Q&A column

Editor: Frederick L. Kiechle, MD, PhD

Submit your pathology-related question for reply by appropriate medical consultants. CAP TODAY will make every effort to answer all relevant questions. However, those questions that are not of general interest may not receive a reply. For your question to be considered, you must include your name and address; this information will be omitted if your question is published in CAP TODAY.

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Q. What is the appropriate way to measure or identify microcytosis or macrocytosis?

A.December 2022—Mean corpuscular volume (MCV) is a measure of the average or mean volume or size of the total RBC population. MCV is used to indicate microcytosis (average RBC size smaller than the normal range) or macrocytosis (average RBC size larger than the normal range).

In contrast, modern hematology analyzers that are based on the principle of flow cytometry provide the volume and hemoglobin content of individual RBCs. On the Siemens Healthineers Advia 120 analyzer, having greater than 2.5 percent of the RBCs with a volume of less than 60 fL indicates microcytosis. The degree of microcytosis is graded as 1+ for 2.5 to 6.4 percent, 2+ for 6.5 to 10.5 percent, and 3+ for more than 10.5 percent of RBCs with a volume of less than 60 fL. Macrocytosis, on the other hand, is defined as greater than 2.5 percent of the RBCs with a volume of more than 120 fL but graded 1+, 2+, or 3+ in the same manner.1

The routine evaluation of peripheral smears involves grading microcytosis and macrocytosis based on MCV ranges, such as normal, 80 to 99 fL; mild microcytosis, 70 to 79 fL; moderate microcytosis, 60 to 69 fL; or severe microcytosis, less than 60 fL. RBCs are usually not counted per microscopic field for grading purposes. A 2015 article by Constantino describes a similar MCV grading system as well as a 1+/2+/3+ grading system.2

  1. Kakkar N, Makkar M. Red cell cytograms generated by an ADVIA 120 automated hematology analyzer: characteristic patterns in common hematological conditions. Lab Med. 2009;40(9):549–555.
  2. Constantino BT. Reporting and grading of abnormal red blood cell morphology. Int J Lab Hematol. 2015;37(1):1–7.

Archana Agarwal, MD
Medical Director, Hematopathology and Special Genetics
ARUP Laboratories
Clinical Professor
University of Utah School of Medicine
Salt Lake City, Utah
Member, CAP Hematology/Clinical Microscopy Committee

Tim Skelton, MD, PhD
Medical Director, Core Laboratory and Laboratory Informatics
Lahey Hospital and Medical Center
Burlington, Mass.
Member, CAP Hematology/Clinical Microscopy Committee

Q. A six-year-old female with B-cell acute lymphoblastic leukemia and Rh-negative blood is being treated with myeloablative chemotherapy to achieve durable remission or as a bridge to stem cell transplantation, during which supportive transfusions will include repeated platelet transfusions over many weeks. Clinicians are concerned that the patient could become alloimmunized to the D antigen, which, in turn, could affect her ability to eventually bear children.
Apheresis platelets contain a small but finite amount of RBC contaminants, which are not usually quantitated. An optimal strategy to prevent anti-D alloimmunization is to use Rh-negative platelets, but they are often in short supply and cannot be ordered stat in a timely enough manner to ensure every platelet transfusion episode is Rh-negative. We considered using Rh immune globulin (RhIg). However, we recognize that commercial RhIg is designed to prevent D alloimmunization in the setting of obstetric fetal-maternal bleeding.
Is there an optimal dose for RhIg or suggested timing of administration to prevent anti-D alloimmunization in this setting?

A.The risk of rhesus D (RhD) alloimmunization in RhD-negative patients who receive RhD-positive platelets is very low. However, alloimmunization may be significant for females with childbearing potential because it could lead to hemolytic disease of the fetus and newborn.1,2 Because of this low but possibly impactful risk, it is preferable to provide RhD-negative platelets for RhD-negative patients who have childbearing potential. Some transfusion services will consider implementing alloimmunization mitigation strategies if RhD-positive platelets are transfused, while others will not because the risk of alloimmunization is low.3

RhIg has been evaluated and used successfully to prevent D alloimmunization in RhD-negative women who are carrying an RhD-positive or RhD-unknown fetus. Clinical studies of RhIg have defined dosing, timing of administration, and efficacy in this context.4 However, to our knowledge, clinical studies have not evaluated RhIg dosing, timing, and efficacy in the context of preventing RhD alloimmunization after platelet transfusions.

