Platelets and Hemostasis Flashcards
Thrombocytosis:
Def
Causes
Def: increased platelets
Causes:
- inflammation (IL-6, TNF-alpha, IL-1) => cause inc thrombopoietin production
- Iron deficiency anemia
- Cushing’s disease 4. Steroid therapy
What doesn’t cause a thrombocytopenia (usually)
hemorrhage bleeding
Platelets
Have calcium in them (most clotting rxns need Ca2+)
Use purple top tube with K/EDTA
Purple top tube
Has K/EDTA EDTA => chelates Ca2+ and inhibits clotting
Calcium in clotting
Positively charged
Mediates binding of coagulation factor enzyme complexes via their negatively charged residues to negatively charged phospholipid surfaces of platelets
Ca is a bridge of positivity, allows platelets to act as scaffolds for clotting reactions
Alpha granules in platelets
Adhesion proteins (like Von Willebrands factor)
Common breed for Von Willebrand’s factor deficiency
Dobermans
Cat Platelets
Larger with a higher mean platelet volume (MPV) Especially sensitive to activation during blood sample collection
Snakes
Have thrombocytes Their platelets are very large and are an actual cell type
How to do a platelet estimate
- Count number of platelets in 10-100X oil immersion fields and average. 2. For dogs average count in 10 fields x 15,000/microLiter 3. For cats average count in 10 fields x 20,000/microLiter
Hemostasis
The arrest of bleeding.
Requires integrated response from blood vessels:
- endothelial cells
- collagen
- smooth muscle cells
- fibroblasts…..
and platelets, circulating clotting factors, and fibrinogen becoming fibrin.
Primary Hemostasis
Peripheral vasoconstriction occurs
Blood starts flowing again after momentary stoppage
Platelets start degranulating and releasing Ca2+ to form a platelet plug
(Endothelial defect exposes subendothelial collagen)
End result = platelet plug
Secondary Hemostasis
Stimulation of the coagulation system
Thrombin (Factor 2a) converts fibrinogen (Factor 1) to the active fibrin (Factor 1a)
Fibrin is incorporated into the clot and clot is stabilized
Coagulopathy
Excessive bleeding due to abnormal function/lack of presence of a coagulation factor.
Defect in secondary hemostasis usually leading to more severe bleeding than a platelet or primary hemostasis problem.
Coagulopathies lead to
Hemoabdomens, hemothoraxes, hemarthrosis, hematuria, and of course petechiae and purpura.
If there is a clot in the blood tube….
Don’t trust values from a hematology analyzer.
Automated platelet counts in cats….
Are NOT reliable.
Platelet counts
Lower in King Charles Spaniels and greyhounds.
King Charles Spaniels have a high prevalnce of
inherited asymptomatic thrombocytopenia with macrocytic platelets:
60-80,000 platelets can be normal
MPV
Large platelets are often young platelets
Platelet clumps will falsely decrease a plately count and falsely increase the MPV
Increased MPV
Seen in enhanced thrombopoiesis in response to thrombocytopenia
Thrombocytopenia often leads to
Petechiae, purpura, or ecchymoses.
-usually when less than 50,000/microLiter or less than that
Facial petechiae around the eyes
May be caused by a tight leash or a rabies pole.
Do a blood smear to look for platelets.
In a thrombocytopenic animal…
Do not use the jugular vein for blood, too close to the heart.
Do not poke organs, they may not stop bleeding.
Hall mark of tick-borne disease
Thrombocytopenia
Evan’s syndrome
Immune mediated hemolytic anemia (IMHA) and immune-mediated thrombocytopenia (ITP) simultaneously.
Thrombocytopenia differentials
1. Decreased platelet production.
2. Increased platelet consumption.
3. Increased platelet destruction (most common cause)
Decreased platelet production
To assess do a bone marrow aspirate to look for sufficient platelet precursors, megakaryocytes.
Not enough megakaryocytes
Myelopthisis: space occupying lesion of bone marrow
Myeloproliferative diseases: Aplastic anemia, bone marrow not making anything ie: estrogen knocks out bone marrow in ferrets and dogs
Increased platelet consumption
DIC and thrombosis, vasculitis, hemangiosarcoma in dogs.
These animals will likely show changes in PT/PTT since platelets used up for clotting.
Increased platelet destruction
Most common cause of thrombocytopenia
Primary immune thrombocytopenia (ITP)
Secondary immune-mediated thrombocytopenias (drugs, infectious agents, neoplasia, SLE)
Vitamin K dependant clotting factors
2, 7, 9, 10
Vitamin K works by
inhibiting the enzyme, vitamin K epoxide reductase in the liver
-makes these factors hypofunctional
Which clotting test is prolonged first?
PT expected to be prolonged before PTT due to the short half-life of Factor 7.
Prolongation of PTT first is possible as well, depending on the species involved.
