It has been known for a long time that platelets play a vital role in hemorrhagic, thrombotic, and ischemic disorders. While clinical laboratories over the years have performed many different tests of platelet function, there has been very little, if any, standardization of procedures globally.1 The Clinical and Laboratory Standards Institute (CLSI) recently published a document, Platelet Function Testing by Aggregometry; Approved Guideline (H58-A), to address this void.

Standardizing Platelet Function Testing

Dr. Douglas Christie, chairholder of the CLSI subcommittee that developed this document, indicated that this is a long-awaited and vital document for various reasons. It is the first comprehensive document designed to help laboratory professionals in standardizing platelet function testing. This effort is vital internationally, Dr. Christie said, because of the well-known role of platelets in hemorrhagic and thrombotic disorders as well as the ever-growing interest in using antiplatelet therapy as a standard of care in managing cardiovascular disease (which, as an entity, is increasing in incidence).

“Because of the lack of agreement concerning which tests to use and the performance variables of such tests, H58-A aims to provide a sound, valid basis to establish and standardize platelet function testing in any clinical setting where such testing is performed,” Dr. Christie continued.

This document does not address all platelet function tests, nor does it claim to be the one standard of platelet function testing. Dr. Alvin Schmaier, another member of the CLSI subcommittee, said aggregometry is a classic method of assessing platelet function and has been around for decades. “However, until now, there has been no unanimity or consensus in assessing platelet function generally, or by aggregometry specifically. This consensus document, although not a standard in the classic sense, serves to provide standardization and may well become the de facto method for performing this procedure. It is a good place to start, if nothing else.”

Focus on Platelet Aggregometry Methods

Committee member Marlies Ledford-Kraemer explained that the growing interest in platelet function testing and increased availability of instrumentation, coupled with the many methods available, have only contributed to the variables affecting patient results and comparability. She explained why the subcommittee concentrated on developing a guideline for platelet function testing by aggregometry as opposed to other methods.

The classical bleeding time test wherein a standardized cut is made on a patient’s forearm does tell us something about how platelets function, but other variables such as the vessel wall and coagulation factors are also present in this in vivo test method. Isolating platelets is difficult but can be done via methods such as flow cytometry (assesses platelet membrane surface markers) or electron microscopy (measures platelet granule content).

With platelet aggregometry, the laboratory can analyze how particular stimulators (agonists) affect platelets in either a milieu of plasma (platelet-rich plasma) or whole blood. Aggregometry can be performed at low- or high-shear rates (velocity of a fluid expressed as inverse seconds). Light transmission aggregometry (LTA) and whole blood aggregometry are both performed by analyzers using a low-shear environment. The platelet function analyzer is one example of a high-shear system. Instruments providing both these environments are commercially available, making aggregometry readily accessible to a clinical laboratory.

“This guideline is long overdue,” said Ledford-Kraemer. “Platelet function testing by aggregometry has been performed in clinical laboratories worldwide for more than 40 years, yet no globally accepted performance standards have been available.”

Performance of platelet aggregometry has been increasing steadily with the use of antiplatelet agents and as manufacturers produce smaller, easier-to-use, and more-accessible aggregometry instrumentation.

Nevertheless, lack of test standardization can lead to inconsistency in test results. Part of the regulatory process, according to Ledford-Kraemer, is to achieve test result reproducibility, and performance guidelines are a means to those ends. Furthermore, to date, one cannot assess performance of platelet aggregometry methods by participating in an external quality assessment program.

Due to the nature of the sample, laboratories must exchange material within a limited time and geographic area. By standardizing methods, it is hoped that this will lead to improvement in test results and, hence, comparability on these types of “interlaboratory proficiency” samples. In a nutshell, the goal of this guideline is for laboratories to achieve greater uniformity with any of their test results.

Document Organized for Ease of Use

H58-A is written in sections, each addressing a different method of platelet aggregometry testing. All the experts interviewed agree that rather than being repetitious, this format makes the document more user-friendly. A user can simply turn to an area of interest and find all the relevant information from specimen collection to testing to important variables related to the user’s particular method or area of interest.

With any one method, there are many variables, including procedure, reagents, specimen collection and storage, and test principle. For example, just in terms of preanalytical (preexamination) variables, Ledford-Kraemer explained that for any one method, one has to consider the matrix (plasma or whole blood), specimen collection and storage, as well as agonist type and concentration. There are also analytical (examination) and post-analytical (post-examination) considerations: reference intervals, assay validation, quality control, result analysis, and troubleshooting for starters. “By attempting to bring all these components to light in each section, the guideline helps laboratorians identify and avoid pitfalls, prepare patient samples appropriately, and identify the full range of considerations for that particular method without having to read the entire document.”

CLSI Guideline Fills the Void

Dr. Christie points out that platelets, regardless of their absolute numbers, may be suboptimal in function and lead to a clinical bleed. They may be too active and lead to in vivo thrombosis with sequela such as strokes, or their function may be deliberately suppressed by the use of medication as part of a treatment regimen. Regardless of the particular interest in platelet function, in order to ensure effective patient care and safety, it is important to measure platelet activity accurately and in a standardized form that can be compared among doctors, patients, and clinical settings.

Therefore, this guideline should be of interest to clinical laboratorians, clinicians who treat patients, researchers, manufacturers (instruments and reagents), and even regulatory agencies.

It is equally important to point out the approved guideline H58-A is precisely that: a consensus guideline. “It is not to be taken as a standard specifying the only way to perform platelet function testing or even platelet aggregometry,” Dr. Schmaier said. “It is not mandated by any regulatory agency, so it is entirely voluntary.”

Dr. Christie emphasized that the committee members want to make it clear they gave no preference to any one method, and the guideline is not to be used for the widescale diagnosis of platelet disorders or choosing a therapeutic agent. According to Ledford-Kraemer, “The guideline does not address therapeutic guidance for antiplatelet agents, or offer direction on the clinical interpretation of test results. Similarly, the guideline is not intended for use with global hemostasis, platelet counting, flow cytometry, or research systems.”

Currently available from CLSI, Platelet Function Testing by Aggregometry; Approved Guideline (H58-A) is a document developed through a consensus process and is the result of a collaborative effort by representatives from government, industry, and the laboratory community. It fills a void and provides clear minimal parameters of how both long-term and new users may perform platelet aggregometry in a consistent, reproducible, standardized manner.1

This CLSI document is also significant in that it is new and does not supersede a previous document. CLSI committee members hope it will form the basis of further discussion and standardization. According to Dr. Schmaier, this unique and comprehensive consensus document is already generating interest internationally.


Platelet Function Testing by Aggregometry; Approved Guideline
 . CLSI document H58-A.
Wayne, PA
Clinical and Laboratory Standards Institute

The Experts

Douglas J. Christie, PhD, FAHA, is Principal Clinical Scientist of Siemens Healthcare Diagnostics.

Marlies Ledford-Kraemer, MBA, BS, MT(ASCP)SH, is president of CLOT-ED.

Alvin H. Schmaier, MD, is chief, Division of Hematology/Oncology and Robert W. Kellermeyer Professor of Hematology/Oncology of Case Western Reserve University.

Glen McDaniel is a clinical laboratory scientist, speaker, and freelance writer based in Atlanta.

CLSI is a global, nonprofit organization that promotes the development and use of voluntary consensus standards and guidelines within the health care community.