Coagulation I Flashcards
What is reflex vasoconstriction?
occurs immediately after an injury to restrict blood flow to the injured area
What initiates reflex vasoconstriction? Augmented?
- Initiated: reflex neurogenic mechanisms
- Augmented: transient local secretion of endothelin
What are the 3 overall steps to primary hemostasis?
- Adhesion
- Activation
- Platelet aggregation
What happens during the adhesion stage of primary hemostasis?
exposure subendothelial collagen & vWF
vWF binds to both collagen & platelet receptor GPIb/IX/V complex
this activates the platelets & initiates a signaling cascade
What are the two components to the activation stage of primary hemostasis?
- Platelets undergo shape change (increase SA)
- smooth disks to spiky & conformational change surface GPIIb/IIIa → increases affinity for fibrinogen & PL on membrane
- PL serve to assemble coagulation factor complexes
- Release of platelet granule contents
- recruit additional platelets & helps to activate multiple coagulation factors
Platelet activation is triggered by what substances?
thrombin (through activation PAR)
ADP also recruits platelets
Activated platelets produce what potent inducer of platelet aggregation?
thromboxane A2 (TxA2)
Describe the process of platelet aggregation
When GpIIb/IIIa binds to fibrinogen, this forms bridges between adjacent platelets, leading to aggregation
Is platelet aggregation reversible?
the initial round is reversible
when thrombin stabilizes the platelet plug, this leads to irreversible platelet contraction
How do ADP & thromboxane A2 help platelet aggregation?
binding of platelet GpIIb/IIIa receptor to fibrinogen
What process is shown in the provided image?
platelet activation
How is secondary hemostasis initiated?
same time platelets are being activated, local activation of coagulation cascade is initiated by exposure of tissue factor (TF)
How does TF initiate coagulation cascade? This is what pathway?
extrinsic pathway
- TF binds to circulating VII → VIIa
- VIIa activates IX → IXa & X → Xa
What molecules initiates the intrinsic pathway? When?
- circulating factor XII autoactivates when exposed to collagen/activated platelets
What is the result of the intrinsic & extrinsic pathways?
activation of series of proteolytic enzymes → generation thrombin & soluble fibrinogen → insoluble fibrinogen → fibrin polymerizes & cements platelets into hemostatic plug → plug is crosslinked by factor XIII
What is the most important coagulation factor?
thrombin
many enzymatic controls of hemostasis & link clotting to inflammation and repair
What are thrombin’s 4 most important functions?
- conversion fibrinogen → cross-linked fibrin = insoluble clot
- activates XI, V, & VII → amplify coagulation cascade
- activate XIII → cross-links fibrin
- activation platelets (via activation PARS)
- Pro-inflammatory effects
-
Anticoagulant effect
- in normal endothelium changes from pro-coagulant to anticoagulant
Why is it important that thrombin acts as an anticoagulant in normal endothelium?
prevents clotting form spreading beyond the site of injury
What are the 4 major counter-regulatory mechanisms that limit the hemostatic process to the site of injury?
- Dilution of factors by blood flowing past the injury (& washed by liver)
- Requirement for negatively charged phospholipids for activation of many factors
- activation fibrinolytic system - via t-PA (most active when bound to fibrin)
- free palsmin in blood is rapidly inhibited by alpha2-antiplasmin inhibitor to limit fibrinolysis
What is the function of t-PA?
t-PA converts inactive plasminogen to active plasmin
plasmin breaks down fibrin & fibrinogen
What are the 3 groups of anticoagulant properties of normal intact vascular endothelium?
- Platelet inhibitory effects
- Anticoagulant effects
- Fibrinolytic effects
What platelet inhibitory effects are exhibited by normal intact vascular endothelium?
- shields platelets from vWF & collagen
- prostacyclin (PGI2), nitric oxide (NO), adenosine diphosphatase (degrades ADP) inhibit platelet adhesion & aggregation
- endothelial cells bind & change activity of thrombin from procoagulant to anticoagulant
What anticoagulant effects are exhibited by normal intact vascular endothelium?
-
thrombomodulin & protein C receptor bind thrombin & protein C in a complex - thrombin can no longer activate coagulation factors & platelets
- thrombin cleaves & activates protein C
- Heparin-like molecules bind & activate antithrombin
- Tissue factor pathway inhibitor (TFPI) most potent inhibitor of factor VIIa-tissue factor complex
What is antithrombin? It is enhanced by what molecules?
protein that inhibits factors IIa, Xa, VIIa, IXa, Xia, kallikren, and XIIa
enhanced by heparin or heparin-like molecules → give antithrombin its anticoagulant properties
What fibrinolytic effects effects are exhibited by normal intact vascular endothelium?
synthesize t-PA
key component fibrinolytic system
What is the main initiator of the clotting cascade in the body?
Tissue factor - amplified by feedback loops involving thrombin
Clotting in vitro is initiated by what substances?
adding phospholipids, calcium & either negatively-charged substance (intrinsic pathway) or source of tissue factor (extrinsic pathway)
Hemostasis testing should occur at what ratio of what substances?
citrulated plasma (blue top)
9:1 ratio blood:antcoagulant
How is a PT acquired?
tissue thromboplastin & calcium added to citrulated plasma
time to form a clot in tube recorded in seconds
PT asses what factors?
integrity of extrinsic & common pathway
PT is useful for what type of monitoring/screening?
monitoring warfarin-type anticoagulants
screening for vitamin K deficiency
A prolonged PT can be due to deficiencies in what factors?
- fibrinogen (<100mg/dl)
- prothrombin (II)
- VII
- X
- V
Warfarin causes a decrease in what factors?
- II
- VII
- IX
- X
What is the most useful test to monitor Warfarin therapy?
