Coagulation in the lab and bleeding disorders Flashcards
How the traditional coagulation pathway is different from what happens in the body
It dIntrinsic pathway doesn’t happen in people!! only in the lab
The two pathways join together in the middle to form the common pathway
APTT
Activated partial thromboplastin time
- venous blood sample collected into citrate
- Citrate stops blood from clotting in the tube because it removes calcium (need for folding proteins) (chelation)
- Spin the sample down to collect the plasma
- Add phospholipid as an activator
- Add known amount of calcium to overcome the citrate
- Length of time to clot formation
examples of contact factors
Kaolin
Silica
Ellagric
Prothrombin time or ratio?
The prothrombin time (in seconds) is often covered to the prothrombin ratio
This standardises the test to account for different laboratory methods and different normal ranges for the prothrombin time
Prothrombin time (patient) / Prothrombin time (normal plasma, an average of heaps of different plasma samples)
The normal ratio is around 1.0 (plus or minus a small amount)
Explain the 1:1 mixing studies
1:1 mixing with normal plasma and intubation
Sample corrects to normal and remains normal = factor deficiency
Sample doesn’t fully correct (anything above the normal range) then an INHIBITOR is present
Because… IF a person has no closing factor whatsoever their assay will be prolonged, if they have 40-50% of the factor the APTT will almost be normal. So if I add 50% normal plasma to that childs sample, even if he has none of a particular factor e.g. no factor VIII, if you add normal plasma it will supply 50% of clotting factors and APTT will be normal
The types of inhibitors in coagulation
Antithrombin, protein C and protein S are called inhibitors as they shut down coagulation
They do not affect the APTT
Different inhibitors affect the APTT
The lupus anticoagulant
Prolongs the APTT
2 different scenarios
- anti-phospholipid syndrome, where you get excess clotting
- Present transiently in patients who are very unwell, e.g. marked inflammation or infected patients in hospitals
DO NOT CAUSE BLEEDING
–> antibodies involved interfere with phospholipid in APTT assay, because we only supply a little phospholipid to trigger clotting in the assay. In the body theres masses of phospholipid, so not restricted
Autoimmune antibodies against a clotting factor - usually VIII
RARE
Associated with bleeding including bone bruising
Can be life threatening
Causes acquired haemophilia
Heparin
Also causes prolonged 1+1
confirmed by addition of protamine
Anticoagulant that works as an inhibitor by up regulating antithrombin –> blocks activated thrombin, therefore adding normal plasma doesn’t over come the effect of the normal inhibitor
Biological glycosaminoglycan chains
Natural glycosamino glycans activate antithrombin in vivo
Common uses of heparin to be aware of when taking a blood sample
To stop central lines from clotting is not used for a while
Donttake blood sample from central line unless you draw a lot of blood off first
Dabigatran
Oral, to treat venous thrombosis and atrial fibrillation
Direct inhibitor of thrombin
Prolonged 1:1 doesn’t correct with protamine, whereas heparin will
APTT Prolonged, PT normal
deficiencies fo factors VIII, IX, XI, XII (intrinsic)
PT prolonged, APTT normal
deficiency of factor VII (extrinsic)
occasional mild deficiencies of II, V, X, I
–> how can you get deficiencies of common pathway factors with normal APTT? Because APTT less sensitive than PT, PT prolonged if factor deficiency decreased by 40-50% whereas APTT needs big factor deficiency to be affected
Causes of a prolonged TCT
- Deficiency of fibrinogen
- Thrombin inhibitor (e.g. dibigatran or heparin)
Multiple factor deficiencies: most common scenario
Warfarin or Vit k deficincy (II, VII, IX, X)
Massive blood loss, loss of coagulation factors and dilution with fluids
DIC
- Widespread activation of coagulation causing thrombosis, followed by bleeding as clotting factors and platelets used up. Low fibrinogen often seen
- Liver disease: lack of production of coagulation factors and inhibitors (except VIII, as not made in the liver)
What is the most sensitive test for liver disease?
prothrombin assay, because bit K dependant factors get used up first, issues with biliary cycle and Vit K deficiency common in liver disease
what is the INR for?
= international normalised ratio.
To standardise Prothrombin ratio, PR, because all labs are different
About warfarin
general name for several hydroxy-coumarin derivatives
Inhibits the recycling of vit K
Lack of carboxylated factors
Dose adjustments via the INR
Reversed by vit K (or more rapidly by replacing clotting factors in plasma products)
Used in atrial fibrillation, venous thromboembolism, other thrombotic disorders
How to approach hereditary factor deficiencies
they're less common Usually a single factor is deficient First do a bleeding history Basic coagulation tests Depending on these, single factor assays
Systemic effects of haemophilia
spontaneous joint bleeds - chronic athropathy - joint dysfunction - Deformity - arthritis Soft tissue bleeds - Tissue damage - nerve damage - deformity
Haemophilia A
factor VIII deficiency most common hereditary disease with serious bleeding 1 in 5000 live male births Inheritance is X-linked recessive mild, moderate, severe
Haemophilia B
Deficiency of factor IX (christmas factor)
Inheritance is X linked recessive
Incidence: 1 in 30,000 live male births
clinical features identical to haemophilia A
Haemophilia treatment
replace missing factor Recombinant in NZ Prophylaxis in children and teenagers when severe Normal lives and joint outcomes except patients who develop inhibitors
How to test for haemophilia
prolonged APTT with normal PR
single factor assays
Genetic analysis
- sequencing of the factor VIII or IX gene
- Restriction fragment analysis in PCR in selected cases
What else other than haemophilia can cause a low factor VIII level?
Patients who lack VW factor will not always but often lack factor VIII as well
Describe VW factor
Plasma glycoprotein
Synthesised in megakaryocytes and endothelial cells
promotes platelet adhesion at vessel wall
Serves as a carrier for factor VIII
Complexes with factor VIII to stabilise and prevent degradation
epidemiology of VW disease
Most common inherited bleeding disorder
Worldwide inheritance 1-3%
Autosomal dominant
Qualitative of quantitative abnormality of VWF
Test for VW disease
Abnormal platelet function screen
Marginally prolonged APTT
Low factor VIII
Low level and function of VW factor with specific tests
Symptoms of VW disease
Mucosal bleeding (lack of platelet tethering) - Epistaxis - Gum bleeds - GI blood loss Brusing Menorrhagia Postpartum or preoperative bleeding Autosomal family history Can be mild through to severe
Factor XII deficiency??
Hereditary XII deficiency is common with no bleeding sequelae, but shows up in the lab test and corrects in the 1+1 test
We don’t need factor XII to clot, is why she has no bleeding history
DIC mild vs severe complications
It can originate from and cause damage to the microvasculature which if sufficiently severe can cause organ dysfunction
Causes of DIC
sepsis - lots of different bacteria e.g. meningicocal septicaemia –> purpuric rash
Malignancy
Organ damage e.g. pancreatitis
Trauma
