The indirect antiglobulin test is designed to detect antibodies present in which sample?

• A. Antibodies present in patient serum
• B. Antibodies present in plasma only
• C. Antibodies bound to red cells without serum
• D. Antihuman globulin only

Answer: (A)

The key idea is that the indirect antiglobulin test looks for antibodies circulating in the patient’s serum that can react with red cell antigens. In the test, patient serum is mixed with red cells carrying known antigens and incubated so any present antibodies can bind to those cells. After washing away unconjugated components, anti-human globulin is added; if antibodies have attached to the cells, the antiglobulin reagent bridges the bound antibodies and causes agglutination. This confirms that antibodies are present in the serum. This differs from direct antiglobulin testing, which detects antibodies already bound to red cells in the patient, not free in the serum.