Coagulation and Clotting Flashcards
GPIa - GPIIa
Receptor for collagen
GPIb
Receptor for Von willebrand factor
Platelet alpha granules
Fibrinogen, platelet derived growth factor, vWF, P-selectin, platelet factor 4 (PF4)
Platelet dense granules
ADP, ATP, serotonin (5-HT), calcium
Thrombin promotes these in coagulation pathway
Fibrinogen to fibrin V to Va XIII to XIIIa VIII to VIIIa XII to XIIa
Factors in extrinsic pathway
VII
Factors in intrinsic pathway
XII,XI,IX,VIII,prekallikrein, high molecular weight kininogen (HMWK)
Factors in common pathway
X,II (thrombin), I (fibrinogen)
Tenase complex
Forms on surface of platelets.
IXa,VIIIa, calcium
Activates X to Xa
Intrinsic pathway
Anchors the Tenase and prothrombinase complexes to platelet
Phophatidleinositol and phophatidlyserine
Prothrombinase complex
Va and Xa
Antithrombin
Inactivates thrombin, IXa,Xa,XIa,XIIa
Stimulated by heparin
Activated protein c (APC)
Inactivates Va and VIIIa with protein S as cofactor
tPA
Tissue plasminogen activator. Coverts plasminogen to plasmin. Released from vasculature endothelial cells with injury.
Bleeding time
Elevated in Von Willebrand disease, other inherited platelet disorders, uremia, aspirin use, low platelet count
PFA-100
Instrument used for platelet disorder screening. Uses artificial vessel. Measures the time for anticoagulated whole blood to occlude aperture under standardized flow control. Aperture is covered with either col/epi or col/adp membrane. Adp membrane is sensitive to drug induced platelet defects (aspirin)
Platelet aggregometry
Sample of platelet rich plasma stirred while being exposed to agonist including ADP,epinephrine, arachidonate, collagen, ristocetin. Turbid mixture clears with aggregation. Medication is most common reason for abnormal results.
Response to everything but ristocetin is hallmark of vWD.
Activated clotting time
Point of care test to monitor high dose heparin therapy (aPTT is unmeasurable >150 seconds)
Activated partial thromboplastin time (aPTT)
Phospholipid, a contact activator of factor XII (e.g., silica, kaolin), and excess calcium are added to citrated plasma. The tim to clot formation is the aPTT.
Elevated PT, normal aPTT
Factor VII liver disease Vitamin K deficiency Warfarin Inherited factor deficiency
Normal PT, Elevated aPTT
Factors: VIII, IX, XI, XII, HMWK, prekallicrein, LAC
Clinical: Acquired inhibitors
Heparin
Lupus anticoagulant
Inherited factor deficiency
Elevated PT, elevated aPTT
Factors: V, X, Fibrinogen, LAC
Clinical: DIC Heparin Amyloidoisis (acquired factor X deficiency) Liver disease Prematurity Polycythemia Inherited factor dificiency
Normal PT, Normal aPTT
XIII, LAC
Bleeding (with XIII)
Normal or thrombophilic (with LAC)
Anti-Xa assay (heparin antifactor Xa assay)
- used to monitor either unfractionated or low molecular weight heparin (LMWH).
- test of choice for monitoring LMWH and danaparoid
- patient’s plasma is added to a known amount of factor Xa with excess antithrombin. Heparin in the patient’s plasma stimulates antithromin to inhibit factor Xa. The quantity of residual factor Xa is measured with a chromogenic substrate which Xa cleaves to produce a colored compound that is detected by a spectrophotometer. The residual Xa is subtracted from the initial Xa to determine the anticoagulant concentration. Thus when there has been no heparin administered, the anti factor Xa should be zero The higher the heparin, the higher the anti factor Xa.
Bethesda assay
- also called factor VIII inhibitor assay, anti-factor VIII antibody assay
- Factor VIII is the most common factor to which antibodies arise
- can be adapted to any factor
- Anti-factor VIII antibodies have characteristic properties
- they behave as inhibitors in coagulation screen assays (fails to correct with 1:1 mix, especially after 1-4 hour incubation - time dependance)
- if factor assays are performed, factor VII will appear decreased and serial dilutions wills how progressively more factor VIII activity
procedure:
- make several dilutions of patient plasma with citrated saline
- each mixed 1:1 with normal plasma and incubated for 1-2 hours
- Factor VIII activity assays are then performed on each dilution
- the dilution at which the factor VIII activity is 50% represents the inhibitor titer.
- Bethesda units are the dilution factor for 50% activity (e.g.. 1:16 = 16 Bethesda units)
- 5 the antibody cannot be overcome with high doses of human factor VIII. (use porcine factor VIII, activated prothrombin complex concentrate APCC, or recombinant factor VII NovoSeven
D-dimer & fibrin-degradation products (FDPs)
- D-dimers are formed by factor XIII when it cross links D regions of fibrin molecules
- later, the fibrin clot is disassembled by plasmin, which is incapable of breaking up D-dimers
- NOT formed in the plasmin-mediated degradation of fibrinogen, but rather released into circulation
- Presence of D-dimer indicates that fibrin has been formed and then degraded
- Assays for FDPs are a little less specific because they detect fibrin degrading products, fibrinogen degradation products, and D-dimer
- most often increased in thrombosis and significant bleeding
- may be elevated in atrial fibrillation and CHF
- cirrhotic patients may have persistent elevation because of difficulty clearing
- some cancers cause elevation (mucin-secreting adenocarcinomas)
- DIC
- widely applied to the diagnosis of venous thromboembolism and PE, negative predictive value is most useful, especially in patients with low pretest probability
Factor assays
- II, V, VII, VIII, IX, X, XI, XII
- patient plasma is mixed with reagent plasma having a known factor deficiency
- resulting PT or PTT is compared to standard curve, generated by plotting known factor levels against clotting times.
- Factor VIII, IX, XI, and XII assays are PTT based
- Factor II, VII, and X assays are PT based
- Factor V can be either
- serial dilutions are usually used
- an inhibitor is suggested if serial dilutions result in an apparent increase in factor activity
- isolated inherited factor deficiencies are less common than multiple acquired factor deficiencies (DIC, liver disease, coumadin)
Fibrinogen activity
- very similar to Thrombin time assay
- excess thrombin is added to diluted patient’s plasma
- use standard curve to determine activity
von Willebrand Dz Types
Type 1
- quantitative decrease in vWF
- mild to moderate bleeding
- most common 70-80% of cases
- normal molecules of factor
Type 2
- qualitative deficiencies with functional abnormalities of vWF
- quantity is often reduced too
- 20-30% of cases
- can be severe bleeding, but usually mild to moderate
- Four subtypes
- 2A - loss of high-molecular-weight multimers (defective assembly and secretion, or increased rate of proteolyzed)
- 2B - mutations lead to increased binding of vWF to GP1b (platelet receptor) and resulting increased clearance of factor and platelets
- “Platelet type” or “pseudo vWD” is rare disorder that is similar to type 2B, except the mutation in on the platelet GP1b gene instead of VWF gene. leads to increased binding and increased clearance
- 2M - decreased function with normal sized multimers. impaired binding of vWF to GP1b
- 2N - (normandy) mutations are located withing factor VIII binding site. Factor VIII half-life is shorter. vWF is normal. Often misdiagnosed as hemophilia A
Type 3
- severe bleeding
- very rare .5- 5.3 per million
- quantitative deficiency so severe that vWF is often undetectable
