[Gel agglutination test--a new test system for semiquantitative detection of feto-maternal transfusion in Rhesus incompatibility]

The postpartum administration of an adequate amount of anti-D immunoglobulin to the mother in cases of Rhesus incompatibility requires the exact quantification of the amount of Rh-positive fetal cells that may be present in the Rh-negative maternal circulation. The classical methods to detect an intrapartum fetomaternal hemorrhage are either time-intensive (such as the Kleihauer-Betke test), of low specificity (such as the indirect Coombs test), or technically cumbersome (such as flow cytometry). The goals of our study were to develop a simple screening test that may be used routinely to quantify fetomaternal hemorrhage in cases of Rhesus incompatability and to evaluate this test in clinical practice.

In cases of Rhesus-negative mothers of Rhesus-positive neonates, 2.5 ml of maternal Rhesus negative blood was sampled in an EDTA tubes immediately postpartum and was incubated with anti-D antibodies. Thereafter, a semiquantitative determination was made of the amount of antibody that remained unbound in the serum via a gel agglutination test (GAT) (DiaMed., Switzerland) after mixing with test red blood cells. The amount of anti-D consumed (bound to fetal cells in the first phase) is the semi-quantitatively indicated by the degree of positivity in the second phase the weaker reaction--the more anti-D absorbed in the first phase--the more Rhesus-positive fetal cells present in the maternal sample--the larger the fetomaternal hemorrhage. Following the development of a discrimination zone using this GAT which could ascertain an Rhesus-positive erythrocyte concentration of over 0.2%, the test was applied in a clinical setting. Between September 1995 and April 1998 in unselected postpartum blood samples from 603 Rhesus negative parturients, the GAT was used to test the same blood samples as those requiring evaluation for HbF concentration using the traditional Kleihauer-Betke test.

In 585 of the 603 cases (97%) there was no evidence of a fetomaternal transfusion following testing using both methods. Furthermore, both tests showed significant evidence for a fetomaternal transfusion in five cases. The Kleihauer-Betke test was false-positive in three cases of mothers who had a hereditary elevation of the HbF concentration. The GAT showed three false-positive reaction due to a Dweak maternal varient. In two cases, the disparity between the GAT and the Kleihauer-Betke test could be attributed to an antecedant dose of anti-D antibody. In the two cases, the Kleihauer-Betke test results were 0.3% while the GAT was only 0.2%.

The GAT may be used as a screening method in routine clinical practice. This is a quick test that allows for the specific determination and semiquantitative evaluation of the Rh-positive erythrocyte concentration in clinically relevant concentrations. Thus, following a positive GAT screening test, a further specific test such as the Kleihauer-Betke test may be utilized to absolutely quantify the amount of blood transfused from fetus to mother. It is also possible to perform such a quantification test with the GAT by eventually using a diluted maternal blood sample.