Tests of coagulation Flashcards
What is the activated clotting time used for
Used for monitoring of anticoagulation with unfractionated heparin
Describe how the activated clotting time test is done and how it works
It is a point of care test
Add blood to well containing a contact activator
Warm blood sample in the well to body temp
Rotate the sample continuously until clot is detected automatically
Record time taken
What are the different activators used for activated clotting time. Give the normal activated clotting time with each activator
- No activator - ACT = 190 - 300 seconds
- Celite - ACT = 100 - 170 seconds
- Glass - ACT = 110 - 190 seconds
4 Kaolin - ACT = 90 - 150 seconds
What are the modern uses of the activated clotting time
To measure the anticoagulant effect of heparin in CABG, ECMO, haemodialysis, PCI.
What are the targets for activated clotting time under normal conditions and for cardiac bypass
Normal 107 ± 13 seconds
Cardiac bypass > 400 seconds is desirable
Other situations > 200 seconds is adequate
What are the limitations of activated clotting time
The test is sensitive to multiple variables which may affect the interpretation of the results
What does the prothrombin time measure and how is the test performed
It was designed to measure the extrinsic and common coagulation pathways.
Platelet poor, decalcified plasma is added to
- tissue factor
- phospholipid (utilised to replace platelet membrane)
- Calcium Chloride
at 37 deg Celsius.
The prothrombin time is the time from initiation of coagulation until fibrin clot formation which is detected electronically.
What is a normal prothrombin time
12 - 15 seconds (but depends on the source of synthetic tissue factor)
List common causes of a prolonged PT
Factor 7 deficiency
Early Vit K deficiency (Factor 7 has shortest half life)
Early DIC
Liver disease
Direct thrombin inhibitors (Dabigatran)
Heparins (unfractionated and LMWH)
Dilutional coagulopathy
What is the international standardised index ISI?
THE TYPE OF THROMBOPLASTIN used in the performance of a prothrombin time has a bearing on both the result and the sensitivity of the test.
The difference in sensitivities is termed the sensitivity index.
All thromboplastins are calibrated according to a standardised thromboplastin which has been termed the INTERNATIONAL REFERENCE PREPARATION by the WHO.
Calibration provides each thromboplastin with an International Standardized index
How is the patient’s INR calculated in the lab
- Calculate Prothrombin ration (PT patient / PT reference) and raise this to the power of the ISI (International Standardized Index to get the INR
INR = [PTR}^ISI
What does the activated partial thromboplastin time measure? Why is it named as such
APTT measures the activity of the intrinsic and common pathways.
Thromboplastin is the term given to phospholipid in the presence of tissue factor which acts as the catalyst in converting prothrombin to thrombin.
The ‘partial’ thromboplastin refers to phospholipid being present in the absence of tissue factor
How is the APTT performed
3.8% citrated tube with 9 parts blood to 1 part citrate
Plasma + Contact Activator (Kaolin) + Phospholipid + platelet source –> factor 12 activated –> intrinsic pathway.
Clot formation is detected by automated machines by analysis of clot density
List common causes of prolonged APTT
Heparin therapy
Factor deficiencies (2, 5, 13, 9, 10, 11, 12)
Vitamin K deficiency
Liver disease
DIC
Massive transfusion (calcium + CF deficiency)
Direct thrombin inhibitors
What is the normal range of APTT
25 - 29 seconds (depending on type of activator utilised)
