Platelet Assays

Question 1

what is a platelet count?

Answer 1

a lab test used to determine the number of platelets in the blood

Question 2

what can cause a hematology analyzer to give an inaccurate platelet count?

Answer 2

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

Question 3

what is a normal platelet count?

Answer 3

150,000 and 450,000 platelets/mm3

Question 4

what platelet count would classify as thrombocytosis?

Answer 4

platelet count over 450,000

Question 5

what happens when someone has thrombocytosis?

Answer 5

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.

Question 6

what platelet count would classify as thrombocytopenia?

Answer 6

below 150,000

Question 7

at what platelet count would you experience severe uncontrollable bleeding?

Answer 7

usually not seen until the platelet count dips below 30,000 platelets/mm3

people with mild thrombocytopenia don’t usually have abnormal bleeding

Question 8

what is HIT?

Answer 8

heparin-induced thrombocytopenia

leads the body to rapidly consume platelets

you have to monitor platelet levels during anticoagulant therapy with heparin products

Question 9

what does heparin do?

Answer 9

heparin binds to antithrombin and activates it

activated AT inactivates thrombin and factor Xa in the coagulation pathway which prevents fibrin formation

Question 10

are blood smears used to diagnose platelet disorders?

Answer 10

not really

most diseases that involve platelets have normal looking platelets

Question 11

what is the bleeding time?

Answer 11

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

Question 12

what are the things to take into consideration when doing a bleeding time test?

Answer 12

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

Question 13

what are the limits of the bleeding time test?

Answer 13

1. hard to standardize like the size of lancet, the skin depth and durability, and the technician’s blotting technique
2. you can’t tell what’s causing the clotting to be messed up

Question 14

what is the platelet function assay?

Answer 14

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

Question 15

what are platelet aggregation studies?

Answer 15

platelet aggregation studies measure how well the patient’s platelets aggregate in response to different aggregating agents

Question 16

how do you do a platelet aggregation study?

Answer 16

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

Question 17

what are the different aggregating agents that are used in a platelet aggregation study?

Answer 17

• 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

Question 18

what are the results of a platelet aggregation study for a Bernard-Soulier patient?

Answer 18

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

Question 19

what is Bernard-Soulier?

Answer 19

platelets lack the normal receptor for von Willebrand factor aka GP Ib receptor

Question 20

what is von willebrand disease?

Answer 20

patient has no vWF or it’s abnormal

Question 21

what are the results of a platelet aggregation study for a von willebrand patient?

Answer 21

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