Editors: Olga Pozdnyakova, MD, PhD, Geoffrey Wool, MD, PhD, David Bernard, MD, PhD & Raul S. Gonzalez, MD
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Q. What is the percent acceptability when conducting a comparison test of activated clotting times (ACT)?
A. September 2026—For ACT, there is no single universally mandated percent acceptability criterion. Instead, acceptability depends on clinical context, device manufacturer recommendations, and local laboratory validation practices. Manufacturers offer cartridges with different heparin sensitivities and indications for use.
CAP checklist requirement COM.04300 Comparability Criteria—Nonwaived Testing indicates that acceptability criteria are “defined for comparability of nonwaived instruments and methods used to test the same analyte.” The acceptability criteria are determined by the laboratory and can vary based on the specific analyte and clinical impact of the analyte’s measurement variation. “These criteria may be developed from in-house data or published literature and must be vetted by the laboratory director to ensure that they are appropriate for the clinical application of the test,” according to the checklist requirement.
The Clinical and Laboratory Standards Institute document H57-A, “Protocol for the Evaluation, Validation, and Implementation of Coagulometers; Approved Guideline,” emphasizes clinically meaningful agreement for comparison tests. We have found that labs typically use a range of 10 to 20 percent difference.
In this context, one should consider communicating any major biases to providers, especially if the institution is transitioning to a different analyzer manufacturer and/or analyzer cartridge, which can lead to significant variations in ACT results.
Eric Salazar, MD, PhD
Professor and Vice Chair, Clinical Pathology
Department of Pathology and Laboratory Medicine
University of Texas Health San Antonio
Medical Director of Clinical Laboratories
UT Health Multispecialty and Research Hospital
Section Director, Hemostasis and Thrombosis Laboratory
University Hospital, San Antonio
Vice Chair, CAP Hemostasis and Thrombosis Committee
Kristi J. Smock, MD
Professor of Pathology
Associate Division Chief of Clinical Pathology
University of Utah School of Medicine
Medical Director, Hemostasis/Thrombosis
ARUP Laboratories, Salt Lake City
Chair, CAP Hemostasis and Thrombosis Committee
Q. Is there any circumstance in which a manual bone marrow differential would be more accurate than flow cytometric analysis?
A. Indeed, flow cytometry allows fast and accurate classification of marrow nucleated cells based on the expression of certain markers. Because flow cytometry assesses a large number of events, it is reasonable to assume that this methodology would be the best approach for differential cell counts. However, despite the disadvantages of manually counting cells from marrow aspirate smears (significantly fewer cells included, technical complexity, time demands, subjectivity, etc.), morphology remains the gold standard.
Flow cytometry has fundamental limitations when it comes to differential counts. First, flow cytometers lyse red blood cells, including erythroid precursors, before analysis. Because nucleated erythroid precursors must be included in a manual count, the percentages of all countable events on a flow cytometer are skewed relative to a manual count.
Second, several samples are taken during bone marrow aspirate collection. The first pass is most frequently used to prepare the smear and is often more cellular. Flow cytometry specimens are taken next and may be less cellular and more hemodiluted, which can further skew the differential counts.
Third, because flow cytometry panels include a limited number of markers, it may be challenging to detect some important cell populations. For example, distinguishing mature from immature monocytes (blast equivalents), which is important for establishing a true blast count, is unlikely to be accurate on many flow cytometry panels.
Ultimately, manual morphologic and flow cytometric evaluation assess different cell characteristics, making these methods complementary but not interchangeable. The definition of a blast for morphology (high nuclear-to-cytoplasmic ratio, immature chromatin, etc.) is not the same as for flow cytometry (lower scatter characteristics, CD34+, etc.).
Importantly, both the College of American Pathologists and International Council for Standardization in Haematology continue to recommend a manual aspirate differential as the gold standard for cell enumeration. However, it must be emphasized that the methodologies complement each other, and their results should be correlated in each case.
Lee SH, Erber WN, Porwit A, Tomonaga M, Peterson LC. ICSH guidelines for the standardization of bone marrow specimens and reports. Int J Lab Hematol. 2008;30(5):349–364.
Torlakovic EE, Brynes RK, Hyjek E, et al.; International Council for Standardization in Haematology. ICSH guidelines for the standardization of bone marrow immunohistochemistry. Int J Lab Hematol. 2015;37(4):431–449.
Ryan C. Shean, DO
Clinical Pathology Resident
Department of Pathology
University of Utah Health, Salt Lake City
Member, CAP Hematology/Clinical Microscopy Committee
Anton V. Rets, MD, PhD
Associate Professor of Pathology
Department of Pathology
University of Utah Health
Medical Director of Hematopathology
ARUP Laboratories, Salt Lake City
Member, CAP Hematology/Clinical Microscopy Committee