Platelets and Primary Hemostasis Flashcards
What is hemostasis? What factors are important?
stoppage of blood without obstruction of blood flow
- prothrombin
- Factor VII
- Factor IX
- Factor X
- Factor XI
- Factor XII
What balances hemostasis?
concentrations of procoagulants (coagulation factors) vs anticoagulants (natural anticoagulants)
What are the 2 things that imbalances of hemostasis can lead to?
- THROMBUS leading to obstructed blood flow and hypoxia - increased procoagulant activity or decreased fibrinolysis (pulmonary thromboembolism)
- HEMORRHAGE - decreased procoagulant activity, decreased platelet numbers or loss of function, fibrinolysis (immune-mediated thrombocytopenia)
What is required for primary hemostasis? Secondary hemostasis?
formation of platelet plug - platelets and endothelium
coagulation - clotting factors
What is the morphology of platelets like?
- anucleated
- blue cytoplasm with reddish cytoplasmic granules
What 2 cytoplasmic granules are present in platelets?
- ALPHA GRANULES - seen on blood smear and contains coagulation factors and proteins (fibrinogen, Factor V, vWF, thrombospondin, PDFG)
- DENSE GRANULES
What are equivalent to platelets in non-mammalian species? How does their morphology compare?
thrombocytes
- rounder and denser nucleus
- clear cytoplasm in a smaller amount
What is thrombopoiesis?
platelet production from megakaryocytes in bone marrow
Where are megakaryocytes found? How do they multiply?
bone marrow sinus
replicate DNA without cell division (endomitosis)
How do megakaryocytes produce platelets? What is their maturation time?
proplatelets (cytoplasm) processes extend into sinus lumen and break off into individual platelets
4-5 days
What regulates thrombopoiesis? Where is it produced?
thrombopoietin regulates production and differentiation
liver, kidney, muscle
What is the circulating lifespan of platelets? Where are most circulating platelets found?
5-9 days
spleen
What is primary hemostasis? What 4 things do platelets do?
vasoconstriction and platelet plug formation
- adhesion
- activation
- aggregation
- granule secretion
Primary and secondary hemostasis happen simultaneously and are mechanistically intertwined. What are imbalances in these processes due to?
- too much clotting = thrombosis
- bleeding = not enough clotting
What 3 steps cause the formation of the primary hemostatic plug?
- platelets adhere to endothelium
- platelets undergo activation
- platelets aggregate to form the plug
What 5 things are required for platelet adhesion?
- von Willebrand Factor (vWF)
- calcium
- ADP
- thromboxane A2 (TXA2)
- thrombin
What does von WIllebrand Factor do?
binds to GP1b on platelet surface and creates a bridge between platelet and exposed collagen from injury on endothelium
What 2 things are required for platelet activation?
- platelet shape change
- secretion of granule products to activate more platelets
How do platelets change shape when activated? What causes this?
shift from smooth discs into spheres with many filopodia to increase surface area
collagen exposure and release of thrombin (FII)
What 4 things does platelet activation lead to?
- recruitment of more platelets
- further platelet activation
- facilitated coagulation
- vessel repair
What 4 things are secreted by platelets to cause aggregation?
- thromboxane A2 - irreversible platelet aggregation
- thrombin - product of secondary hemostasis
- ADP
- calcium
What causes clot retraction? What does this facilitate?
actinomyosin filaments
wound closure and vessel patency
What is the correct order of platelet plug formation?
adhesion —> activation —> aggregation
What are samples collected in to evaluate platelet concentration? What are 2 methods of evaluation?
EDTA purple top tubes
- evaluation on a blood smear
- hematology analyzer (part of routine CBC)
Why is venipuncture technique especially important for platelet evaluation?
rough handling can activate platelets and cause coagulation
How do you manually count platelet concentration by blood smear?
- count platelets in 10 HPT at 100x under immersion oil
- calculate the average
- multiply average by 20,000 to approximate platelet concentration/uL
In what 2 species is it common to find physiologic platelet clumps?
