PFA-100
What is the PFA-100?
What are the normal values?
How long are blood samples stable for PFA-100 testing?
How do I interpret the results?
How is the PFA-100 quality-controlled?
What are the common status messages produced by the PFA-100?
Can the PFA-100 be used as a pre-operative screening test?
We frequently see prolonged closure times in the ADP cartridge in patients with aortic or mitral valve disease. Why is this?
How does the PFA-100 compare to a Template Bleeding Time?
What are the advantages of the PFA-100?
Why is the Template Bleeding Time a poor test of platelet function?
Can the PFA-100 be used to test for aspirin resistance?
I need to run PFA-100 tests on mouse blood, but I get repeated flow obstructions and the blood often clots. Is there a workaround?
How do I order a PFA-100 test?
Where can I get more information?
What is the PFA-100?
The PFA-100 test is a new test of platelet function that measures the time taken for blood, drawn through a fine capillary, to block a membrane coated with collagen and epinephrine (CEPI) or collagen and ADP (CADP). This is referred to as the Closure Time (CT) and is measured in seconds. The test is therefore a combined measure of platelet adhesion and aggregation.
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What are the normal values?
In the Hemostasis and Thrombosis Laboratory, normal ranges are CEPI <164 s; CADP <116 s. The ranges are dependent on citrate concentration and, most likely, the choice between glass and plastic collection tubes. Each laboratory should determine its own normal range.
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How long are blood samples stable for PFA-100 testing?
The instrument manufacturer, Dade-Behring, Inc., recommends that samples be tested within 4 hours of collection. In house studies at the Hemostasis and Thrombosis Laboratory suggest that normal blood may be stable for up to 6 hours, but we have no information on the behavior of samples with abnormal platelet function.
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How do I interpret the results?
The PFA test result is dependent on platelet function, plasma von Willebrand Factor level, platelet number and (to some extent) hematocrit. It is important to remember that significantly anemic (Hct <28%) individuals may exhibit a prolonged CT even if their platelet number and function is normal. In the Hemostasis and Thrombosis Laboratory we typically intpret as follows:
CEPI is <164 s - Normal Platelet Function
The PFA test is initially performed with the CEPI membrane. A normal CEPI value excludes the presence of a significant platelet function defect.
CEPI >164; CADP <116 s - "Aspirin Effect"
If the closure time with CEPI is prolonged, the CADP test is automatically performed. If this result is normal, the most likely explanation is that the patient has ingested aspirin or similar medication. The CEPI test is very sensitive to aspirin and in many patients, the result will be greater than 250 s. In contrast, the CADP test is relatively insensitive to the aspirin-induced platelet defect. If the patient denies aspirin, we suggest follow up with platelet aggregation/release studies to exclude a 'real' defect. There is evidence that platelet storage pool and release defects can give the "aspirin pattern" on the PFA. Such defects may be more common than previously recognized, even exceeding vWD (~1% of the population) in their prevalence. These defects can be distinguished from aspirin by platelet aggregation/release studies.
CEPI >164; CADP >116 - Abnormal Platelet Function
The finding that both CEPI and CADP are prolonged suggests that platelet function is abnormal. However, thrombocytopenia (<100K) and anemia (Hct <0.28) should first be excluded. If platelet count and hematocrit are normal, the PFA indicates an abnormality of platelet function that should be followed up. Note, however, that some patients taking long-term aspirin may also have a modest prolongation of the CADP time. In our experience, the most common causes of an abnormal CADP test are heart valve disease, severe vessel stenosis, renal failure and von Willebrand's disease. Since the first three will be clinically obvious, appropriate follow up is a platelet aggregation/release profile and von Willebrand Factor activity and antigen determination.
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How is the PFA-100 quality-controlled?
The PFA-100 instrument performs a self-test on start up that tests the electronics and system vacuum. There are no commercially available controls since the required sample is fresh whole blood. Each cartridge is essentially a self-contained test system, so running a normal control really tells you nothing about the performance of the next cartridge containing the patient's sample. In practice, it is only necessary to run a normal control with each new lot of cartridges.
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What are the common status messages produced by the PFA-100?
The PFA-100 may produce any of several messages during a test run. The event is flagged with a '>' sign, followed by the time the event occurred, a status code letter (A-E) and a brief description of the event. For example: > 300 SEC MAX TEST TIME EXCEEDED. The following status codes may occur:
A. Maximum test time exceeded.
B. Air leak
C. Flow obstruction
D. Insufficient sample
E. Maximum syringe travel reached.
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Can the PFA-100 be used as a pre-operative screening test?
