Queen's University School of Medicine Mississauga, Ontario, Canada
Background/Case Studies: Fetal maternal hemorrhage (FMH) testing is an important tool to estimate the volume of a bleed and provide appropriate dosing of Rh immunoglobulin to prevent alloimmunization during pregnancy. Heterogeneity exists in the laboratory tests used to estimate the volume of FMH.
Study
Design/Methods: A practice survey was administered to all Canadian hospitals and transfusion services that perform perinatal testing (2019). Additionally, a scoping review (1946-2022) of laboratory methods for FMH testing was performed seeking evidence to support optimal testing practice.
Results/Findings: Of the participating institutions, 580/597 (97%) completed the survey and 146 (146/580; 25%) perform FMH testing. Of the 4467 citations, 283 articles met the scoping review inclusion criteria and were summarized. Approximately half of the sites (70/146; 48%) use the rosette test as the primary FMH screening test, while others (66/146; 45%) utilize the Kleihauer-Betke test (KBT). The literature highlights benefits of the rosette test including avoidance of resource intensive KBT but also limitations including false positive results with maternal RhD variants or positive direct antiglobulin test. The survey noted that when FMH quantification is indicated, the KBT is most widely used. Limitations of the KBT include its labour-intensive methodology, subjectivity in counting, inter observer variability and propensity to underestimate FMH due to the decreasing fetal red blood cells (RBC) with advancing sample age or secondary to antibody mediated clearance (e.g. ABO incompatibility). The KBT may also overestimate FMH volume when maternal F-cells are elevated (e.g. hemoglobinopathies). The survey found variability in source of reagents, number of fields scanned/cells counted, and whether positive tests are verified by a second technologist. Flow cytometric (FC) quantification of FMH can circumvent the subjectivity of the KB method as well as other limitations. Few (14/112; 13%) institutions reported using FC to quantify FMH. Of those that do, the majority use anti-F antibody methodology (10/14; 71.4%). Some studies suggest that anti-F FC underestimates FMH due to the variability of hemoglobin F concentration within fetal RBCs; however, it remains more accurate than KBT in estimating FMH volume. Limitations of implementing FC to quantify FMH include high cost and limited availability of validated equipment and trained technologists. Conclusions: There are various tests and methodologies employed to quantify FMH. Each test has inherent advantages and limitations that restrict utility in certain clinical contexts. Institutions should consider maternal/fetal antigenicity, presence of abnormal hemoglobin, technologist expertise, and optimal process efficiency to select the most appropriate FMH test.
Importance of research: Proficiency surveys show that transfusion services often over/underestimate the volume of FMH. The consequence is administration of an incorrect anti-D immunoglobulin dose. Labs differ in their FMH testing methods, contributing to variability in dosing. By combining the results of a practice survey and a systematic review, we highlight areas for practice improvement and testing standardization. Additionally, we present limitations of FMH tests to ensure the correct test and method are employed.
