A word must also be said regarding a few patients whose blood presents unexplained difficulty in crossmatching. The laboratory should be allowed to solve the problem and possibly to obtain aid from a reference laboratory. During this time 5% serum albumin or saline may temporarily assist the patient. In the absence of complete crossmatching, blood is given as a calculated risk.

If blood is needed for emergency transfusion without crossmatch, a frequent decision is to use group O Rh-negative blood. The rationale is that no recipient (whether group O or any other ABO group) could have ABO group antibodies against group O cells, and any unexpected antibodies in donor serum against recipient RBCs would be diluted by the blood volume of the recipient. Even so, there is risk involved, since the recipient may possess antibody to some RBC non-ABO blood group antigen of the donor (e.g., anti-Rh or anti-Kell). A crossmatch would detect this. Moreover, the anti-A or anti-B antibodies in group O blood may be in high titer, and transfusion reactions may occur when this blood is used inrecipients who are group A, B, or AB. Many blood banks maintain a certain amount of low-titer O-negative blood for use in emergencies. Titers over 1:50 are considered too high for this purpose. In addition, A and B group–specific substance (Witebsky substance) may be added to the donor blood to partially neutralize anti-A and anti-B antibodies. These substances are A and B antigens manufactured from animal sources and, being foreign antigens, may sensitize the patient.

Rather than use group O Rh-negative blood in a blind fashion for emergencies, a better method is to use blood of the same ABO and Rh type as the patient’s blood. ABO and Rh typing can be done in 5 minutes using anticoagulated specimens of the patient’s blood. This avoids interpretation problems produced by putting group O cells into a group A or B patient and subsequently attempting crossmatches for more blood.

When repeated transfusions are needed, a new specimen should be drawn from the patient (recipient) for crossmatching purposes if blood was last given more than 48 hours earlier. Some patients demonstrate marked an amnestic responses to RBC antigens that they lack and may produce clinically significant quantities of antibody in a few hours. This antibody is not present in the original specimen from the patient.