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 VWF
3. Coagulation factors Factor Factor XI IX Factor 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 Factor XI IX
Factor VIII Coagulation factors
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + VIII
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh https://www.youtube.com/watch?v=_yQD0U3ZtCs
Bleeding disorders
1. Platelets • Low platelet number • Abnormal platelet function
2. von Willebrand factor • von Willebrand disease
3. Coagulation factors • Haemophilia • Other factor deficiency Coagulation factors
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + VIII
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Coagulation factors
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + VIII Haemophilia A
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Coagulation factors
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII Haemophilia B
IX + VIII
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Coagulation factors
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII Factor XI deficiency (AKA haemophilia C) IX + VIII
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh How do we measure blood clotting? Coagulation factor levelAPTT PT Fibrinogen VWF Activity
One stage factor level Chromogenic factor level
Platelet count Inhibitor level Thrombin time Bethesda units Key players
1. Platelets
2. von Willebrand factor VWF
3. Coagulation factors Factor Factor XI IX Factor 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 Coagulation screen – PT and APTT
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + VIII
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Prothrombin Time (PT) Tissue (HMWK, prekallikrein) factor (Tissue Factor) (XII) Calcium XI VII
IX + VIII
X + V
II Thrombin Normal range (prothrombin) 11 to 18 seconds Fibrinogen Fibrin Fibrin mesh https://www.youtube.com/watch?v=GmgVVCwm9IY Prothrombin Time (PT) (HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + VIII
X + V
II Thrombin Normal (prothrombin) 11 to 18 seconds Fibrinogen Fibrin Fibrin mesh Activated Partial Thromboplastin Time (APTT)
(HMWK, prekallikrein) Contact (Tissue Factor) (XII) activator XI Calcium VII Phospho lipid IX + VIII
X + V
Normal range II Thrombin 24 to 38 seconds (prothrombin) Fibrinogen Fibrin Fibrin mesh Activated Partial Thromboplastin Time (APTT)
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + VIII
X + V
Longer time to clot II Thrombin >38 seconds (prothrombin) Fibrinogen Fibrin Fibrin mesh Why is the APTT abnormal (prolonged)?
• Factor VIII deficiency • Factor IX deficiency • Factor XI deficiency • Factor XII deficiency APTT in haemophilia A
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + FVIII
X + V
Longer time to clot II Thrombin >38 seconds (prothrombin) Fibrinogen Fibrin Fibrin mesh APTT in haemophilia A + replacement factor
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + FVIII rFVIII
X + V
Normal/shorter APTT II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh 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 Factor VIII level
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + 1. Add plenty of all the ? other factors (apart from FVIII) X + V
2. Measure time to clot II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Factor VIII level
Step 3. Compare clotting time to factor VIII level on a reference curve
100
50
30s APTT(seconds)
0 0 70% 100 Factor VIII level (%) Factor VIII level
Step 3. Compare number of seconds to factor VIII level on a calibration curve 100
60s
50 APTT(seconds)
0 0 5% 100 Factor VIII level (%) New therapies and blood tests Extended half-life factors
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + rFVIII PEG Normal/shorter APTT X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Gene therapy
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + FVIII Normal/shorter APTT X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Hemlibra (emicizumab)
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII Emicizumab: • Shortens APTT IX + FVIII • Lab needs to perform a special FVIII assay
X + V
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Fitusiran
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
IX + FVIII Antithrombin X + V (more bleeding)
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Fitusiran
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII
• No effect on APTT IX + FVIII • No effect on factor Antithrombin Fitusiram VIII level (less bleeding) • Can measure X + V antithrombin level
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh Factor inhibitor Factor inhibitor
(HMWK, prekallikrein) (Tissue Factor) (XII) XI VII ↓ Factor VIII level IX + VIII ↑ APTT
X + V Bethesda assay = inhibitor level
II Thrombin (prothrombin) Fibrinogen Fibrin Fibrin mesh 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.