Platelets

Question 1

Why are platelet function tests performed and when?

Answer 1

Why:

• to monitor patients with bleeding disorders
• the monitor the response to anti-platelet treatments
• to evaluate the perioperative haemostasis
• in transfusion medicine

When:
- if a person bruises very easily or experiences prolonged or excessive
bleeding
- when a person is on medication that can affect platelet function (Eg
aspirin is variable in patients so use ths to see how effective it is being
on their platelets)
- prior to or during certain surgeries

NO ONE TEST MEASURES ALL FUNCTIONS

Question 2

What is light transmission aggregometry?

Answer 2

The basis of aggregometry is:
- plasma without platelets, following fast centrifugation, acts as
the comparison
- platelet rich plasma in the resting state is set as 0
- an agonist is added that causes the platelets to change shape
and begin to agglutinate. This will only occur if the plasma is stirred.
- agglutination prevents the transmission of light through the
sample.

There are lots of different agonists available that can diagnose lots of different conditions.

Question 3

What are the activation markers?

Answer 3

Also known as agonists.

• ADP
• adrenaline
• collagen
• ristocetin

Question 4

What type of sample is used for a platelet function test?

Answer 4

A citrate tube
- mild pH
- mild Ca chelator (stop unnessecary clotting, but still allow sufficient
amounts of Ca so that when stimulated by an agonist platelets will still
work properly)

Question 5

What disorders may aggregometry diagnose?

Answer 5

Lack of agglutination with ristocetin as the agonist indicates not enough serum VWF or not enough VWF receptors, hence Von Willebrand Disease or Bernard Soulier Syndrome.

Lack of agglutination with ADP, adrenaline or collagen agonists indicates a severe platelet deficiency downstream of fibrinogen action, hence Glanzmann’s thrombasthenia or afibrinogenaemia.

A shoulder mid way through the curve or complete reversal of curve direction indicates agglutination has not gone passed the second stage. This suggests the patient may have a storage deficiency, so a failure to release a and b granules. This is consistent withplatelet storage pool disorder or a defect in nucleotide release (though is also seen in patients taking aspirin)

Question 6

What is lummi-aggregometry?

Answer 6

Same type light transmittance aggregometry, measured agglutination, but in this case dense granule release is also measured.
Add luciferin/ luciferase, from fireflies which need ATP to glow.
When platelets release dense granules they also release ATP, the amount of granule release is proportional to the light produced.

Question 7

What is whole blood aggregometry?

Answer 7

Activated platelets adhere to electrolytes
This impedes the electrical signal between the electrodes
Impedance of the signal is proportional to agglutination

This requires no sample preparation.

Question 8

What point of care versions of aggregometry are available?

Answer 8

Due to necessity to asses prior to operations or to measure the effectiveness of drugs. Also platelet function tests must be done in 4 hours since collection.

VerifyNow:
Light transmission through whole blood. Platelets aggregate and stick to beads then allowing MORE light through.
More expensive. Use whole blood = no sample processing. But less flexible, cant look at lots of agonists (usually these related to bleeding out), therefore not diagnostic or investigative, main use is in theatres.

PFA-100:
Standardised version of bleeding test.
Measure time it takes to form platelet plug = closure time.
Is the of occlusion within a given reference range?

Question 9

How is Flow Cytometry used to assess platelet function?

Answer 9

• Different fluorescent antibodies are used to measure different
  receptors and determine whether they are they present or not.
• Flexible technique. Used to look at surface expression of
  molecules and whether they are increased upon activation. Can also
  look at intracellular components.