The rosette test is a screening test for FMH that detects fetal D+ red cells in maternal Rh negative blood. If the rosette test is positive, follow-up testing is done to quantitate the FMH, e.g, a Kleihauer-Betke acid elution test or flow cytometry.
Note: The rosette test cannot be done if the fetus is weak D as false negatives may result. In such cases, a Kleihauer-Betke test or flow cytometry can be done.
General description (example only):
- Incubate a maternal 3-5% red cell suspension with IgG anti-D at 37°C. The anti-D will bind to any infant D+ cells that are present.
- After washing to remove unbound anti-D, add indicator red cells. Indicator cells are ficin-treated R2R2 cells that will bind to the antibody-coated infant rbc causing agglutination ("rosettes") that can be detected microscopically.
- A specified number of agglutinates (e.g., 3 or more in 10 fields or 7 or more in 5 fields) is designated a positive and suggests a significant FMH (>30 mL) requiring more RhIg.