45
In which diseases is the buccal mucosal bleeding time altered?
Decreased in thrombocytopaenia
| Prolonged with von Willebrands disease and disorders of platelet function
46
What can happen if platelet numbers are decreased or their function is impaired?
Haemorrhage in the form of eccymoses or petechiae
47
What can cause thrombocytopaenia?
Immune-mediated thrombocytopenia
Increased consumption
Decreased production due to bone marrow disease, neoplasia or drugs
Infection with FeLV, BVD, Ehrlichia and Leishmania
Sequestration is rare but may occur with splenomegaly/large cavitated mass
48
What is the most common cause of thrombocytopaenia?
Immune-mediated thrombocytopaenia with numbers reaching below 10x10^9/L
Megakaryocytes in the bone marrow may also be affected
49
What is Eva's syndrome?
Concurrent immune-mediated thrombocytopaenia and anaemia
50
What can cause immune-mediated thrombocytopaenia?
Primary, other immune disease, drugs, vaccine, neoplasia or infection
51
What are the clinical findings of immune-mediated thrombosytopaenia?
Profound thrombocytopenia, petechial/eccymotic haemorrhage, beeding from gums/mucosal surfaces/prolonged bleeding from wounds
52
How is immune-mediated thrombocytopaenia diagnosed?
Diagnosis of exclusion but can use bone marrow examination, anti-platelet antibodies and response to treatment
53
What signs are seen if thrombocytopaenia due to haemorrhage?
Evidence of haemorrhage either into a cavity or out of the body
Numbers shouldn't be lower than 100x10^9/L
54
What is seen if thrombocytopaenia is due to DIC?
Numbers can be very low with concurrent signs of another coagulation defect so need to check PT, PTT to see if increased and see if FDPs are increased
55
What is Glanzmann's thrombasthenia?
Defect in GP IIbIIIa found in Otterhounds, Great Pyrenees and Quarter Horses causing defective platelet aggregation and abnormal clot retraction
56
What is Canine thrombopathia?
Abnormal GP IIbIIIa exposure and impaired degranulation in Basset Hounds
57
What is Bovine Thrombopathia?
Occurs in Simmentals causing mild to severe bleeding although the exact defect isn't known
58
What causes physiological thrombocytosis?
Transient increase due to epinephrine induced splenic contraction
59
What is reactive thrombocytosis?
Secondary increase due to increased thrombopoietin/IL-6 or inflammation, haemorrhage or iron deficiency
60
What is essential thrombocythemia?
Myeloproliferative disorder causing a marked persistent increase in platelets
Bone marrow megakaryocytes are increased and may have abnormal morphology
Function of platelets is variable as may see petechiae and eccymoses or thrombosis
TPO levels normal or increased
61
What is vWF?
A plasma glycoprotein needed for platelet adherence to collagen and formation of primary haemostatic plug
62
How does vWF circulate?
Bound to factor VII which is protective against accidental activation and can see concurrent disease in factor VII
63
What are the different multimers of vWF?
There are three, small, medium and large with large being the most active in haemostasis
64
What are the clinical signs of von Willebrands disease?
Mucosal bleeding as GI/epistaxis/haematuria, no petechiae (differentiates from other platelet disorders), prolonged buccal mucosal bleeding time without thrombocytopaenia, clotting times usually normal but PTT may be prolonged due to decrease in factor VII
65
What animals has von Willedbrands disease been diagnosed in?
Most common in dogs and rare in cats, horses and pigs
66
What is type I von Willebrands disease?
All multimers present but at decreased concentrations
Variable severity of bleeding but not until concentration of vWF are less than 20%
Accounts for 90% of cases especially in Dobermanns
Autosomal inheritance so equal males and females
67
What is type II von Willebrands disease?
Qualitative abnormalities in vWF structure and function often with disproportionate decrease in large multimers
Severe and uncommon but seen in German shorthaired and wirehaired pointers with one horse case
Autosomal recessive
68
What is type II von Willebrands disease?
Absence of all vWF multimers with less than 0.1%
Seen in Scottish Terriers, Chesapeake Bay retrievers, Shetland Sheepdogs and Dutch Kooiker
Autosomal recessive
69
How do you perform a test to compare vWF:Ag ratio?
Collect blood into EDTA or citrate (dilution of 1:9)
| Separate plasma immediately and send overnight with ice
70
How is an ELISA used to diagnose von Willebrands disease?
Quantitative measurement of vWF using species specific antibodies with <50% considered decreased = carrier/affected
71
How is immunophoresis used to diagnose von Willebrands disease?
Separates out relative amounts of different multimers and is required for diagnosis of type II disease
72
How are carriers identified?
Genetic testing
73
How is a vWF ELISA interpreted?
<35% = clinical disease
vWF:Ag 0% = type III disease
50-69% vWF:Ag = borderline, transmit but not affected
70-100% vWF:Ag = free from trait
74
What is the treatment for vWF?
Transfusion to supply vWF using cryoprecipitate as 5-10 times greater concentration of vWF given at dose of 1U/10kg
Plasma given at 6-12 ml/kg if cryoprecipitate isn't available and whole blood if anaemic as well
75
What can be used prior to surgery to reduce risk of bleeding out?
Desmopressin for dogs with type I at a dose of 1ug/kg SQ 30 minutes prior to surgery
Also an intranasal preparation available
76
What do most blood tests require blood to be collected in?
Citrated plasma with a ratio of anticoagulant:blood of 1:9 and filled up to the line
77
Why should trauma whilst taking blood be reduced as far as possible?
