Post on 20-Aug-2020
The Basics about blood clotting and how this interacts with factor
replacements/novel therapies
Yvonne Brennan
How does blood clot?
Key players
1. Platelets
2. von Willebrand factor
3. Coagulation factors
VWF
Factor
XIFactor
IXFactor
VIII
Platelets
von Willebrand factor
“Platelet plug” initially plugs hole
Coagulation factors help make blood clot firm
Fibrin mesh
Coagulation factors
• Proteins in blood
Factor
XIFactor
IX
Factor VIII
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Coagulation factors
Bleeding disorders
1. Platelets
• Low platelet number
• Abnormal platelet function
2. von Willebrand factor
• von Willebrand disease
3. Coagulation factors
• Haemophilia
• Other factor deficiency
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Coagulation factors
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Coagulation factors
Haemophilia A
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Coagulation factors
Haemophilia B
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Coagulation factors
Factor XI deficiency(AKA haemophilia C)
How do we measure blood clotting?
APTT
PT
Coagulation factor level
Fibrinogen
VWF Activity
Inhibitor level
Bethesda units
Chromogenic factor level
One stage factor level
Platelet count
Thrombin time
Key players
1. Platelets
2. von Willebrand factor
3. Coagulation factors
VWF
Factor
XIFactor
IXFactor
VIII
Platelets
1. Platelet number 2. Platelet function
von Willebrand factor
1. VWF amount 2. VWF function
Coagulation factors
1. Screening tests = PT, APTT
2. Individual coagulation factor levels
Coagulation screen“Coags”= time for the blood to clot
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Coagulation screen – PT and APTT
Calcium
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Prothrombin Time
(PT)Tissue factor
Normal range11 to 18 seconds
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Prothrombin Time
(PT)
Normal 11 to 18 seconds
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX VIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Activated Partial
Thromboplastin Time (APTT)
Calcium
Contact activator
Phospholipid
Normal range24 to 38 seconds
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
VIII
Longer time to clot>38 seconds
Activated Partial
Thromboplastin Time (APTT)
Why is the APTT abnormal (prolonged)?
• Factor VIII deficiency
• Factor IX deficiency
• Factor XI deficiency
• Factor XII deficiency
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
FVIII
Longer time to clot>38 seconds
APTT in haemophilia A
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
FVIII
Normal/shorter APTT
APTT in haemophilia A + replacement factor
rFVIII
Measuring individual coagulation factor levels
Factor VIII level in haemophilia A
• For diagnosis
>5 - 40% Mild haemophilia A
1 - 5% Moderate haemophilia A
<1% Severe haemophilia A
• To monitor treatment• For major surgery/severe bleeds = aim 80% to 100%
• For prophylaxis = keep FVIII level above 1-3% (sometimes higher)
Pharmacokinetic (PK) curve diagram
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
2. Measure time to clot
Factor VIII level
? 1. Add plenty of all the other factors (apart from FVIII)
Factor VIII level
Step 3. Compare clotting time to factor VIII level on a reference curve
Factor VIII level (%)
AP
TT (
seco
nd
s)
10000
50
100
30s
70%
Factor VIII level
Step 3. Compare number of seconds to factor VIII level on a calibration curve
Factor VIII level (%)
AP
TT (
seco
nd
s)
10000
50
100
60s
5%
New therapies and blood tests
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Extended half-life factors
rFVIII
Normal/shorter APTT
PEG
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Gene therapy
FVIII
Normal/shorter APTT
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX FVIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Hemlibra (emicizumab)
Emicizumab:• Shortens APTT• Lab needs to perform a
special FVIII assay
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX FVIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Fitusiran
Antithrombin(more bleeding)
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX FVIII+
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Fitusiran
Antithrombin
(less bleeding)Fitusiram
• No effect on APTT• No effect on factor
VIII level • Can measure
antithrombin level
Factor inhibitor
(Tissue Factor)
VII
X V+
II(prothrombin)
Fibrinogen Fibrin
IX +
Thrombin
(HMWK, prekallikrein)
(XII)
XI
Fibrin mesh
Factor inhibitor
VIII
↓ Factor VIII level↑ APTT
Bethesda assay = inhibitor level
Conclusions
• Normal blood clotting requires normal amount and function of: • Platelets• von Willebrand factor• Coagulation factors
• Reduced amount or abnormal function of the key players can increase bleeding.
• The laboratory can perform tests to measure blood clotting.
• The laboratory tests help us monitor treatment.
Acknowledgement
I thank Professor Chris Ward, Royal North Shore Hospital, for allowing me to use his “how does blood clot” slides.