Some interfering substances that lead to ABO discrepancies are more pronounced when a gel or solid phase testing method is used. If this is the case, the effect of the interfering substance may be controlled by repeating the ABO group testing using the tube method (ie, "stepping back"). For example, if a patient is demonstrating significant rouleaux formation in the forward type, this could be seen more readily in gel testing. By stepping back to tube testing, washing the patient cells in physiological saline, and repeating the forward typing, the rouleaux formation may be eliminated so that the ABO reactions can be observed.
