1. Lacking an antibody of a specific type in serum; used to mean absence of prior infection with a specific agent (e.g., rubella virus), disappearance of antibodies after treatment of a disease (e.g., syphilis), or absence of antibody usually found in a given syndrome (e.g., rheumatoid arthritis without rheumatoid factor).
2. Showing no significant level of serum antibodies, or other immunologic marker in the serum, that would indicate previous exposure to the infectious agent being tested.