A laboratory test in which the lack of agglutination (clumping) of red blood cells (RBCs) indicates that antibodies are present in the patient’s blood. Certain viruses (e.g., mumps, measles, rubella, adenovirus) bind with RBCs and cause clumping. However, antibodies, if present, quickly bind with the virus, preventing viral binding to RBCs and the resulting agglutination.