Prevent activating platelets and coagulation
78
How soon should serum be separated from blood?
Centrifuge within 1 hour and analyse within 4 hours or freeze it
79
What colour are citrate tubes?
Blue, purple or green (always check for clots in lids and tubes)
80
What is needed to calculate the activated clotting time (ACT)?
2ml of whole blood into an ACT tube containing diatomaceous earth and incubate for 60 seconds at 37C and check for clots every 5-10 seconds
81
What does ACT test?
Intrinsic and common pathways
82
How is the ACT interpreted?
ACT = time to initial signs of a clot
| Similar to PTT but less sensitive and thrombocytopaenia will prolong
83
What does PTT stand for?
Partial thromboplastin time
84
What is it used to evaluate?
Intrinsic and common pathways of coagulation
85
How is PTT performed?
Incubate citrated plasma with excess phospholipid, contact activator and calcium
86
How is PTT interpreted?
Prolonged PTT indicates a defect in intrinsic (factors XII,XI,IX,VIII) or common pathways (X,V,II,fibrinogen)
Factor activity must be <30% of normal to prolong the PTT
87
What causes a prolonged PTT?
Haemophilic (factors VIII or IX), factor XII deficiency, DIC, vitamin K deficiency and liver disease
88
What pathology doesn't affect the PTT?
Thrombocytopenia
89
What does PT stand for?
Prothrombin time
90
What does PT screen for?
Defects in extrinsic and common pathways of coagulation
91
How is a PT performed?
Incubate citrated plasma with tissue thromboplastin (TF) and calcium and measure time to clot formation
92
How is a PT interpreted?
Prolonged PT indicates defect in extrinsic (factor VII) or common pathways of coagulation
Factor activity must be <30% normal to prolong PT
93
What causes a PT to be prolonged?
Factor VII deficiency, DIC, vit K deficiency and liver failure
94
What test results are found in early vitamin K deficiency?
PT may be prolonged with a normal PTT
95
What does specific factor analysis detect?
Specific factor deficiencies usually to detect hereditary deficiencies
96
What does latex agglutination test evaluate? How is it performed?
Fibrinolysis
| Special test kit using serum separated within 30 minutes and needs to be tested immediately or frozen
97
What do increased FDPs indicate and what are they seen with?
Enhanced fibrinolysis and seen with DIC, haemorrhage, jugular vein thrombosis and liver disease
98
What do D-dimers detect?
Plasmin mediated degradation of cross-linked fibrin = fibrinolysis
99
What is the mechanism of vitamin K deficiency?
Factors II, VII, IX, and X produced in liver activated by vitamin K dependent carboxylase which requires reduced vit K which requires vit K reductase which is inhibited with coumarin = rodenticide
Lack of active FII, VII, IX, X so all pathways affected but FVII has shortest half life so PT prolonged first
100
What are the clinical signs of vitamin K deficiency?
Mainly haemorrhage
101
What do test results of animals with vitamin K deficiency show?
Prolonged PT and PTT
Mild thrombocytopenia is possible due to consumption associated with haemorrhage
FDPs may also be elevated
Same results if animal has coagulopathy secondary to hepatic disease
102
What are the treatments for vitamin K deficiency?
Emetics, cathartics, activated charcoal if ingestion of rodenticide is recent
Transfusions of whole fresh blood/fresh frozen plasma and packed cells if severe anaemia
Vitamin K orally/SC with loading dose then lower doses every 8 hours
103
How long will a patient with rodenticide poisoning need treatment for?
Warfarin/1st gen = 1 week treatment
2nd/3rd gen = 3-6 weeks
Check PT 24-48 hours after last dose
104
What are some hereditary defects of coagulation?
Factor VII deficiency, Haemophilis A (Factor VIII), Haemophilia B (Factor IX), factor XI deficiency, factor XII deficiency
105
What is the treatment for a hereditary defect of coagulation?
Transfusions of whole blood or plasma to replace factor deficiency and red cells
Fresh/frozen plasma gives a small amount of factor
Cryoprecipitate
106
What is DIC?
Disseminated intravascular coagulation is a mixed haemostatic defect resulting from excessive coagulation leading to widespread thrombosis and haemorrhage eventually results as all coagulation factors are consumed
Can be secondary to other disease, acute or chronic
107
What haemostatic abnormalities are seen in DIC?
Thrombocytopenia, prolonged PT and PTT, elevated FDPs, decreased fibrinogen, decreased antithrombin III
108
What is the prognosis for DIC?
Poor
109
What is the treatment for DIC?
Stop coagulation process using heparin/blood transfusion as a source of antithrombin III/asprin to stop platelet activation
Correct underlying abnormalities
110
What is the first thing to do if presented with a patient with a bleeding abnormality?
Petechial/eccymoses - if yes then likely platelet defect if not coagulation defect
Check CBC and evaluate HCT and platelet numbers
111
What do you do if a patient has thrombocytopenia?
Check for clots, check smear and recheck numbers
Assess degree of thrombocytopenia
If platelets 100x10^9/L consider haemorrhage as a cause but if below 25x10^9/L thrombocytopenia may be cause of haemorhage
112
What diseases and tests should be considered if platelet numbers normal but haemorrhages/eccymoses/petechiae present?
vWD or platelet function defects
| Check BMBT, vWF:Ag assay, clotting function
113
What disease is likely if both PT and PTT are prolonged?
Vitamin K deficiency or DIC
114
What disease is likely if PTT alone is prolonged?
Intrinsic pathways, haemophilia A or B
115
What disease is likely if PT alone is prolonged?
Vitamin K deficiency, liver disease or early DIC
