An indirect Anti-Human Globulin Test is positive when:

The indirect Anti-Human Globulin Test (IAT) is a laboratory procedure primarily used to detect the presence of antibodies against red blood cells (RBCs) in the recipient's serum. A positive result indicates that these antibodies are present and have bound to the donor's RBCs when mixed with the recipient's serum. This binding is what leads to the positive result, typically observed as agglutination when tested with anti-human globulin serum.

When antibodies against donor RBCs are present, it signifies that the recipient's immune system has been sensitized to the donor antigens, which can happen if the recipient has previously been exposed to those antigens through blood transfusion or pregnancy. This sensitization is crucial for compatibility assessments prior to transfusions, as it can inform clinicians about the risk of hemolytic reactions.

The other choices do not align with the concept of a positive IAT: compatibility between blood types implies the absence of a significant immune response (which would produce negative results), while the presence of no antibodies or no clumping suggests that the test would yield negative results, indicating no immune reaction. Thus, the presence of antibodies against donor RBCs is what confirms a positive result in the context of this test.