To date, there is no evidence that a pre-operative PFA-100 test can predict bleeding during a surgical procedure. However, because it is more sensitive to drug-induced defects and other platelet abnormalities than the Template Bleeding Time (TBT), abnormal results will be seen more frequently. Indeed, if the intent of the test is to detect the presence of potentially anti-platelet drugs prior to surgery, the PFA-100 is the test of choice. Aspirin use in patients undergoing epidural anesthesia increases the risk of neurological complications. The PFA-100 test can identify such individuals much more reliably than the TBT. In general, however, the significance of abnormal PFA-100 results for bleeding during subsequent surgery is not known at this time.
If the PFA-100 results suggest aspirin ingestion (Prolonged CEPI but normal CADP) and the history confirms this, the surgeon should make the determination whether or not to proceed with surgery. As noted above, the presence or magnitude of the PFA (CEPI) abnormality is not necessarily related to the possibility of bleeding during surgery.
If the PFA results indicate abnormal platelet function (prolonged CEPI and CADP) and the patient is not anemic, thrombocytopenic or has cardiovascular or renal disease, we suggest that surgery be postponed until the underlying reason for the abnormality is determined.
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We frequently see prolonged closure times in the ADP cartridge in patients with aortic or mitral valve disease. Why is this?
Our laboratory reported this connection in a paper published in 2000. An acquired form of type 2A von Willebrand disease is common in patients with severe aortic stenosis. Von Willebrand factor abnormalities are directly related to the severity of stenosis and are improved by valve replacement.
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How does the PFA-100 compare to a Template Bleeding Time?
Our published studies (Francis et al., Platelets 10: 132-136, 1999) indicate that the PFA-100 and TBT give equivalent results in about 75% of cases. Most of the patients that gave discrepant results had a normal BT with abnormal PFA-100. In these cases, the PFA-100 result was usually supported by platelet aggregation, the "gold standard" of platelet function tests. Most discrepancies between TBT and PFA-100 are a result of aspirin ingestion since the PFA-100 is much more sensitive to this effect.
Several studies have now shown that the PFA-100 has excellent (near-100%) sensitivity to the presence of von Willebrand's Disease. In contrast, the BT may miss as many as one third of cases. The PFA-100 can also more conveniently be used to monitor treatment with DDAVP in Type 1 von Willebrand's Disease.
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What are the advantages of the PFA-100?
- Tests performed on regular blood (blue top) sample
- Blood samples stable for 4 hours after collection
- Reproducible and rapid (10 minutes) to perform
- Correlates closely with platelet aggregation
- Sensitive to acquired and congenital platelet defects (including von Willebrand's Disease)
- Can distinguish aspirin-induced platelet dysfunction from other platelet defects
- Cheaper and faster than platelet aggregation tests
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Why is the Template Bleeding Time a poor test of platelet function?
Despite attempts at standardization, the TBT remains poorly reproducible and is relatively insensitive to platelet function defects. For example, over one-third of patients with von Willebrand's Disease have normal results. The TBT has been widely used as a pre-operative screening test, despite the fact that numerous studies have pointed to the unsuitability of the test for this purpose: Here are some excerpts from published papers:
P. Peterson, T. E. Hayes, C. F. Arkin, E. G. Bovill, R. B. Fairweather, W. A. Jr Rock, D. A. Triplett, and J. T. Brandt. The preoperative bleeding time test lacks clinical benefit: College of American Pathologists' and American Society of Clinical Pathologists' position article. Arch Surg 133 (2):134-139, 1998.
"(1) In the absence of a history of a bleeding disorder, the bleeding time is not a useful predictor of the risk of hemorrhage associated with surgical procedures. (2) A normal bleeding time does not exclude the possibility of excessive hemorrhage associated with invasive procedures. (3) The bleeding time cannot be used to reliably identify patients who may have recently ingested aspirin or non-steroidal anti-inflammatory agents or those who have a platelet defect attributable to these drugs. The best preoperative screen to predict bleeding continues to be a carefully conducted clinical history that includes family and previous dental, obstetric, surgical, traumatic injury, transfusion, and drug histories. In the absence of a history of excessive bleeding, the bleeding time fails as a screening test and is, therefore, not indicated as a routine preoperative test"
A.S. Gewirtz, M. L. Miller, and T. F. Keys. The clinical usefulness of the preoperative bleeding time. Arch Pathol Lab Med 120:353-356, 1996.
