Laboratory assessment of fetomaternal haemorrhage and Rh immune globulin management: Canadian practice and scoping review.

Fetomaternal haemorrhage (FMH) in RhD-negative individuals can lead to alloimmunization with future antibody-mediated destruction of fetal red blood cells. Accurate estimation of FMH is essential for guiding the dose of Rh immune globulin and mitigating alloimmunization. We conducted a national survey and performed a scoping review to determine the availability, technical characteristics and clinical limitations of FMH tests. We describe the evolution of FMH testing, including the qualitative methods (alpha-fetoprotein level, microscopic weak D test, enzyme-linked antiglobulin test, rosette test, gel agglutination cards) and the quantitative methods (Kleihauer-Betke test, flow cytometry assays). Although the rosette test is the most commonly used qualitative method, it may yield false-negative results with fetal RhD variants or false-positives with maternal RhD variants or a positive direct antiglobulin test. Kleihauer-Betke test is the most commonly used quantitative test but has limitations: It is labour-intensive, prone to interobserver variability and can overestimate FMH when maternal F-cell levels are elevated (e.g. haemoglobinopathies). While modifications to the Kleihauer-Betke test can enhance accuracy, flow cytometry remains the most accurate quantification method. With limitations in flow cytometry availability, efficient sample prioritization and pragmatic referral protocols are required. We propose an algorithm for FMH test selection to support decision-making across clinical contexts and resource settings.