Forward typing is done using known antisera to detect ABO antigens present on the patient’s red cells. In the tube test, known antisera and patient cells are placed in labeled test tubes, centrifuged, and observed for agglutination. Each manufacturer has specific instructions for its own antisera, detailing the percent of cell suspension, number of drops of cell suspension versus number of drops of antisera, and the rate and length of centrifugation. Though the details differ, the theory behind the tests is the same.