Transfusion services should develop clinical pathways outlining mitigation strategies for RhD sensitization with input from clinicians and taking into consideration a patient’s clinical context. A 300-µg dose of RhIg (preferably in an intravenous formulation for patients with thrombocytopenia) within 72 hours of exposure to RhD-positive platelets is reasonable. This dose will cover 15 mL of RBCs. It is advisable to obtain from your blood collection facility the average number of RBCs present in apheresis and pooled platelet units to calculate the number of platelet units that will be covered. The instructions for RhoGAM (Kedrion Biopharma) and HyperRHO (Grifols Therapeutics) state that in obstetric patients, one 300-µg dose covers a pregnant woman for up to three weeks if less than 15 mL of RhD-positive RBCs are present. This timing could be extrapolated to coverage for platelet transfusions.

  1. Karafin MS, Westlake M, Hauser RG, et al. Risk factors for red blood cell alloimmunization in the Recipient Epidemiology and Donor Evaluation Study (REDS-III) database. Br J Haematol. 2018;181(5):672–681.
  2. Cid J, Lozano M, Ziman A, et al. Low frequency of anti-D alloimmunization following D+ platelet transfusion: the Anti-D Alloimmunization after D-incompatible Platelet Transfusions (ADAPT) study. Br J Haematol. 2015;168(4):598–603.
  3. O’Brien KL, Haspel RL, Uhl L. Anti-D alloimmunization after D-incompatible platelet transfusions: a 14-year single-institution retrospective review. Transfusion. 2014;54(3):650–654.
  4. McBain RD, Crowther CA, Middleton P. Anti-D administration in pregnancy for preventing Rhesus alloimmunisation. Cochrane Database Syst Rev. 2015(9):CD000020.

Monica Beatriz Pagano, MD
Associate Professor of Laboratory Medicine
Medical Director, Transfusion Services
University of Washington
Seattle, Wash.
Vice Chair, CAP Transfusion, Apheresis, and Cellular Therapy Committee

Lynne Uhl, MD
Vice Chair, Division of Laboratory and Transfusion Medicine
Beth Israel Deaconess Medical Center
Boston, Mass.
Member, CAP Transfusion, Apheresis, and Cellular Therapy Committee

Glenn E. Ramsey, MD
Professor of Pathology
Feinberg School of Medicine
Northwestern University
Medical Director, Blood Bank
Northwestern Memorial Hospital
Chicago, Ill.
Chair, CAP Transfusion, Apheresis, and Cellular Therapy Committee

How many blocks?

To readers: A response to the question of how many blocks a histotechnologist working in a semiautomated laboratory should cut per day (October issue, https://bit.ly/QA_102022) raised questions. Vinita Parkash, MBBS, MPH, of Yale School of Medicine and the CAP Surgical Pathology Committee, provided the answer published in the October issue and now the following more detailed response.

Three studies have tried to assess work productivity for pathology labs,1-3 only two of which attempted to assess individual productivity metrics for histotechnologists.1,2 These produced vastly different results. The CAP-NSH study reported a median output of 6,433 blocks per year per histotechnologist (range, 200–16,649), which computes to 26 blocks per day, while the Buesa study reported a median of 23 blocks per hour per histotechnologist (range, 5–70).1,2 Since the CAP-NSH study also reported that histotechnologists spent only 25 percent of their time on cutting, one could theoretically impute that histotechnologists cut a median of 13 blocks per hour.1

However, there are many reasons to be cautious about using these data to set benchmarks for contemporary practice. Both reports are more than a decade old, with data collected between 2005 and 2009. Much has changed in the histopathology laboratory in that time, including increasing automation and specialization of labor. Furthermore, neither study was a true-timed study, where a task is observed and timed simultaneously. These were both survey-based assessments that calculated work productivity per unit-time. The CAP-NSH study also reported having to discard its entire data subset for individual timed activities, due to irreconcilable inconsistencies in the data.1

An extremely wide variation in the laboratory environments in which histotechnologists work limits the identification of a reliable and valid work-productivity benchmark for microtomy. Thus, the CAP-NSH study included laboratories with annual block volume ranging from 260 to 280,000 blocks (5th–95th percentile), and the Buesa study range was 600 to 116,000 blocks.1,2 Although the CAP-NSH study divided laboratories into small, medium, and large, the ranges remain extremely wide (e.g. 55,000–280,000 for large laboratories). Laboratories also have extremely varied levels of automation, job tasks (grossing, accessioning, frozen sections, managerial tasks), and workflows. As a rule, larger laboratories have higher productivity values likely due to efficiencies of scale, greater automation, and task specialization.

  1. Kohl SK, Lewis SE, Tunnicliffe J, et al. The College of American Pathologists and National Society for Histotechnology workload study. Arch Pathol Lab Med. 2011;135(6):728–736.
  2. Buesa RJ. Productivity standards for histology laboratories. Ann Diagn Pathol. 2010;14(2):107–124.
  3. Dwyer K, Siena D, Wanner AMJ, Wildeman CI. National Society for Histotechnology workload study. J Histotechnol. 2020;43(1):38–46.