Platelet Function Testing
Bruckner, Terrie, Journal of Continuing Education Topics & Issues
Platelets or thrombocytes are small cells that are vital in the blood clotting process. Platelets are produced in the bone marrow and circulate in the blood until they are activated by collagen, thrombin, ADP receptors (P2Y1 and P2Y12) expressed on platelets, a negatively charged surface as well as many other activating factors." They are rapidly deployed to injury or infection sites and potentially modulate inflammatory processes by interacting with leukocytes and secreting cytokines and chemokines. (3)
When there is an injury to a blood vessel, the circulating platelets will adhere to the injury site, aggregate with other platelets and form a loose platelet plug. The high concentration of myosin and actin filaments in platelets are stimulated to contract during aggregation, which help to reinforce the plug. During this time, they will also release a number of different coagulation factors and platelet activating factors to ensure further reinforcing of the platelet plug. This process will end in the formation of fibrin strands woven through the plug and cross linked to form a fibrin net. The fibrin net will remain in place until the injury has healed. Upon healing, other factors break down the clot and remove it. Normal platelet count in a healthy adult is between 150,000400,000 per mm of blood. (3) However, an adequate number of platelets will not guarantee adequate function. (2,5)
The discovery of platelet dysfunction in a patient may not always be associated with venous injury. Many times the dysfunction is linked to an inherited or acquired bleeding disorder or may be discovered during a routine workup. (5) Suggested indications for platelet function testing include the following:
--Patient who experiences easy bruising
--Women experiencing menorrhagia
--Hospitalized patients experiencing bleeding
--Anti-platelet therapy monitoring
--Uremia (circulating substances impair platelet interactions with the vessel wall) (1)
--Cirrhosis (thrombocytopenia, coagulopathy and vascular abnormality) (1)
Platelet Function Tests
In years past, the primary test used to evaluate platelet function was the bleeding time (BT). It is performed by making a standard incision on the forearm and measuring the time required for bleeding to stop. The BT is currently being phased out of most hospitals nationwide due to the fact that is it not sensitive or specific and does not reflect the risk or potential severity of a surgical bleed. The BT is rarely reproducible and varies by the technologist performing the test. It can be affected by ingestion of aspirin or other drugs containing aspirin and leaves a small scar on the forearm. (5)
Continuous research and emerging technology have created new instrumentation in the platelet function testing. Microprocessor controlled instrumentation and disposable test cartridges have enabled medical technologist to perform platelet function testing on whole, citrated blood obtained from a single venipuncture. Improved diagnostic value, patient acceptance and cost are reason enough to evaluate platelet function on new instrumentation over the use of the BT.
Platelet Function Analyzers
The PFA-100[R] analyzer was the first commercially available in-vitro test that incorporated high-shear flow to provide a realistic hemodynamic environment to measure platelet function. Blood is aspirated into the instrument under a constant pressure through a capillary in a membrane that is coated with either collagen and epinephrine or collagen and ADP. This being the stimuli for platelet aggregation and adherence, activation and aggregation will occur in the instrument to result in the formation of a platelet plug. This plug will occlude the flow in the instrument and the time taken to occlude the flow is called the "closure time" (CT). The analyzer requires just 800[micro]L of whole citrated blood and reports the CT in 4-8 minutes. The Collagen-Epinephrine Cartridge is the primary test cartridge to identify mild to severe platelet dysfunction, von Willebrand factor deficiency or the effect of aspirin. If the CT is prolonged, the Collagen-ADP cartridge will be run to provide further information and confirm the Collagen-Epi test. (4, 5)
Another new test would be the Verify Now[R] Test. It is a rapid platelet function analyzer that uses three different test cartridges testing for the following drug effects: (1) GP IIb/IIIa dependent aggregation, (2) Aspirin effects, and (3) P2Y12 blockade. There are many other platelet function tests that measure specific aspects of platelet function or clot formation. While some methods are only used in research, others are being used only by specific physicians for specific purposes." Listed below are several other Platelet Function tests available:
1. Clot Signature Analyzer[R], Global Haemostasis High Shear platelet function
2. Cone and Plate Analyser[R], High shear platelet adhesion/aggregation
3. Flow Cytometry, Platelet glycoproteins, activation markers
4. ICHOR[R], Platelet counting (simple point of care)
5. Haemodyne[R], Platelet contractile (measures clot properties)
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In the following, choose the one best answer for each question.
1 Platelets are activated by:
B. negatively charged surface
C. fibrin net
2 Which of the following help to reinforce the platelet plug?
A. myosin and actin
B. myoglobin and troponin
3 Discovery of platelet dysfunction is always associated with venous injury.
4 Which of the following would not be an indication for Platelet Function Testing?
A. Patient easily bruises
5 A normal platelet count in a healthy adult is:
A. 150,000-450,000/mm blood
B. 50,000-400,000/mm blood
C. 350,000-450,000/mm blood
D. 500,000-950,000/mm blood
6 Automated Platelet Function Testing is performed on which of the following specimens?
A. whole citrated blood
B. EDTA plasma
C. serum with no additive
D. Lithium Heparin
7 Ingestion of aspirin will not affect a bleeding time.
8 Which of the following is not a reason for phasing out the Bleeding Time?
A. it is not sensitive
B. it is not specific
C. leaves a scar on the patient
D. provides reproducible results
9 The PFA-100[R] analyzer was the first commercially available in-vitro test measuring platelet function. The analyzer measures:
A. bleeding time
C. collagen time
D. closure time
10 The Collagen-Epi cartridge used in the PFA100[R] will help identify all of the following except:
A. von-Willebrand Factors
B. aspirin effects
C. mild to severe platelet dysfunction
D. Chronic Obstructive Pulmonary Disease
(1) http://asheducationbook. hematologylibrary. org/cgi/contentlfull/ 2005
(2) http://www.labtestsonl i ne. org/understanding/analytes
(3) http://www.righthealth.com/Health/plateletl-od-definition wiki Platelet-s
(4) http://www.urmc.rochester.edu. Path/newsletter1 03
(5) Laboratory Communique. Billings Clinic Laboratory. Department of Coagulation. Volume 1 (3): October 2007
Second Place Winner 2008 AMT Technical Writing Contest
Terrie Bruckner, MLT(AMT), generalist and responsible for Hematology and Coagulation Depts. at Washakie Medical Center Laboratory, Worland, WY…
Questia, a part of Gale, Cengage Learning. www.questia.com
Publication information: Article title: Platelet Function Testing. Contributors: Bruckner, Terrie - Author. Magazine title: Journal of Continuing Education Topics & Issues. Volume: 10. Issue: 3 Publication date: August 2008. Page number: 106+. © 2007 American Medical Technologists. COPYRIGHT 2008 Gale Group.
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