Platelet Assays Flashcards
what is a platelet count?
a lab test used to determine the number of platelets in the blood
what can cause a hematology analyzer to give an inaccurate platelet count?
platelet counts can be falsely elevated or decreased when other small cells are misread as platelets or when platelet clumps are present
in these instances, platelet counts must be performed manually by examining a blood smear
blood smear may also reveal subtle morphologic abnormalities in the platelets that the analyzer device cannot detect
what is a normal platelet count?
150,000 and 450,000 platelets/mm3
what platelet count would classify as thrombocytosis?
platelet count over 450,000
what happens when someone has thrombocytosis?
patients with extremely high numbers of platelets may suffer ischemic events, because the blood gets “sludgy” and flows slowly when there are so many platelets present.
what platelet count would classify as thrombocytopenia?
below 150,000
at what platelet count would you experience severe uncontrollable bleeding?
usually not seen until the platelet count dips below 30,000 platelets/mm3
people with mild thrombocytopenia don’t usually have abnormal bleeding
what is HIT?
heparin-induced thrombocytopenia
leads the body to rapidly consume platelets
you have to monitor platelet levels during anticoagulant therapy with heparin products
what does heparin do?
heparin binds to antithrombin and activates it
activated AT inactivates thrombin and factor Xa in the coagulation pathway which prevents fibrin formation
are blood smears used to diagnose platelet disorders?
not really
most diseases that involve platelets have normal looking platelets
what is the bleeding time?
doctors used to cut people and measure the time it took the patient to stop bleeding
the time it takes from beginning to end represents the time it takes a person to perform primary hemostasis = the platelet plug
usually 2-9 minutes
what are the things to take into consideration when doing a bleeding time test?
bleeding time does not reflect the activity of fibrin and the formation of a mature clot
this is the only test we have that tests the function of platelets in vivo. All other tests are in vitro
what are the limits of the bleeding time test?
- hard to standardize like the size of lancet, the skin depth and durability, and the technician’s blotting technique
- you can’t tell what’s causing the clotting to be messed up
what is the platelet function assay?
PFA-100
in vitro test
blood is fed into a machine with specific chemicals that cause platelets to form a plug over a standardized aperture
it’s basically bleeding time within a machine
what are platelet aggregation studies?
platelet aggregation studies measure how well the patient’s platelets aggregate in response to different aggregating agents
how do you do a platelet aggregation study?
add an aggregating agent to a tube containing platelet-rich plasma
as the platelets clump together, the plasma will become less cloudy, or turbid
a decrease in turbidity means that the platelets are aggregating properly in response to that particular agent
what are the different aggregating agents that are used in a platelet aggregation study?
- ristocetin,
- collagen
- ADP
- epinephrine
normal platelets will respond to every aggregating agent but in platelet disorders platelets don’t respond normally to every agent
the pattern of response to the different agents is what’s really helpful in narrowing down a diagnosis
what are the results of a platelet aggregation study for a Bernard-Soulier patient?
the platelets respond normally to every aggregating agent except ristocetin
this is because ristocetin works by making platelets express their GP Ib receptors
so you can give all the ristocetin you want to platelets from patients with Bernard-Soulier, and the platelets just won’t aggregate properly because they can’t express normal GP Ib
what is Bernard-Soulier?
platelets lack the normal receptor for von Willebrand factor aka GP Ib receptor
what is von willebrand disease?
patient has no vWF or it’s abnormal
what are the results of a platelet aggregation study for a von willebrand patient?
they respond to all aggregating agents except ristocentin
this is because in vWF disease, there’s no vWF
so if you mix platelets with ristocetin, they will go ahead and express their little GP Ib receptors, but there’s no normal vWF around
so the platelets won’t aggregate in response to ristocetin