PT
Other than factor deficiencies, what are the other reasons for a prolonged PT?
FDP (fibrin degradation products)
D-dimer (plasmin breakdown product)
act as circulating anti-thrombins
Can you use PT to monitor heparin?
yes, but less sensitive than APTT or TT
PT is not affected by deficiencies in what factors?
- XII
- XI
- IX
- VIII
- HMWK
- prekallikrein
What is the INR?
calculated value using the measured PT result, the ISI (international sensitivity index) of the reagent & the MRI (mean reference interval)
developed to better standardize monitoring of warfarin therapy - consistency from one lab to another
What is the purpose of the lines on the test tubes used to collect blood to screen for hemostasis?
max fill & min fill
overfill - too much blood per anticoagulant
under-fill - too much anticoagulant per blood
want ration 9:1 blood:anticoagulant
What type of blood is used to screen for hemostasis?
platelet poor plasma
(has been spun down)
want to get normal platelets out of there so they do not confound the test
Even if you fill the tube to the correct level for 9:1 blood:anticoagulant, what other variable can alter test results?
significant increases & decreases in Hct
Why is calcium added to the sample when performing PT?
sodium citrate binds calcium in blood as its mechanism as an anticoagulant
adding a bunch of calcium overcomes this anticoagulant & allows the clotting process to begin
How is an Activated Partial Thromboplastin Time (APTT) acquired?
an activator or “contact” factors is added with phospholipids (“partial thromboplastin”) to citrated plasma & incubated for specified time at 37 degrees C
calcium is added & the time to formation of clot is measured in seconds
APTT assess what factors?
integrity of intrinsic & common pathway
APTT is prolonged by deficiencies in what factors?
- XII
- XI
- IX
- X
- VIII
- V
- thrombin (II)
- fibrinogen (I)
- prekallikrein & high molecular weight prekallikrein
What is the most common cause of prolonged APTT in hospitalized patients?
heparin (antithrombin)
What is the new standard for assessing heparin therapy?
heparin assay
anti-Xa or Xa inhibition test
APTT is prolonged by what type of inhibitors?
- lupus anticoagulant
- fibrin degradation products (FDP)
- specific factor inhibitors
- direct thrombin inhibitors
How can improper storage lead to increased APTT time?
factor VIII is labile & will deteriorate if has been sitting at room temp >4 hr
APTT is not affected by deficiencies in what factors?
VII
Shortened APTT or PT may be due to what probable causes?
- traumatic blood draw & subsequent activation coagulation cascade prior to testing
- may indicated elevated risk of thrombosis due to elevated levels one or more factor
How is a Thrombin Time (TT) acquired?
thrombin is added to plasma & the time to form a clot is measured in seconds
TT asses what factors?
the time it takes thrombin to convert fibrinogen to fibrin
What is the most sensitive test to use for detection of heparin?
Why is it not used to monitor therapy?
TT
too sensitive - if heparin is present, TT will often be >60s
Other than heparin, TT may be prolonged by what situations?
- afibrinogenemia
- hypofibrinogenemia
- dysfibrinogenemia
- inhibitors
- paraproteins (inhibit polymerization)
- fibrin degradation products
- direct thrombin inhibitors
- argatroban
- lepirudin
What is the clotting factor in the highest concentration in plasma?
fibrinogen
(largely absent in serum)
What are the two types of fibrinogen assays?
functional or antigenic
What are the possible causes of decreased fibrinogen levels?
-
afibrinogenemia or hypofibrinogenemia
- dysfibrinogenemia if functional assay is performed
- heparin
What are the possible causes of increased fibrinogen levels?
inflammation or malignancy
What is the minimal fibrinogen level for hemostasis?
75-100 mg/dL
How are fibrin degradation products formed?
plasmin-mediated degradation of fibrin & fibrinogen
Fibrin degradation products may be elevated in what situations?
- DIC, thrombosis, significant bleeding
- mucin-secreting adenocarcinomas
- cirrhosis
- diseased clearance by liver
What is a D-dimer? How is it formed?
subtype FDP
formed from plasmin-mediated degradation of mostly cross-linked fibrin
D-Dimer may be elevated in what situations?
- DIC, thrombosis or significant bleeding
- mucin-secreting adenocarcinomas
- cirrhosis
- decreased clearance FDP
What is bleeding time used to assess?
screen for capillary integrity & platelet function
not great- poor predictor bleeding risk
USELESS as pre-op screen bleeding risk
How is a bleeding time acquired?
shallow cut on arm of patient at rest
measure time until bleeding stops
Bleeding time can be affected by what variables?
- age
- hydration
- temperature of room
- direction of cut
- location on arm
- etc
When are mixing studies used?
evaluation prolonged PT, APTT or both
differentiate factor deficiency from inhibitor as cause of prolongation
How is a mixing study performed?
mix patient plasma 1:1 with normal pooled plasma to replace clotting factors to at least 50% normal
this is sufficient for normal hemostasis
D-dimer is not specific, but a normal D-dimer effectively rules out what issue?
thrombosis
(PE/DVT)
If the PT normalizes after the mixing study, what is likely the problem with the patient?
factor deficiency
I, II, V, VII, X
If the PT corrects after the mixing study, but elongates again after incubation, what is likely the issue with your patient?
Factor V inhibitor
If the APPT normalizes after the mixing study, what is likely the problem with the patient?
factor deficiency
VIII, IX, XI, XII
If the APPT corrects after the mixing study, but elongates again after incubation, what is likely the issue with your patient?
Factor VIII inhibitor
If the PT remains elongated after the mixing study, what is likely your patient’s issue?
inhibitor
(e.g. specific factor inhibitor)
If the APPT remains elongated after the mixing study, what is likely your patient’s issue?
inhibitor - lupus anticoagulant
