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Subject: Rosette Test
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The rosette test detects D-positive red cells in the blood of a D-negative mother, whose fetus or recently delivered baby is D positive.
When anti-D reagent is added to the mother's blood, fetal D-positive red cells become coated with anti-D on incubation and exhibit mixed-field agglutination when antiglobulin reagent is added. Because the mixed-field agglutination may be difficult to detect, D-positive RBCs are added to the mixture to demonstrate rosettes of several cells clustered against antibody-coated D-positive cells.
Normal value: Absence of rosettes is considered negative for major fetomaternal hemorrhage in an Rh-negative mother with an Rh-positive fetus.
The test is used to determine the presence of fetal–maternal hemorrhage by testing for the presence of D-positive fetal red cells in the circulation of a D-negative mother. If >30 mL of fetal (whole) blood or >15 mL of fetal red cells are present in the maternal circulation, the assay has a sensitivity of ≥99%.
The presence of a positive result indicates that the fetal blood is admixed with that of the mother. This happens when the fetus has hemorrhaged into the mother's circulation and may require intrauterine transfusion or obstetric intervention.
The rosette test is a qualitative screening test. If it is positive, the amount of fetal blood in the maternal circulation must be quantified using flow cytometry or the Kleihauer-Betke (acid-elution) assay.