A serological laboratory test is performed on the serum or plasma of a patient receiving blood. It aims to detect the presence of irregular antibodies against red blood cell antigens not present in the patient's own blood. Various techniques (such as tube, gel, or cytology) are used to mix the patient's serum with a range of red blood cells with known antigens. If the result is positive, it indicates the presence of antibodies that necessitate antibody identification testing to determine their identity and provide compatible blood units free of the target antigen.