"The positive predictive value of the preoperative bleeding time was 5%, and the negative predictive value was 95%. In conclusion, screening for preoperative bleeding time is not a reliable test for assessing the risk of clinically significant perioperative bleeding and should not be used for this purpose".
R. De Caterina, M. Lanza, G. Manca, G. B. Strata, S. Maffei, and L. Salvatore. Bleeding time and bleeding: an analysis of the relationship of the bleeding time test with parameters of surgical bleeding. Blood 84: 3363-3370, 1994.
"Thus, in patients with a negative history of bleeding and no recent intake of non-steroidal anti-inflammatory drugs, higher values for bleeding time and bleeding time-related parameters are not associated with higher indices of perioperative and postoperative bleeding at coronary bypass surgery. Therefore, we do not recommend the use of the test in this setting to predict perioperative or postoperative bleeding".
R. P. Rodgers and J. Levin. A critical reappraisal of the bleeding time. Semin. Thromb. Hemost. 16:1-20, 1990.
"Two ROCs [Receiver Operating Curves] from surgical studies, in which the bleeding time was used to try to predict abnormal bleeding, were statistically indistinguishable from that of a completely non-informative test. In no instance was there evidence that the bleeding time significantly altered a priori estimates (based on prevalence) of the risk of bleeding".
E. R. Burns and C. Lawrence. Bleeding time. A guide to its diagnostic and clinical utility. Arch.Pathol.Lab.Med. 113:1219-1224, 1989.
"Although the bleeding time is the most widely used clinical test employed to assess platelet function, its indications are poorly defined. The test is commonly used as a preoperative screen to predict hemorrhage, but the data supporting this indication are contradictory at best. Its utility in providing helpful clinical information in patients who do not have a known bleeding diathesis by history is minimal."
E. R. Burns, H. H. Billett, R. W. Frater, and D. A. Sisto. The preoperative bleeding time as a predictor of postoperative hemorrhage after cardiopulmonary bypass. J Thorac Cardiovasc Surg 92:310-312, 1986.
"There was no correlation between bleeding time and either fall in hemoglobin level (r = 0.04) or chest tube drainage (r = 0.004). In addition, bleeding time did not correlate with the number of units of platelet concentrate (r = 0.12) or packed red cells (r = 0.2) infused. The bleeding time, which has been recommended as an essential screening test before all cardiopulmonary bypass procedures, need not be performed as a preoperative screen in otherwise healthy patients with no history of bleeding abnormalities and a normal coagulation profile".
S. E. Lind. Prolonged bleeding time. Am.J.Med. 77:305-312, 1984.
"The bleeding time appears to have its greatest utility in evaluation of a patient with active bleeding or one with a well documented bleeding history. It should not be used as a substitute for a clinical history, since there is insufficient information available to calculate its sensitivity, specificity, or predictive value with regard to peri- or postoperative hemorrhage."
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Can the PFA-100 be used to test for aspirin resistance?
This is currently a controversial area. Published data suggest that a significant number of patients taking prophylactic aspirin do not respond with the typical prolongation of the CEPI closure time. There is also some evidence that such patients may have a worse outcome. However, other factors, such as elevated von Willebrand Factor levels, may give rise to normal closure times in patients taking aspirin. In addition, it remains unclear whether changing patient management based on the results of any aspirin resisance testing will improve patient outcomes.
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I need to run PFA-100 tests on mouse blood, but I get repeated flow obstructions and the blood often clots. Is there a workaround?
We find the same problem. However, anticoagulating the mouse blood with a direct thrombin inhibitor, such as lepirudin, allows us to get consistent closure times.
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How do I order a PFA-100 test?
Within the Florida Hospital system, the PFA-100 test is available through the computer ordering system and is available from the Hemostasis and Thrombosis Laboratory between 6 am and 5 pm Monday to Friday, and 8 am to 4 pm on weekends. Tests outside these hours are limited to emergency requests only. Local clients of the Hemostasis and Thrombosis Laboratory may obtain a PFA-100 test as long as we receive the blood sample within 4 hours of collection.
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Where can I get more information?
If you have a questions about the PFA-100 that is not addressed here, please e-mail it to us and we will try to answer it for you. You may find the answer in our Slideshow on PFA-100 or in our list of Publications on the PFA-100. Alternatively, you can contact the manufacturer, Dade-Behring Inc. or visit their educational website, PlateletPerspectives.
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