Pathology Associates Of Lexington, P.A.
Pathology Associates Of Lexington, P.A.
Pathology Associates Of Lexington, P.A.
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        Platelet Function Testing at LMC
      
[NewsPath, November 2002, Dr. John Carter]
A reasonably comprehensive range of platelet function testing, including platelet counts, platelet morphology, a standardized bleeding time and platelet aggregation profiling, has previously been available at LMC.  Several years ago platelet aggregation profiling was discontinued due to a significant fall-off of clinical utilization.  With the transfer of Dr. Will Armstrong's Hematopathology practice to LMC, platelet function testing services are significantly upgraded to include an automated Platelet Function Analysis (PFA) screening test and an expanded Platelet Aggregation Profiling service as noted in the following summaries:

LMC's Platelet Function Services:

  1. Platelet count & morphology (as part of CBC).

  2. Platelet Function Analysis (PFA): An automated screening test available 24/7, replacing Bleeding Time test.

  3. Platelet Aggregation Profiling for evaluation of:

    • Abnormal bleeding problems.
    • Abnormal clotting disorders.
    • Thrombocytosis: Etiology & Clinical significance.

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Automated Platelet Function Screening  2002

    The Bleeding Time has been the traditional screen for platelet dysfunction for many years.  It is an in vivo test and in many ways simulates the surgeon's scalpel, but is subject to a number of technician and patient variances.  Recently technology has advanced with the appearance of the Platelet Function Analyzer (PFA-100, Dade).  We are pleased to announce that this new system is now in use at LMC and is scheduled to replace the Bleeding Time test. 

   The PFA uses citrated blood obtained by venipuncture, the same specimen as for routine coagulation tests.  Whole blood is passed under pressure through a narrow channel in a test cartridge causing hemodynamic shear stress and platelet activation.  These activated platelets pass through a small pore impregnated by either collagen/epinephrine or by collagen/ADP.  The test is complete when a platelet plug occludes the pore.  This "closure time" is reported in seconds.  The PFA testing will be available 24/7 with the same turnaround time as routine coagulation tests, does not need to be scheduled, and will replace the Bleeding Time in the near future. 

   There are several disadvantages to the Bleeding Time, chiefly its lack of standardization and reproducibility.  This is especially critical on 2nd & 3rd shifts.  It has long been recognized that there is poor correlation between bleeding times and the propensity to bleed due to platelet function abnormalities.  Some patients do not test accurately with a Bleeding Time, including the elderly with fragile skin and the young who squirm.  We will be able to perform more accurate assessment of clotting status in pediatric patients with the PFA.  Skin fragility does not affect the PFA.  The PFA has been shown to more accurately predict platelet abnormalities which will cause bleeding. 

   The PFA is standardized according to our customary quality control system and reproduces well.  It will be abnormal in essentially all patients with vonWillebrand's syndrome, whereas the bleeding time will be normal in approximately 35% of cases. 

   Because of its standardization and ease of use, the PFA is useful in monitoring some types of antiplatelet therapy.  More information on anti-platelet therapy monitoring will be provided when available. 

   With the use of the two different reagent cartridges, differentiation can usually be made between anti-platelet drug effect, such as aspirin, and platelet dysfunction due to other causes.  Aspirin-like defects will cause prolongation of collagen/epinephrine, but will be normal with collagen/ADP.  Other defects will be abnormal with both.  As with the Bleeding Time, accuracy of the PFA test diminishes with platelet counts less than 100,000/ul or hematocrits <30%.  The PFA will occasionally be abnormal in high-dose heparin therapy. 

Possible uses for the PFA include:

  1. Evaluation of a history or presence of easy bruising,

  2. Family history of heritable bleeding problems,

  3. Pre-operative screen for major surgical procedures,

  4. Post-operative bleeding with normal platelet count,

  5. Prior to renal and liver biopsies, and

  6. Prior to invasive CNS procedures. 

   It is generally accepted that a normal PFA test result precludes further platelet function work-up such as platelet aggregation profiling.

   For any questions concerning the use or interpretation of this test please contact Dr. Armstrong at 791-2410.  References are available upon request. (Sem. Thrombosis & Hemostasis. 24(2) 195-202, 1998.)

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More Platelet Function Testing at LMC

Platelet Aggregation Profiling

    Platelet Aggregation Profiling (PAP) is a standard and definitive test for specific variants of abnormal platelet function.  After several years of absence, this testing service has returned to LMC with the recent purchase of the latest generation Platelet Aggregometer.  Platelet Aggregation studies are the next step in evaluation of platelet function after an abnormal PFA screen has been documented.  LMC is one of only three institutions in South Carolina performing this test. 

   Traditional indications for ordering Platelet Aggregation studies primarily concern the evaluation of elevated platelet counts (thrombocytosis), diagnosis and monitoring of anti-platelet medication effect, and evaluation of platelet-related bleeding or clotting disorders. Congenital platelet dysfunction, with the exception of vonWillebrand's disease is uncommon.  Variants of vonWillebrand's disease are relatively common, however, particularly the mild form (Type IIa) which may exist below the clinical threshold for bleeding problems.  It is particularly important to identify which of these problems is responsible for a hemostatic defect since specific therapy differs for each.  Defining a precise platelet function defect will minimize a "shotgun approach" to therapy when abnormal bleeding is encountered.  Unnecessary transfusion of blood products can possibly be avoided.  In the case of vonWillebrand's variants, human blood products can often be avoided entirely.  A precise diagnosis thus may provide safer, more efficient, and less costly care for these patients. 

