Practical 7: Coagulation Flashcards
What are the three coagulation tests carried out in the lab?
Prothrombin Time (PT)
Activated Partial Thrombin Time (APTT)
Thrombin Time (TT)
What coagulation test may have to be carried out if there is an abnormality/a prolonged time?
Correction/mixing studies
How many phases are there to haemostasis?
Three phases
What are the three phases of haemostasis?
Primary
Secondary
Tertiary
What does the APTT and the PT measure
They measure the time elapsed from activation of the coagulation cascade at different point to the generation of fibrin
What must always be checked before running a coagulation test?
Check the patients platelet levels
How should a sample be taken for coagulation testing
(2)
Blood must be taken into a bottle which contains sodium citrate anticoagulant
The sample must be analysed within 4 hours of being taken
What is the preferred ratio of anticoagulant to blood?
1:9
Why is the ratio of anticoagulant to blood so important?
The sample must not have clotted or haemolysed as this will interfere with the accuracy of the coagulation tests
What exactly does the PT measure
Measures the time necessary to generate fibrin after activation of factor VII
It measures the integrity of the extrinsic and common pathways
What factors are examined in the PT test
Fibrinogen (1), Prothrombin (2), 5, 7, 10,
What exactly does the APTT measure
Measures the time necessary to generate fibrin from initiation of the intrinsic pathway
What factors are examined in the APTT test
Fibrinogen (I)
Prothrombin (II)
V
VIII
IX
X
XI
XII
How is the APTT test ran without activating the PT
By using an external agent (e.g. kaolin) capable of activating factor XII without activating factor VII
Since platelet factors are necessary for the cascade to function normally, the test is performed in the presence of a phospholipid emulsion that takes the place of these factors
What exactly does thrombin time measure
It measures the time necessary to drive the reaction of fibrinogen to fibrin in the presence of thrombin
It measures the intensity of this reaction and isolates an abnormality to either a decrease in normal fibrinogen or an inhibitor to its activation
What causes haemophilia A
Loss of factor VIII
What causes haemophilia B
Loss of factor IX (9)
What would cause a deficiency of fibrinogen
(5)
A consumptive coagulopathy
Severe liver disease
Hereditary deficiencies
Afibrinogenaemia
Dysfibrinogenaemia -> acquired (liver disease) or inherited
What can affect TT -> fibrinogen to fibrin
Inhibitors such as heparin and dabigatran
How can you reverse the affects of anticoagulants on TT
Catalyse the reaction with reptilase
Unlike thrombin, reptilase is insensitive to heparin and dabigatran
When and why is a mixing/correction study carried out
(2)
Used for the investigation of prolonged blood coagulation screening tests
Purpose is to determine if the prolongation is due to a factor deficiency or an inhibitor
Comment on the results of a mixing/correction study
(2)
If a factor(s) is deficient in the patients plasma there will be a shortening of the prolonged screening test
The presence of an inhibitor should result in no correction or minimal correction of the screening test
How is a mixing/correction study carried out?
(3)
Plasma from a patient with an abnormal PT and/or aPTT is mixed in equal volumes (1:1) with normal pooled plasma (a source of normal levels of all coagulation factors)
The sample is then analysed by the required method i.e. PT or APTT
Use the Rosner Index to determine if there has been a correction or not
What should you do if your mixing/correction study gives a corrected result
Incubate the sample for 1 hour at 37 degrees and reanalyse to determine if there is a time or temperature dependent inhibitor e.g. Haemophilia or lupus anticoagulant
What should you do if your mixing/correction study identifies a factor deficiency
A factor assay is then carried out to determine the quantity of the factor that is present
What is Lupus anticoagulant?
Antiphospholipid antibodies usually IgG and rarely IgM directed against plasma proteins e.g. prothrombin bound to anionic phospholipids
How common are lupus anticoagulants
Seen in 5-10% of systemic lupus patients
How do lupus anticoagulants affect coagulation tests
(2)
They block the in vitro assembly if the prothrombinase complex
This results in a prolongation of protein assays such as APTT and rarely PT
What is the prothrombinase complex
Factor Xa and Factor V assembled on negatively charged phospholipid membranes in the presence of calcium ions
Why is thrombosis so common in lupus patients even if they cant for the prothrombinase complex?
Its not really understood but it may involve IgG binding to phospholipids that are essential for the normal activating and degrading effects of protein C and protein S, thus shifting the balance in favour of thrombus formation
List six errors that can occur when taking a blood sample
Short draw -> not enough blood
Failure to mix specimen after collection
Excessive vigorous mixing
Haemolysis
Improper storage
Chilling in refrigerator or placing on ice
What factors are vitamin K dependent
II
VII
IX
X (Protein C, protein S)
What anti-coagulant affects vitamin K dependant factors?
Warfrin
What is the equation for the Rosner Index
(1:1 mix clotting time result - NPP clotting time result)/ initial prolonged clotting of patient sample
What is a high Rosner Index and what does it mean
High index > 15% indicates inhibitor
Low index < 15% indicates deficiency
What diseases would cause a normal PT and ATPP
(5)
Von Willebrand’s disease
FXII deficiency
Dysfibrinogenemia
Platelet dysfunction
a-antiplasmin deficiency
What three diseases would cause a prolonged PT, a normal APTT and a corrected mixing study?
Hereditary = isolated FVII deficiency
Acquired = vitamin K deficiency or severe liver impairment
What two diseases would cause a prolonged PT, a normal APTT and no corrected mixing study?
Lupus anticoagulant
FVII inhibitor (rare)
What five diseases would cause a normal PT, a prolonged APTT and a corrected mixing study?
Bleeding:
- FVIII/vWD deficiency
- FIX/XI deficiency
No bleeding:
- FXII
- HMWK
- prekallikrein
What 8 diseases would cause a normal PT, a prolonged APTT and no corrected mixing study?
Bleeding:
- FVII
- FIX
- FXI inhibitor
- heparin
No bleeding:
- FXII inhibitor
- lupus anticoagulant
- HMWK
- prekallikrein
What five diseases would cause a prolonged PT, a prolonged APTT and a corrected mixing study?
FII, FV, FX deficiency
FV and FVII deficiency
Severe liver impairment
Vitamin K deficiency
DIC
What two diseases would cause a prolonged PT, a prolonged APTT and no corrected mixing study?
FII, FV, FX inhibitor
Lupus anticoagulant
What is the normal range for PT
10-14 seconds
What is the normal range for APTT
21-35 seconds
What is the normal range for TT
10-16 seconds