- cats*
- cattle
What species have high variation in platelet size?
cats
How does the morphology of platelets compare in horses?
pale staining
What are macroplatelets? What does an increased number indicate?
large platelets the same size or larger than RBCs
active production of platelets - regeneration
How do hematology analyzers count platelets? How can this give incorrect counts?
categorized by size
macroplatelets and platelet clumps will not be counted, making platelet counts seem decreased
How do hematology analyzers calculate mean platelet volume? What does it mean if it is increased?
sized, counted, and calculated the average size of all those counted
increased thrombopoiesis, a sign of platelet regeneration
When are platelet production tests done? How is it evaluated?
in cases of thrombocytopenia
bone marrow aspirate (BMA) counts megakaryocytes and analyzes morphology
How should platelet concentration be interpreted?
THROMBOCYTOPENIA = lower PLT count below RI
- mild = 80,000/ul
- moderate = 30,000-80,000/ul
- severe = <30,000/ul
THROMBOCYTOSIS - higher PLT count above RI
When are patients at risk for spontaneous hemorrhage?
when platelet counts are below 30,000/uL
What are 4 clinical signs of thrombocytopenia?
- platelet-associated bleeding patters from mucosal membranes (epistaxis, gingival bleeding, GI bleeding, hematuria)
- petechia or ecchymosis
- prolonged bleeding post wound or surgery
- excessive cutaneous bleeding
What are 5 causes of thrombocytopenia?
- decreased production by bone marrow
- increased destruction
- sequestration
- increased consumption/use
- pseudothrombocytopenia
What is a condition in dogs that causes thrombocytopenia due to decreased production?
pure megakaryocytic hypoplasia
What is bone marrow panhypoplasia commonly present with? What 3 external sources can cause this?
leukopenia (before the thrombocytopenia)
- drugs: estrogen, chemotherapy
- toxins: mycotoxins
- radiation
What is myelophthisis? What are 5 causes?
decreased production of platelets due to replacement of thrombopoietic tissue
- disseminated inflammation
- primary neoplasia: leukemia
- secondary neoplasia: mast cell tumor, histiocytic sarcoma
- myelofibrosis/necrosis
- infection agents: FeLV, FIV, EIA, BVD
What are the 2 types of immune-mediated thrombocytopenia?
- PRIMARY: antibodies against platelet antigens (GPIIb/ IIIa)
- SECONDARY: antibodies secondary to another underlying condition, like systemic lupus erythematosus, neoplasia, drugs, vaccines
What 3 vaccines have the possibility of causing immune-mediated thrombocytopenia?
- Distemper
- Parvovirus
- Panleukopenia
What are the expected results on CBC, coagulation tests, bone marrow aspirate, and serum biochemistry in patients with immune-mediated thrombocytopenia?
- severe thrombocytopenia +/- acute hemorrhagic anemia (regenerative, hypoproteinemia) and leukocytosis
- normal
- increased megakaryocytes
- normal
What clinical signs are seen with immune-mediated thrombocytopenia?
platelet-assoicated bleeding patterns:
- epistaxis
- gingival bleeding
- GI bleeding
- hematuria
Where are platelets typically sequestered? In what 4 situations is this seen?
large vascular beds
- splenomegaly, splenic torsion
- hepatomegaly, portal hypertension
- vasodilation in endotoxic shock
- severe hypothermia
What are 2 common causes of increased platelet consumption? When will hemorrhage occur?
- disseminated intravascular coagulation
- vasculitis (Rickettsial disease, FIP)
coagulation defects or leakage of blood from vessels
What is pseudothrombocytopenia?
hematology analyzer does not measure present platelets because they are too big or clumped
In what dog is macrothrombocytopenia commonly seen? What causes this?
Cavalier King Charles Spaniel
mutation in B1-tubulin gene that alters megakaryocyte proplatelet formation and release
Do Cavalier King Charles Spaniels present clinically for macrocytopenia? How does it appear on CBC?
no bleeding tendencies - platelets function normally
- total platelet mass (plateletcrit) is normal
- giant platelets (high MPV) with low platelet count
What bleeding test is done to test platelet function? What does it test? What are the 2 possibilities that cause abnormal (prolonged) times?
buccal mucosal bleeding time (BMBT) - tests the ability of platelets to form a platelet plug (NOT fibrin plug)
- decreased platelet function
- decreased platelet numbers
How is the buccal mucosa bleeding time test done? What 2 things cause bleeding times to vary?