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Platelet Aggregation Profiling: 

The Consultative Report

    Platelet aggregation profiling is not an automated test with a simple quantitative result.  Rather it involves expert manual processing of platelet-rich plasma in reaction with at least four stimulating agents, often in varied concentrations, followed by detailed analysis of the resulting aggregation patterns.  The Final Report is presented as a consultation requiring close correlation with the patient's clinical situation. 

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The Absolute Functional Platelet Count

    Over the years, platelet aggregometry has been expanded to other areas of platelet function assessment, particularly those related to abnormal thrombotic disorders.  One platelet function study offered at LMC is the Absolute Functional Platelet Count (AFPC).  The finding of a chronically elevated platelet count is common, though most often clinically insignificant below the level of 550,000-600,000/ul.  A platelet count over 600,000/ul can be associated with increased risk of abnormal clotting.  Some of these cases of elevated platelet count are associated with a post-splenectomy response, a diagnosis clearly evident by history.  Other cases may reflect a chronic myeloproliferative process, particularly Essential Thrombocythemia (ET).  Many patients with an elevated platelet count have a reactive thrombocytosis (response to infection, surgery, traumatic injury, neoplasia, etc.) in which platelet counts may range up to 1.2 million.  A precise diagnosis is essential when considering anti-platelet therapy in these patients.  While a platelet aggregation profile will often clarify the precise diagnosis in these patients, consideration of anti-platelet therapy is facilitated by determination of the number of functional platelets.  In Essential Thrombocythemia the number of functional platelets is often in the normal or low-normal range.  Some ET patients may actually have a functional thrombocytopenia, even with an elevated total platelet count.  The neoplastic platelets in this disorder are often largely non-functional and in these cases anti-platelet therapy would be contraindicated.  Since patients with Essential Thrombocythemia usually have defective epinephrine-induced platelet aggregation, the PAP will clarify that diagnosis and the AFPC will determine the need for or utility of anti-platelet therapy. 

   Absolute functional platelet counts can be determined by a straightforward calculation involving pre- and post-aggregation platelet counts on the same patient sample.  The percent of platelets that have aggregated is the functional platelet count.  Normal patients have greater than 95% functional platelets. 

   From a clinical perspective there are no solid guidelines as to when the absolute functional platelet count warrants anti-platelet therapy.  This decision must involve the global clinical picture and other risk factors such as severe atherosclerosis, ischemic heart disease, diabetes, obesity, family history of thrombosis, etc.

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Hyperactive Platelets & Anti-Platelet Therapy

    Some patients have circulating platelets which have increased tendency to aggregate, especially in response to aggregating agents or exposed sub-endothelium. This has been called the Super Sticky Platelet Syndrome. It is a very common syndrome, and is associated with a wide variety of ischemic heart diseases, diabetes, obesity, auto-immune disorders, chronic renal disease, pregnancy-induced hypertension, neoplasia, and as an idiopathic disorder. Patients with Super Sticky Platelets have a high incidence of thromboses, especially arterial. It is common for them to present with a thrombosis related problem. To evaluate hyperactivity, we use platelet aggregation with progressive dilutions of epinephrine and collagen. The degree of hyperactivity can be quantitated as mild, moderate, or severe. With this test, a pre-thrombotic state can be diagnosed when platelets are a significant contributor. A finding of hyperactive platelets gives support to the addition of anti-platelet drugs to the treatment regimen. In addition, this test can also give some quantitation to the success of anti-platelet therapy. Simply adding an anti-platelet drug does not guarantee adequate platelet suppression, since some patients will be non-responders. (Clin. Appl. Thrombosis/Hemostasis 4 (2) 77-81, 1998.)

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Ordering and Scheduling:

    Platelet Aggregation Profiling and related tests are available M-F during the day.  Samples must be collected in the morning.  These tests must be pre-scheduled through the Hematology Lab @ 791-2403 and mention of a specific clinical question will help to focus testing and will greatly facilitate diagnosis interpretation. 

   A written Clinical Pathology consultation will be given with Platelet Aggregation studies.  This report will include aggregation data, diagnostic impression and clinical suggestions where appropriate. 

Platelet Aggregation tests to order include:

  1. Platelet Aggregation: Abnormal bleeding.

  2. Platelet Aggregation: Abnormal clotting/hyperactive platelets. 

  3. Platelet Aggregation: Thrombocytosis -- Etiology and Absolute Functional Count.

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Platelet Aggregation Profiling

The Process

  1. The platelet aggregometer records light transmission thru platelet-rich plasma. 

  1. Platelet aggregation is externally induced by the addition of stimulating agents.  Four platelet aggregating stimulants are used in a routine study:

  1. ADP

  2. Collagen

  3. Epinephrine

  4. Ristocetin

  1. Platelet aggregation in response to these stimulating agents occurs in progressive phases:

  1. Addition of the stimulant

  2. Primary wave of aggregation in response to the stimulant.

  3. Termination of primary platelet aggregation response prior to:

  4. Induction of platelet secretory response and:

  5. Secondary wave of aggregation in response to the endogenous platelet secretory products;

  6. Complete irreversible aggregation of platelets.

  1. The platelet aggregation wave below is typical of a normal platelet response to ADP stimulus.  Aggregation patterns in response to the other stimuli may be slightly different.  The various congenital and acquired platelet function abnormalities produce varied abnormal aggregation patterns in response to the different stimulating agents. 

(posted Dec. 2002)

 
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