Which statement describes weak D antigens?

• A. It is a variant of the D antigen
• B. It is not expressed
• C. It is the same as D negative
• D. It is not detected by any serologic method

Answer: (A)

Weak D describes a D antigen that is present but expressed at a markedly reduced level on the red cell surface. This reduced expression results from genetic variants in the RHD gene that alter how much D antigen is displayed, so the cells are not completely devoid of D antigen. Because of the low density, a simple routine test may be negative, but more sensitive serologic methods, such as the indirect antiglobulin test, can reveal the D antigen. That’s why this is best described as a variant of the D antigen. It is not simply “not expressed” or “the same as D negative,” since there is some D antigen present, and it can often be detected with appropriate testing rather than being undetectable by all serologic methods.