H. Testing of Primary Hemostasis (L6, L7, L9 & Lab 3) Flashcards
Broadly speaking what are the 2 main aspects of primary hemostasis that are assessed?
Platelet number
Platelet & VWF function
What is the quantitative assessment of platelets and when is it performed?
- Platelet count
* Routinely performed as part of a CBC
How commonly are platelet function tests ordered?
- Relatively infrequently due to high cost and the cumbersome nature of the tests
- Generally only do functional analysis if quantitative tests don’t explain symptoms
Platelet functional analysis can be performed on what 2 types of samples? Which is easier to use?
- Platelet rich plasma
- Whole blood
- Whole blood is easier to use because it does not have to be prepared
How is platelet aggregation tested on platelet rich blood?
- Optical density for the sample is measured –> relatively little light passes through as most is scattered by the platelets that are even dispersed through the sample –> high adsorption –> high OD
- Add platelet agonist –> continuing measuring OD –> platelets aggregate leaving more “open spaces” in the sample –> more light passes through –> plot OD on graph as it lowers overtime
What is the shape of the OD vs. time graph for the platelet aggregation test? How does it look for somebody deficient in agonist response?
- OD decreases over time after agonist is added
- The decrease is in 2 distinct phases. The first phase is due to the agonist which is added and the second phase is due to the agonist being released by platelets themselves
- If there is a deficient response to the agonist the curve begins to decrease and then “flat lines”
There are 2 groups of agonist used in platelet aggregation studies. Name the agonists in both groups and the mechanism they are testing:
- ADP, Epinepherine, Thrombin, Collagen and TxA2 –> test GPIIb/GPIIIa & fibrinogen
- Ristocetin –> GpIb & VWF
How is a whole blood aggregometer different than aggregation studies performed on platelet rich plasma?
• Instead of using spectrophotometry, it measures electrical impedance
What agonists are used in whole blood aggregometry?
- ADP, Collagen, Arachidonic acid and Ristocetin
* Epinepherine is not used
What disorders cause impaired clotting in response to ristocetin?
- Von Willebrand Deficiency
* Bernard Soulier Disease (GPIb deficiency)
What is Benard Soulier Disease?
• Impaired clotting due to a deficiency of GPIb (receptor for VWF) –> impaired coagulation
How is it determined whether impaired clotting to Ristocetin is the result of VWF deficiency or a deficiency of the platelet receptor for WVF (GPIb)?
• Ristocetin cofactor test
What is the Ristocetin Cofactor Test?
• Repeat Ristocentin test but use platelets from another healthy individual –> only the patients WVF is being used
What is suspected with an abnormal Ristocetin Induced Platelet Aggregation (RIPA) and normal Ristocetin Cofactor test?
- Bernard Soulier syndrome
* Normal cofactor test –> VWF from patient is fine –> must be problem with platelet –> def of GPIb
What is suspected with an abnormal Ristocetin Induced Platelet Aggregation (RIPA) and abnormal Ristocetin Cofactor test?
- VWF deficiency
* Abnormal cofactor test –> VWF from patient is not functioning
What 3 supplemental laboratory procedures can test VWF?
- Elisa tests the amount of VWF in the plasma
- Western Blot –> probe with anti-VWF –> VWF size distribution (Larger multimers are more active)
- Can test VWF ability to adhere to collagen to tests its ability to initiation adhesion
What is type 1 Von Willebrand Disease? What is it’s pattern of inheritance? Is it heterozygous or homozygous?
- Normal ratios of different sized VWF but absolute deficiency of all sizes
- Autosomal dominant
- Heterozygous
What is type 2 Von Willebrand Disease?
• Improper ratio of different sized VWF –> deficiency of larger more active VWF
What is type 3 Von Willebrand Disease? Is it heterozygous or homozygous?
- Production of VWF is virtually absent
* Homozygous for the mutation that causes type 1
What is the most common type of Von Willebrand Disease?
Type 1
What is necessary for a normal PFA-100 reading?
- Adhesion due to VWF
* Aggregation due to response to agonist
How is the PFA-100 (Platelet Function analyzer) performed?
- Have a membrane with a 150 micrometer whole –> suck blood through it and measure how long it takes to clot = closure time
- Collagen is present on the membrane (adhesion) and either epinephrine or ADP is given as an agonist (aggregation
What type of blood sample is the PFA-100 test performed on?
Whole blood
Platelet aggregation tests are not performed on patients with what bleeding disorder?
• Thrombocytopenia, already know that coagulation will be delayed by CBC & the aggregation tests are more expensive
