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Coagulation

From MedLabWiki

Factors II, V, VIII, HMWK -> cofactors

Factors II, VII, 9-12, Prekallikrein -> serine proteases

Vitamin K Dependent Factors

  • Serine Proteases
    • Factors II, VII, IX, X
    • Regulatory control proteins C, S, Z
Factor Other Names Pathway(s) Role Location of Synthesis
I Fibrinogen Common
II Prothrombin Common
III Tissue Factor (TF)
IV Ca2+
V Labile factor Common
VII Stable factor Extrinsic
VIII Anti-hemophilic factor Intrinsic
IX Christmas Factor Intrinsic
X Stuart-Prower Factor Common
XI PTA (Plasma Thromboplastin Antecedent) Intrinsic
XII Hageman Factor Intrinsic
XIII Fibrin Stabilizing Factor (FSF) Crosslinks adjacent fibrin strand D-domains to make an insoluble polymer
VWF
HMWK High Molecular Weight

Stages of Hemostasis

  1. Primary hemostasis
  2. Secondary hemostasis
  3. Fibrin clot formation
  4. Coagulation inhibition

Protein Z

  • Z-dependent protease inhibitor (ZPI)
  • Protein Z enhances ZPI activity
    • inhibits factor X and XI

Restoring Blood Flow

  • Fibrinolytic proteins
    • Plasminogen
      • Converted into plasmin by TPA and UPA
      • Plasmin helps restore blood flow
    • TPA
    • UPA

1.85*10^-3 x (100-HCT) x v

300ul citrate

Coagulation Testing

All coagulation testing uses sodium citrate (light blue top) tubes to yield plasma. Centrifuge to separate plasma from platelets, yielding platelet poor plasma (to prevent activation of coagulation).

  • RT: 4 hours
  • Frozen: 2 weeks (-20C) to 6 months (-80C)
What? (Test) Why? How? (Methodology) Associated Conditions RI
PT/INR (Prothrombin Time) Assess deficiencies or inhibitors of extrinsic or common pathways
  • Calcium thromboplastin added to sample (calcium and tissue factor source)
    • Activates coagulation cascade
  • Measured optically or mechanically
    • Optical: light scattered as fibrin strands form
    • Mechanical: magnetic balls oscillates, movement impeded by clot
11.5 - 13.5 s

INR: 0.9 - 1.2

APTT Assess deficiencies or inhibitors of intrinsic or common pathways
  • APTT reagent added to sample (contains contact activator and phospholipids)
    • Activation of factor XII and XI
  • Calcium then added to activate factor IX and VIII
  • Factor X activated, causing cascade to activate Factor II and I, forming fibrin clot
23 - 35 s
Fibrinogen (Clauss Assay) Assess fibrinogen activity
  • Clauss assay uses reference plasma samples with known concentrations of fibrinogen
  • Run thrombin time (high thrombin concentration) and plot results of concentration vs. time
  • Patient samples can then be run and compared to the curve to determine fibrinogen concentration
  • Clot time inversely proportional to fibrinogen activity
Loss or consumption of fibrinogen
  • Bleeding
  • Hyperfibrinolysis
  • DIC

Decreased production

  • Liver disease
  • Congenital hypofibrinogenemia
  • Dysfibrinogenemia
1.5 - 4 g/L
Thrombin Time Assess deficiencies of fibrinogen or the presence of thrombin inhibitors
  • Low concentration of thrombin added to cleave fibrinogen → fibrin
  • Measure time to form fibrin clot
Drugs
  • Heparin
  • Direct thrombin inhibitors (dabigatran, apixaban, etc.)

Conditions

  • Congenital fibrinogen deficiencies
  • Acquired fibrinogen deficiencies
  • Increased clot breakdown (e.g., DIC)
12 - 16 s
D-Dimer Assess presence of fibrinolysis activation
  • Various methods (ELISA, latex immunoassay, etc.)
  • Latex immunoassays: latex beads bind to D-Dimers and decrease measured absorbance
Negative results highly sensitive and specific for ruling out venous thromboembolism (VTE) in certain populations

Positive results non-specific

  • DIC
  • Various diseases and infections
  • Inflammation
  • Recent surgery/trauma
  • Cancer
  • Elderly people
  • Pregnancy (especially later trimesters)
< 600 ug/L FEU*
Unfractionated Heparin/Anti-Xa Assay Assess anticoagulant activity
  • Known value of Factor Xa is added in excess to patient sample
  • Complex formed between drug and coagulation factor
  • Unbound FXa hydrolyzed and abosrbance measured
  • Result converted to a drug concentration
Drugs
  • Unfractionated heparin
  • Low molecular weight heparin
  • Fondaparinux
  • Direct FXa inhibitors
    • Rivaroxaban
    • Apixaban
    • Edoxaban
Varies by drug
Factor Assays

'* FEU or DEU may be used for measurement

Anticoagulant Drugs

Drug Mechanism of Action Monitoring Reversal
Unfractionated Heparin (UFH) INDIRECT
  • Various length chains of glycosaminoglycans
  • Combines with antithrombin (as pentasaccharide)
  • Inhibits IIa, Xa, IXa, XIa, and XIIa
  • APTT
  • Anti-Xa assay
Elective
  • Discontinue IV for 4 hoursEmergency
  • Protamine sulfate
Low Molecular Weight Heparin (LMWH)
  • Dalteparin, enoxaparin, nadroparin, tinzaparin, etc.
INDIRECT
  • Smaller chains of heparin (fractionated out)
  • Combine with antithrombin (pentasaccharide)
  • Inhibits Xa and IIa only
  • Relative inhibition of factors (Xa:IIa ratio) various by formulation
Not usually required
  • Anti-Xa assay only
Elective
  • 12-24 hours discontinuation

Emergency

  • Andexanet alpha pending approval
  • Protamine may reverse effects on IIa only for intravascular LMWH
Warfarin (Coumadin)
  • Vitamin K antagonist
  • Prevents activation of vitamin K-dependent factors
    • II, VII, IX, X, anticoagulant proteins
  • PT/INR
Elective
  • 5 days discontinuation (bridging may be done for some patients)
  • Vitamin K if non-urgent

Emergency

  • Vitamin K - activates factors already present
  • Prothrombin complex concentrates (PCCs - Octaplex)
    • Contain Vitamin K-dependent factors and some heparin
    • Contraindicated in HIT
Fondaparinux INDIRECT
  • Synthetic pentasaccharide
  • Combines with antithrombin
  • Inhibits FXa only
Not usually required
  • Anti-Xa assay only
Elective
  • 1-2 days discontinuation

Emergency

  • Andexanet alpha (pending approval) may be helpful
  • Otherwise, none
Direct Thrombin Inhibitors (DTIs)
  • Dabigatran (VTE, stroke prevention)
  • Argatroban, Bivalirudin, Lepirudin (HIT treatment)
DIRECT
  • Synthetic
  • Directly inhibit thrombin, do not require antithrombin (direct-acting)
Not usually required Elective varies

Emergency (dabigatran)

  • Idarucizumab
  • Activated charcoal (within 2 hours)
  • Hydration
Direct Xa Inhibitors
  • Rivaroxaban
  • Apixaban
  • Edoxaban
  • Synthetic
  • Directly inhibit FXa (don't require antithrombin)
  • Used for prevention of VTE and stroke
Not usually required
  • Anti-Xa assay only
Elective varies

Emergency

  • Andexanet alpha (pending approval)
  • Prothrombin complex concentrates (PCCs - Octaplex)