Thrombotic disorders

Question 1

Briefly describe the 5 major causes of hereditary
thrombophilia (blood forms clots easily)

Answer 1

• Deficiency of naturally occuring anticoagulants:
- antithrombin (AT)
- Protein C
- Protein S

• Factor V Leiden (FVL)
• Prothrombin Gene Mutation (G20210A)

Question 2

a) background on Antithrombin (function, mechanism)
b) pathophysiology Antithrombin deficiency (hereditary thrombophilia)

Answer 2

a) AT inhibits serine proteases: targets activated II (thrombin), X, IX, XI, XII. & enhanced by heparin
b) mutation in reactive site &/ Heparin binding = dec activity = thrombophilia

Question 3

Lab Ix/test of Antithrombin deficiency (hereditary thrombophilia)

Answer 3

*functional-heparin co-factor assay (for Type 2 def)
- Pt plasma mixed w/ heparin & thrombin
- clot based assay
- residual thrombin activity
• Antigenic: ELISA or latex agglutination (for Type 1 def)

Question 4

Describe the protein C / protein S system

Answer 4

Contains:
• Protein C (PC): Vit K dependent >aPC inactivates FVa/FVIIIa
• Protein S (PS): Vit K dependent
• Thrombomodulin (TM) on Endoth.C > binds free thrombin & changes thrombin substrate specificity
• Endothelial Protein C Receptor (EPCR) on EC > binds to PC

Question 5

describe the activity assays of protein C

Answer 5

• *Chromogenic assay: not detect PC w/ abnormal PL / Ca2+ binding
• Clot-based (aPTT): normal plasma prolonged time vs PC def. plasma has normal time (unreliable to chrome. assay)

Question 6

list 3 the assays of protein S

Answer 6

• *Free PS antigenic assay
• Total PS antigenic assay
• PS functional assay

Question 7

Describe the 3 types of deficiency in PS

Answer 7

T1: Normal PS but dec no. made = dec Ag lvl & functional PS
T2: Normal PS w/ dec functional activity = normal Ag lvl & dec functional lvls
T3: Dec free PS Ag, normal TOTAL PS Ag

Question 8

PC and AT have 2 types of deficiencies, what are they

Answer 8

T1: dec Ag lvl of functional protein
T2: normal Ag lvls of protein w/ dec function

Question 9

a) Describe the prothrombin gene mutation (G20210A)
b) type of testing it’s ID

Answer 9

• mutation in 3’ UTR of F2 gene = inc stability of F2 mRNA
• (hetero) elevated plasma prothrombin => inc thrombin generation
  b) molecular testing

Question 10

a) describe how factor V is activated
b) describe how factor V is INactivated

Answer 10

a)Thrombin of fXa cleave fV = remove B-domain
+ Remaining peices of fV joined by Ca2+ = fVa
b) (w/out PS) aPC cleaves afV at 506, 306, 679 aa. OR (w/ PS) aPC can cleave 306aa (skip 506)

Question 11

how can PS & fV act as co-factor for aPC (> inhibit factor VIII)

Answer 11

fV is cleaved at aa506 AND fV retains C-terminal portion of B-domain
*note FVL NOT act as aPC co-factor

Question 12

describe Factor V Leiden (FVL)

Answer 12

• mutation in F5 = changes aa (Arg->Glut) @ 506 in fV protein
• aPC doesn’t recognise Glut @ 506 so not cleaved
  => aFVL will be inactivated by aPC @ slower rate than fVa

Question 13

Lab Ix/test of Factor V Leiden (hereditary thrombophilia)

Answer 13

Molecular testing OR aPC resistance test

Question 14

Briefly describe the APC resistance test and how it can be modified to specifically measure FV Leiden or other causes of APC resistance

Answer 14

1. sample is diluted in fV deficient plasma
2. diluted sample is tested for aPTT, w/ & w/out addition of aPC
   - WT fV: clotting time is prolonged when aPC is added
   - FVL: time is NOT prolonged bc fVL is more slowly activated by aPC

Question 15

Briefly describe the pathogenesis of heparin-induced thrombocytopenia (HIT)

Answer 15

1. activated plt release PF4 from alpha granule
2. PF4 + heparn => PF4-heparin complex
   3*. complex recognised as foreign => Aby made
3. Aby bind to complex => activate plts = aggregation
4. HIT => thrombosis
   OR
   *3. complex binds to a monocyte = release tiss. factor
5. activate endothelial cells = express TF
6. HIT => thrombosis

Question 16

Briefly describe the pathogenesis of thrombotic thrombocytopenic purpura

Answer 16

• Microangiopathic haemolytic anaemia w/ shistocytes & thrombocytopenia
• due to deficiciency in ADAMTS13

Question 17

describe the dx of thrombocytic thrombocytopenic purpura (TTP)

Answer 17

• RBC: Anaemia: schisto, polychrom
• Plt: thrombocytopenia
• DAT neg
• Normal PT & aPTT
• <5% activity in ADAMTS13 assays

Question 18

4 Lab dx of heparin-induced thrombocytopenia (HIT)

Answer 18

• if 50%+ dec of plt count (from Highest plt count to lowest after initiate therapy)
• if thrombus development w/in 5-10 days after treated w/ heparin
• Immunoassays: detect Aby against Hep-PF4 complexes
• Functional plt studies: Serotonin release assay, aggregometry, Flow cyto

Question 19

Treatment of heparin-induced thrombocytopenia (HIT)

Answer 19

• stop heparin therapy & give direct thrombin/Xa inhibitor
• ONLY use warfarin once plt count INC (bc dec PC & PS = gengrene)

Question 20

describe the treatment of thrombotic thrombocytopenic purpura

Answer 20

• transfuse w/ cryoprecipitate- depleted plasma

Question 21

Describe how venous thrombosis can be an issue

Answer 21

1. Endothelial is activated (bc inflammation OR by stasis-> hypoxia)
2. fibrin forms in endothelial surface
3. RBC & plts get trapped by fibrin
4. Clot can break in small pieces and travel through <3 & lodge pulmonary circ.

Question 22

How might stasis contribute to venous thrombosis

Answer 22

• accumulate prothrombotic factors
• hypoxia in WBC, plt & EC => TF expressed in monocytes & produced in TF-bearing microparticles

Question 23

How can you dx Venous thrombosis & Venous thromboembolism (VTE- hereditary thrombophilia)?

Answer 23

measure d-dimer
- elevated = VTE
- normal = Venous thrombosis

Question 24

Difference b/w venous thrombosis & arterial thrombosis

Answer 24

• VT: EC activated = Red clott bc consist of RBC
• AT: due to formation of artherosclerotic plaque (by ox-LDL = inflamm) = white clott bc consist of WBC & plt

Question 25

What is Virchow’s triad?

Answer 25

VT due to 1/+ causes
- Inc blood coagulability
- Changes in BV
- Blood stasis (if static)