- make a standard incision
- blot blood gently
- measure time for bleeding to cease
- species
- lancet used
What are the normal bleeding times on buccal mucosal bleeding time tests on cats, dogs, and horses/cattle? What does a time over 4 minutes indicate?
- CATS = 1-3 mins
- DOGS = 2-4 mins
- HORSES/CATTLE = 8-10 mins
vWB deficiency or thrombocytopathy
When is the buccal mucosa bleeding time test done? In what patients is it not needed?
patients with normal platelet counts with concurrent clinical signs of primary hemostasis disorder —> must rule out possible coagulopathy (secondary hemostasis)
thrombocytopenic patients —-> does not add information
What are 2 mechanisms of thrombocytosis? How do patients usually present?
- increased production
- increased distribution in plasma
asymptomatic
When is thrombocytopenia typically physiologic?
epinephrine-induced splenic contractions due to excitement or fright
(mild or transient)
What causes reactive (inflammatory) thrombocytopenia?
inflammatory IL-6 stimulated TPO release, which causes increased platelet production
In what situation is thrombocytosis common after recovery from previous thrombocytopenia?
rebound response of increased bone marrow megakaryocyte production
What causes thrombocytosis post-splenectomy?
increased thrombopoiesis and TPO production
Essential thrombocythemia and megakaryocytic leukemia is extremely rare. How must they be diagnosed?
evaluate bone marrow (BMA) and rule out other causes first
In what dogs is inherited von Willebrand disease most common? How can it be acquired?
Doberman Pinscher**, German Shorthair Pointer, Chesapeake Bay Retriever, Scottish Terrier
excessively high shear stress conditions, like aortic stenosis or cardiovascular disease
What is the main clinical feature of von Willebrand disease? When do signs typically decrease?
mild to severe bleeding exacerbated by surgery or trauma
with age and successive pregnancies
What platelet count, BMBT, and PTT/ACT is expected with von WIllebrand disease? What must be done for diagnosis?
- normal
- prolonged
- usually normal, but can be prolonged if Factor VIII deficiency is pronounced
analyze plasma for vWF concentration
What is the most common way to test for von Willebrand disease? What other 2 tests can be done?
ELISA
- immunoelectrophoresis determines relative amount of different vWF multimers
- collagen-binding activity assay detects only vWF that can bind to collagen
What is Glanzmann thrombasthenia? In what species is this common?
deficiency of GPIIb-IIa making platelets unable to bind to fibrinogen, causing no platelet aggregation or clot formation
- Otterhounds
- Great Pyrenees
- Quarterhorses
- Thoroughbreds
- Peruvian Paso
What is the clot retraction test? When is it indicated? What patients show abnormal retraction times?
measures the amount of time it takes for a clot to retract from a test tube after thrombin is added
animals with normal platelet count, but questionable platelet function
dogs and horses with Glanzmann thrombasthenia
CASE: 1 y/o Pointer has been treated for neck and back pain with corticosteroids by a referring veterinarian. The dog was thought to have GI parasites due to occult blood in feces and was treated with anthelmintics. The dog returned a month later with a PCV of 15% and MCV of 40fL. At the time the dog has an abdominal effusion.
- Upon physical exam, the dog seems to have a painful abdomen and pale mucous membranes.
- Hematology shows numerous keratocytes, few schistocytes, and some RBCs appear hypochromic.
ERYTHROGRAM:
- PCV, Hgb are both decreased - ANEMIA
- RBC is WNL, suggesting small erythrocytes
- MCV is decreased - ALWAYS caused by iron deficiency , which is indicative of chronic external blood loss in adults (also aligns with small RBCs)
- reticulocytes NOT increased - NON-REGENERATIVE ANEMIA
- morphology alights with IRON DEF. ANEMIA
LEUKOGRAM:
- neutrophilia and increased bands - INFLAMMATORY
- compatible with the presence of inflammation in the peritoneal cavity
PLATELETS:
- thrombocytosis - HALF OF ALL animals with iron def. anemia will have increased platelets, likely due to cytokines and growth factors
NON-REGENERATIVE IRON DEF. ANEMIA
- usually regenerative, but inflammation can exacerbate
