Flashcards in Tests of Coagulation Deck (18):
What are the factors involved in primary hemostasis? What areas are at risk?
Platelets, von Willebrand's, fibrinogen
areas at risk of damage: skin, mucosa, endothelium
Petechiae and ecchymoses associated with primary
What are the factors involved in secondary hemostasis? What areas are at risk?
clotting factors of the cascade
Areas at risk of damage if those proteins not present: joints, deep tissue
What is bleeding time test used for?
A Screening test for the primary platelet plug.
-In vivo measure of time to clot for skin incision
-Indirect assessment of platelet number & effective interaction with subendothelial vWf (IbIX), other platelets (IIbIIIa, fibrinogen)
-Normal adult <9 mins (simulate)
-Not a reliable predictor of procedure-associated hemorrhage in patients without a bleeding hx
-Normal bleeding times do not reliably rule out the possibility of operative hemorrhage
-Not reliably correlated with exposure to aspirin or NSAIDS
What is the PFA-100 test used for?
A screening test for the primary platelet plug
-Anticoagulated whole blood is passed through a narrow tube with a membrane coated with collagen + ADP or EPI
-Closure Time (CT) = time it takes to occlude (clot) the aperture
-Superior to bleeding time as a screen for platelet dysfunction, mod-severe vWD, Aspirin effect
What are the specific tests of platelet function?
1. Platelet aggregometry: a series of tests performed on whole blood or platelet-rich plasma, using several agonists (platelet activators). Measure of platelet clumping is recorded. agonists include thrombin, ADP, etc.
2. von Willebrand factor antigen- quantitative measure of vWf:Ag
3. Ristocetin Cofactor Assay- often used with #2, tests the ability of vWF to bind platelet GP1b. A functional tests of vWF, usually parallels vWF: Ag, if it doesn't, then there's something wrong
What does the activated Partial Thromboplastin Time test? (aPTT) How does the test work?
The intrinsic pathway, also affected by deficiencies in common pathway
-Citrated plasma (which binds calcium=anticoag) is combined with phospholipid and contact activator.
-Calcium is added and the time to clot is measured
-the test is most sensitive for deficiencies in VIII, IX, XI, XII when their concentration is <30%
-mild but significant deficiencies might be missed
-correlation between degree of prolongation and factor level better for VIII than others
Normal range: 23-33 s
What does the Prothrombin time test? How does it work?
The extrinsic pathway
tests for deficiency in factor VII, also affected by deficiencies in common pathway
-brain tissue (thromboplastin)+citrated plasma
-add calcium and measure time to clot
-PT begins to prolong with 50-60% factor deficiency
What are the factors in the common pathway?
X, V, II, I
which affect both PT and aPTT
I = fibrin
What is the order of clotting factors that get activated in the intrinsic pathway?
XII, XI, IX, VIII
12, 11, 9, 8
When can you get spurious prolongation of aPTT?
1. elevated HCT (elevated citrate: plasma)
2. heparin in tube (anticoag)
3. clotted sample (pulls factors out of sample)
How should you workup a prolonged aPTT?
50:50 mixing study where 50% patient's plasma mixed with 50% control plasma
-deficiency: only 30-40% normal factors needed for normal aPTT, so if deficiency, it should be corrected
---> follow up with assay for factors XII, XI, IX, VIII
-inhibitor: if doesn't correct, an inhibitor is present and inhibits the same factor in control plasma and in patient
---->follow up with test for lupus anticoagulant, anticardiolipin antibodies
-if negative, test for specific factor inhibitors
What are inherited factor deficiencies that are not associated with bleeding?
XII, PK, HMWK
How would you workup a prolonged PT?
50:50 mixing study
-prolonged still: inhibitor
What does the thrombin time test for? How does the test work?
Thrombin+citrated plasma, measure thrombin clotting time
-measures rate of conversion of fibrinogen to fibrin
-depends on: quality and quantity of fibrinogen (need min of 75-100mg/dL of fibrinogen for normal time)
What are conditions associated with prolonged thrombin time?
-Congenital afibrinogenemia, dysfibrinogenemia
-Heparin (confirm with Reptilase test which still clots fibrinogen in presence of heparin)
-Excess Fibrinogen degradation products (FDPs – as in DIC)
-Paraproteinemia (inhibits polymerization)
What are tests used to test for defects in fibrinolysis?
1.Euglobulin Lysis time
2. dilute whole blood clot lysis time
-Screening for accelerated fibrinolysis
-Measures the time to clot breakdown in a sample of plasma (ELT) or whole blood (DWBCLT)
-Shortened in: advanced cirrhosis, alpha2-antiplasmin deficiency, plasminogen activator inhibitor-1 deficiency, systemic fibrinolysis
-Occasionally prolonged in venous thrombosis and renal disease (increase PAI-1 levels)
What are the screening tests for defects in primary hemostasis?
-Patient and family hx