Which statement is NOT true about weak D antigens?

• A. They were formerly known as D^u
• B. They are tested by the antiglobulin technique after incubation
• C. They usually appear negative or weakly reactive at initial testing
• D. They are reliably detected by routine forward typing on initial testing

Answer: (D)

Weak D antigens are D variants that express the D antigen at very low density on red cells. Because of this reduced expression, a standard initial forward typing (the immediate spin test) often fails to show agglutination, so these cells can appear negative or only weakly reactive. That’s why these antigens are not reliably detected by routine forward typing upfront. Historically, some weak D variants were described with the name D^u, reflecting their uncertain or variant expression. The reliable detection method is the antiglobulin technique after incubation (the indirect antiglobulin test), which uses anti-D and anti-human globulin to reveal those low-density D antigens that don’t agglutinate in the initial test. So the statement about routine forward typing being reliable for detecting weak D is not true.