This qualitative test detects Rh-positive fetal cells in the Rh-negative maternal circulation. The detection of fetal erythrocytes is important when it is suspected that a severe fetal red cell loss has occurred and when serious risk for the mother becoming immunized against the fetal red cell groups is anticipated. In these instances, the mother’s blood sample should be collected immediately after delivery to be examined for fetal cells. This test can be performed only if the mother is Rh negative and the newborn is known to be Rh positive. The rosette test is 97% accurate for detecting transplacental hemorrhage of fetal blood called fetomaternal hemorrhage (FMH) that enters the maternal separation and exceeds 30 mL of whole blood. This test cannot be performed on patients who have had abortions, miscarriages, injuries, or amniocentesis.

• Negative for FMH

• No Rh-positive fetal red blood cells detected in maternal blood

1. Obtain a 7-mL venous blood EDTA sample from the mother shortly after delivery.

2. Label the specimen with the patient’s name, date, and test(s) ordered and place in a biohazard bag for transport to the laboratory.

3. Perform this test and examine results for rosettes or mixed field agglutinates. Following manufacturer’s guidelines, the presence of rosettes above a predetermined number indicates an FMH that exceeds 30 mL of whole blood.

When the test sample contains few or new Rh₁-positive fetal cells, rosetting or agglutination is absent, and the FMH is less than 30 mL, one dose of parental RhIG will prevent immunization. If the fetal blood loss into the maternal circulation exceeds 30 mL, a quantitative or semiquantitative test (i.e., Kleihauer–Betke) or quantitative flow cytometry (if available) must be performed to calculate the amount of RhIG to administer.

Pretest Patient Care

1. Explain test purpose and procedure.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.