CONTENTS
Platelet Function Testing
Change in Medical Necessity Documentation for Protimes
Winter Virus Info On-Line
Beginning Monday, January 6th, 2003, the Strong Health Clinical Laboratories will start using a new test to evaluate platelet function. There are two reasons for this change. First, there are serious diagnostic and technical problems with the bleeding time, which is the most commonly used test to evaluate platelet function currently. Second, new technology has been developed that offers improved diagnostic information with greater convenience.
At the present time, the bleeding time and platelet aggregation studies are available to investigate possible problems with platelet function. The bleeding time (BT) is performed by making a standard incision on the forearm and measuring the time required for bleeding to stop. The BT can be prolonged by thrombocytopenia, congenital or acquired abnormalities of platelet function, and it is sensitive to the effects of numerous drugs including aspirin. Although it has been used for many years, the BT suffers a variety of problems. First, it is somewhat operator dependent which results in test variability. Second, it is poorly predictive of surgical bleeding when used for preoperative evaluation. Third, it may be normal in patients with significant problems with primary hemostasis. Fourth, it is expensive and difficult to schedule, requiring considerable technician time. Finally, it is inconvenient and unpleasant for patients who dislike having the incision made and the scar that often results. Therefore we believe that initial screening for abnormalities in platelet function will be better performed using new instrumentation that uses a blood sample to analyze platelet function.
The method we have chosen is the Platelet Function Analyzer (PFA)-100. This system is composed of a microprocessor-controlled instrument and disposable test cartridge that contains a biologically active membrane. Blood is aspirated into the instrument under a constant pressure through a capillary in a microscopic aperture in a membrane that is coated with either collagen and epinephrine (CEPI) or collagen and ADP (CADP). These are physiologic stimuli for platelet aggregation and platelet adherence. Activation and aggregation of platelets occur under high shear rates generated in the instrument to result in formation of a "platelet plug" that occludes flow. The time taken to occlude the membrane aperture is termed the "closure time" (CT) and is recorded by the instrument as the endpoint.
Prolonged closure times reflect decreased platelet function. This may be caused by either congential or acquired platelet function abnormalities, including the effects of common drugs such as aspirin. The laboratory will first use the sensitive CEPI cartridge and will report normal platelet function if the CT is within the normal range. If the CT is prolonged, the test will also be run with the CADP cartridge to provide further information. In particular, aspirin will prolong the CT with the CEPI, but not the CADP cartridge. Similar to the BT, closure times may be prolonged in patients with thrombocytopenia and/or anemia.
Platelet function analysis using the PFA-100 will become the primary screening method to detect platelet function abnormalities. We recommend this over the BT because of improved diagnostic value, patient acceptance and cost. The BT will, however, remain available if needed. That is, the BT will be usual, for the first three months after implementation of the CT. After this period, the BT will be available only with Hematology Consult. Lab requisitions with the CT will gradually replace the current requisitions listing the BT. In the meantime beginning 1/6/03, the CT may be ordered by writing in "closure time" on requisitions. Platelet aggregation studies will also continue to be available as will specific tests to evaluate von Willebrand's disease including the ristocetin cofactor assay and von Willebrand antigen.
The PFA-100 closure time is reviewed by Bernd Jilma in: J Laboratory Clinical Medicine, 138(3) 2001: 152-163 (PubMed). If you have any questions please contact Dr. Charles Francis (Vascular Medicine - Charles_Francis@urmc.rochester.edu) or Dr. James Corsetti (Pathology and Laboratory Medicine - James_Corsetti@urmc.rochester.edu). Consultation is also available through the Hematology/Oncology Unit.
With the implementation of the National Coverage Determinations (NCD's) a change in the diagnosis requirement for Protimes is in effect. For dates of service beginning November 25, 2002 and after, Medicare no longer requires two diagnoses for Protimes--indication of the disease, such as Atrial Fib and CAD, or the indicatation of coumadin therapy will satisfy the medical necessity. See www.urmc.rochester.edu/path/Medicare/index.htm for links to policies in their entirety.
This year current information on the winter viruses (Influenza and RSV) circulating in Rochester will be available on the Clinical Laboratories web site.
Check out www.urmc.rochester.edu/path/wintervirus.htm for data from the Clinical Virology Lab at SMH, updated weekly from now through April.
Daniel H. Ryan, M.D., Director, Clinical Laboratories,
585-275-5891
Dan_Ryan@urmc.rochester.edu
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