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The PFA-100 is a system for analysing platelet function in which citrated whole blood is aspirated at high shear rates through disposable cartridges containing an aperture within a membrane coated with either collagen and epinephrine (CEPI) or collagen and ADP (CADP). These agonists induce platelet adhesion, activation and aggregation leading to rapid occlusion of the aperture and cessation of blood flow termed the closure time (CT). This LINK will take you to an animation that will explain this process in more detail.

The advantages of the PFA-100 include:

• Only small volumes of citrated venous blood [800µL) are needed and so the test is useful for investigating platelet function in children.

• Can be used by non-skilled personnel and is both rapid and automated

• The PFA-100 was designed as a screen to detect problems with primary haemostasis and in part to replace the bleeding time and in this respect it is better standardised.

• Measurement of platelet function at high shear [physiological] rates whereas LTA measures platelet function at low shear rates i.e. less physiological.

• Relatively insensitive to clotting factor deficiencies

• High negative predictive value – i.e. if the PFA-100 gives a normal result then with some exceptions primary haemostasis is intact [Exceptions: SPD, Primary Secretion Defects, mild Type 1 VWD]

A number of variables have been shown to affect the results obtained with the PFA-100 and these include:

• Citrate concentration – so laboratories must use a fixed citrate concentration.

• How the sample was collected and transported to the lab.

• Haematocrit – closure times increase progressively with decreases in haematocrit.  Conversely, CTs are shortened in the neonate due to their higher haematocrit. 

• Thrombocytopenia - closure times increase progressively as the platelet counts falls below 100 x 10⁹/L

• Closure times correlate inversely with plasma VWF activity levels and may be increased in blood group O patients for the same reason [Blood group O individuals have lower VWF levels.]

• Drugs:

• COX inhibitors such as aspirin and NSAIDs usually prolong the closure time of the CEPI cartridge but not the CADP cartridge. 

• The effects of ADP receptor blockers such as Clopidogrel is unpredictable.

• Inhibition of the GpIIb/IIIa receptor is associated with a significant prolongation of both cartridges.

• Acquired platelet function defects

• Cardio-pilmonary bypass

• Liver disease

• Uraemia

• Some foods

• Generation of false positives particularly with the CEPI cartridge. This is due probably to ingested foods or drugs.  An abnormal PFA-100 result is not, therefore, diagnostic. 

• The PFA-100 is often used to establish the presence of absence of aspirin resistance. The frequency of aspirin resistance is unknown, but estimates range from 5-60%. The mechanism of aspirin resistance is unknown but proposed mechanisms include poor patient compliance, poor aspirin absorption, increased platelet hypersensitivity to agonists, increased COX activity, and polymorphisms in the Gp IIIa receptor and the COX enzyme.  Aspirin resistance appears to be dose related in some patients and may be overcome with higher doses.

• The PFA-100 CTs are shortened in the neonate due to their higher haematocrit.

• The tests must be performed within 4 hours of collection.

Interpretation

The following table summarises some of the abnormalities that have been reported with the PFA-100.

Reference Ranges

Reported reference ranges for closure times are:

• 78 - 199 seconds for the CEPI cartridge
• 55 - 137 seconds for the CADP cartridge

What Test Next?

1. The PFA-100 has a high negative predictive value i.e. if the PFA-100 gives a normal result then with some exceptions primary haemostasis is intact [Exceptions: SPD, Primary Secretion Defects, mild Type 1 VWD] and so may obviate further screening of platelet function.

2. If the PFA-100 is abnormal then formal platelet aggregation testing will be required to establish the underlying cause.

Useful Links & References

1. Favaloro, E.J., Clinical utility of the PFA-100. Semin Thromb Hemost, 2008. 34(8): p. 709-33.

2. Reny, J.L., et al., Use of the PFA-100 closure time to predict cardiovascular events in aspirin-treated cardiovascular patients: a systematic review and meta-analysis. J Thromb Haemost, 2008. 6(3): p. 444-50.

3. Karger, R., et al., Diagnostic performance of the platelet function analyzer (PFA-100) for the detection of disorders of primary haemostasis in patients with a bleeding history-a systematic review and meta-analysis. Platelets, 2007. 18(4): p. 249-60.

4. Favaloro, E.J., Laboratory monitoring of therapy in von Willebrand disease: efficacy of the PFA-100 and von Willebrand factor:collagen-binding activity as coupled strategies. Semin Thromb Hemost, 2006. 32(6): p. 566-76.

5. Franchini, M., The platelet function analyzer (PFA-100): an update on its clinical use. Clin Lab, 2005. 51(7-8): p. 367-72.

6. Franchini, M., The p latelet-function analyzer (PFA-100) for evaluating primary hemostasis. Hematology, 2005. 10(3): p. 177-81.

7. Harrison, P., The role of PFA-100 testing in the investigation and management of haemostatic defects in children and adults. Br J Haematol, 2005. 130(1): p. 3-10.

8. Favaloro, E.J., Clinical application of the PFA-100. Curr Opin Hematol, 2002. 9(5): p. 407-15.

9. Jilma, B., Platelet function analyzer (PFA-100): a tool to quantify congenital or acquired platelet dysfunction. J Lab Clin Med, 2001. 138(3): p. 152-63.

10. Favaloro, E.J., Utility of the PFA-100 for assessing bleeding disorders and monitoring therapy: a review of analytical variables, benefits and limitations. Haemophilia, 2001. 7(2): p. 170-9.

11. Rand, M.L., M.D. Carcao, and V.S. Blanchette, Use of the PFA-100 in the assessment of primary, platelet-related hemostasis in a pediatric setting. Semin Thromb Hemost, 1998. 24(6): p. 523-9.12.

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.