For the test think PT!!!!
Anticoaculation proteins
Anti-thrombin
Alpha 2 macroglobulin
Anti-thrombin
Stops thrombin from acting.
Thrombin converts fibrinogen to fibrin
Heparin keeps thrombin and anti-thrombin together
PLE
Protein losing enteropathy:
Low albumin and low globulin
PLN
Protein losing nephropathy.
Low albumin only.
Low antithrombin
Hypercoagulable state
DIC
PLN
PLE
Phase of clotting
- Vascular phase:
Exposure of subendothelial matrix and exposure of tissue factor (TF)
- Platelet phase:
Adhesion, activation, shape change, secretion (degranulation), aggregation
=> Platelet adhesion: Von Willebrands factor is how platelets adhere to each other and the vasculature (deficiency in dobermans)
Desmopressin
Stimulates transient release of von Willebrand factor from the endothelial cells.
von Willebrand factor
synthesized and released from endothelial cells and platelets
Major platelet agonists
PAF (Platelet activating factor)-made by cells of immune system, this is one way that inflammation and coagulatio are linked.
Intrinsic clotting cascade
12, 11, 9, 8 (PTT test) + common pathway
Extrinic clotting cascade
VII and TF (PT test) + common pathway
Common pathway
10, 5, 2, 1
How do you evaluate platelet function?
BMBT (buccal mucosal bleeding test)
Which clotting factor has the shortest half life?
Factor VIIa
Which clotting factor test would be prolonged if an animal got into a vitamin K rodenticide?
The animal will have a prolonged PT.
VIIa is an extrinsic pathway factor so use PT.
2, 7, 8, 10 all need vitamin K
Prolonged PT and PTT
Common clotting pathway is the problem
Platelet function disorder
von WIllebrand factor deficiency
Blood tube to test PT/PTT
Blue top=citrated plasma
Function of factor XIIIa
Cross links fibrin
Causes of fibrinolysis
Occurs from crotalase in Eastern Diamondback rattlesnake venom
Cryoprecipitate contains
Von Willebrand factor
Fibrinogen
Factor 8
Factor 13
Cats and platelets…
Cats have largest and most activated platelets of any species.
Thrombocytosis
Increased platelets.
Most common cause: Inflammation
IL-6 leads to increased thrombopoietin
Iron deficiency anemia
- Seen with cushings (inc adrenal secretion of cortisol) or prednisone
=>cortisol is immunosuppressive and prevents macs from phagocytosing old platelets
- Rebout response to thrombocytopenia.
- Withing one week following a splenectomy
Hyper coagulable state
More likely to have a clot
Initial reactions that lead to thrombosis and DIC
Anti-thrombin deficiency
Mast cells in circulation in a dog
IMHA, inflammation, or Parvo virus
Likely not from a mast cell tumor
Mast cells in circulation in cats
Likely has a mast cell tumor in spleen, liver, or intestines (visceral organs)
Snake venoms
Degrade fibrinogen
=>increase FDPs, not increased D-dimers
=>sperocytes may be seen in blood film
Increased FDPs
Snake bite
Breakdown of fibrin monomers by plasmin before they’ve been crosslinked, before Facto13a has crosslinked the fibrin.
=> these usually removed from circulation by the liver.
Doesn’t necessarily mean activation of clotting cascade with active lysis of cross-linked fibrin clot.
Presense of D-dimers
Increased D-Dimers
Means the active breakdown of covalently, crosslinked (via Factor 13a) fibrin
Active coagulation and breakdown of clots
D-dimers specific for
Thrombus formation because it only occurs when soluble fibrin has been crosslinked by Factor 13a and plasmin has cleaved this stable fibrin to form this unique D-dimers
Fibrinogen
Acute phase protein.
Increases with inflammation.
May be the only early sign of inflammation in large animals.
Inflamed animals without elevated fibrinogen
=> liver not making fibrinogen (liver failure)
=> or fibrinigen getting converted to fibrin (look for other signs of DIC
Other signs of DIC
Low fibrinogen
Increased D-dimers AND increased FDPs
Fragmentation morphologies
=> acanthocytes
=> schistocytes
=> keratocytes
Consumption of platelets (thrombocytopenia)
DIC treatment
PLASMA NOW
Then find underlying cause of DIC
Hemophilia A
Factor 8 deficiency
Hemophilia B
Factor 9 deficiency
Factor 12 and testing
Abscense doesn’t cause bleeding disorders
=> Not necessary for secondary hemostatis in vivo
=> Missing in some cats and other species
=> Will cause an increase in PTT b/c test is in a plastic tube, and assess for 12, 11, 9, 8, 10, 5, 2, and 1
If PT/PTT are both prolonged
Think common pathway
Factors 10, 5, 2 (pro-thrombin to thrombin) and 1 (fibrinogen to fibrin)
