Question

1. A patient's DAT is positive. The eluate shows no activity against antibody screening identification red cells. What could explain this result?

2. A newborn baby has a positive DAT. the mothers serum/plasma does not contain any unexpected antibodies. What might be the cause of the baby's positive DAT?

Answer 1

1. DAT is direct antiglobulin test, which estimates antibodies binding to red blood cells. These antibodies, will attach red blood cells and cause hemolysis. Antibodies against red blood cells are seen in autoimmune disorders, drug induced anemia, ABO and Rh incompatibility in mother and baby or during transfusion. Positive DAT indicates that there are antibodies in the patient’s blood. However, there are no specific antibodies against red blood cells, as indicated by the red blood cell antibody screening identification test. False positive DAT test could be due to increased amount of immunoglobin G in the patient. IgG can recruit complement and cause agglutination. Similarly, elevated levels of complement can also give false positive results. False positive reactions can also be due to technical errors such as degradation/clotting of specimens, leading to non-specific binding to assay reagents. Tight packing of red blood cells also causes false positive DAT test. The patient may also demonstrate spontaneous agglutination. Anticardiolipin, antiphospholipid antibodies can cross react with the RBC cell membrane and give false positive DAT test.

2. Hemolytic disease of newborn is caused by the placental transfer of autoantibodies such as antibodies against Rh factor from other to baby. However, the mother has no autoantibodies. Hence, the positive DAT is no due to transplacental transfer of antibodies from mother to child.

Blood samples for DAT test in newborn is from the umbilical cord, while still attached to the placenta during delivery. Wharton’s jelly from cord blood, a gelatinous substance within the umbilical cord causes false positive test. If blood samples are contaminated with Wharton’s jelly, then the DAT will be positive. Paraprotein are polyclonal immunoglobulins that can be present in newborn. Paraproteins are formed by increased clonal maturation of B cells. Spontaneous agglutination and poor washing techniques can also cause false positive DAT. If the mother is Rh negative, then antibodies to anti-Rh (RhoD immunoglobulins) are injected during pregnancy. These antibodies can also give false positive DAT test.