Answering Your Questions

April 1, 2011

Du: Is it D positive or negative?


Our lab is in the practice of reporting the Rh type of weak D patients as Rh neg, Du positive. I think this can be misleading. The presence of the D antigen, whether it is weak or not, still means the person is Rh positive for D. I have talked to my co-workers about reporting such a reaction as Rh pos, Du pos. They say that anyone who is Du positive should be transfused with Rh-negative blood, especially female patients in childbearing years. But they believe that by reporting it as Rh neg, Du pos, blood-bank personnel will be prompted to cross match Rh-neg units, and other medical staff would expect Rh-negative units to be given. How should a weak D officially be reported?


The Standards for Blood Banks and Transfusion Services (Standard 5.8.2) says if the initial test with anti-D is negative, the blood shall be tested using a method designed to detect weak D when either test is positive the label shall read “Rh positive.” Thus, donor blood weak D is Rh positive. For the purpose of patient or recipients, however, according to Standard 5.13.2, Rh type shall be determined with anti-D reagent. The test for weak D is unnecessary when testing the patient. For the purpose of Rh immune globulin prophylaxis, the Standards start with cautioning about the Rh mistyping, and according to Standard 5.20.1, “Interpretation criteria shall be established to prevent the mistyping of an Rh-negative patient as Rh positive due to exposure to Rh-positive red cells.” And to classify a pregnant woman Rh negative, in the standard (criteria for Rh immunoglobulin prophylaxis), it is clearly suggested, “A test for weak D is not required.”

Weak D testing, however, is required when testing the fetus (Standard Thus, according to the standard, women of childbearing age can be classified as Rh negative without further testing for the weak D or Du. Standards avoided this issue and simplified it by avoiding testing for weak D. According to the standard, donors will be tested for weak D and classified D positive if tested positive, but recipients are not required to be tested and are classified D negative. Standards gives an easy answer for the above issue by avoiding testing for weak D and cautioning not to misinterpret a recipient as Rh negative; therefore, leaving it up to the transfusion services to interpret the results of the weak D testing.

Historically, weak D red blood cells (RBCs) are defined as having decreased D antigen levels which requires the indirect antiglobulin test, or IAT, for detection. Weak D expression primarily results from single-point mutation in the RHD gene that encodes amino-acid change in the intracellular or cytoplasmic region of Rh D protein with the decreased number of D antigen sites. Up to 54 (1 to 54) different weak D types have been reported. The weak D types 4.2 and 15 have been reported to make anti-D and recommended to be classified as partial D. Since the difference in the D antigen is quantitative, not qualitative, the majority of weak D patients can be transfused with D positive RBCs. Thus, with few exceptions, from a historical perspective, one can safely classify the weak D as D positive.

The following factors are responsible for the difficulty in the D antigen typing:

  • D antigen is defined by multiple conformational epitopes and their variation (about 175 RHD alleles are described genetically);
  • multiple different methods of testing and monoclonal reagents (different clones and blends); and
  • reactivity of monoclonal IgM to “D-like” epitope on Rhce protein.

Once we take in account above factors, it is not surprising why discrepancies in D typing are observed in routine testing. According to a 1999 CAP survey, 58% (may be lower now) of hospital services were performing weak D testing with striking differences in transfusion practices. The various methods used for D testing include slide, tube test, solid phase, gel columns, and automated analyzer with enzyme-treated RBCs. Of those surveyed, 44% were transfusing D negative units, while 42% were transfusing D positive units, and only 10% were transfusing D negative only to women of childbearing age. These survey results illustrate that various methods of testing cause variation in reactivity of weak D, as well as variation in D status assignment by the transfusion services. As seen at serological-level D typing, discrepancies can arise due to many D variants (see Table 1), and not only due to weak D (formerly Du). Therefore, DNA-based testing, as well as thorough serological testing, may be helpful prior to assigning the D antigen status — with the key consideration being whether it is for a blood donor or transfusion recipient, and what the risk of sensitization and formation of anti-D is.

Table 1. D variants and Rh considerations for transfusions.

Most experts agree that a serologically or molecularly confirmed D-variant blood donor should be classified as D positive and a recipient as D negative. This is good for safer transfusion practices but could be confusing to patients and physicians. Also it creates a dilemma how to report and record the results. But the answer will not always be a simple Rh positive or Rh negative and will require an additional statement of explanation to both the patient and the physician.

One way to report could be the patient is a D variant (classify serologically and molecularly, if possible) and will be considered Rh positive as a blood donor and Rh negative for transfusion and Rh immune globulin administration. This may not apply to each case; and, if possible, full serological and molecular testing should be used to assign the D antigen status in an individual at risk of formation of anti-D and risk of hemolytic disease of newborn.

—Krishna Oza, MD

Hematopathology, US LABS

Brentwood, TN

Further Reading

  1. AABB. Standards for Blood Banks and Transfusion Services, 24th ed. 2006. AABB: Bethesda, MD.
  2. Westhoff CM. Rh complexities: serology and DNA genotyping. Transfusion. 2007;47(1 Suppl):17S-22S.
  3. Westhoff CM. The Rh blood group D antigen … dominant, diverse, and difficult. Immunohematol. 2005;21(4):155-163.
  4. Denomme GA, Dake LR, Vilensky D, Ramyar L, et al. Rh discrepancies caused by variable reactivity of partial and weak D types with different serologic techniques. Transfusion. 2008;48(3):473-478.
  5. Noizat-Pirenne F, Verdier M, Lejealle A, Mercadier A, Bonin P, et al. Weak D phenotypes and transfusion safety: where do we stand in daily practice? Transfusion. 2007;47(9):1616-1620.
  6. Christiansen M, Samuelsen B, Christiansen L, Morbjerg T, et al. Correlation between serology and genetics of weak D types in Denmark. Transfusion. 2008;48(1):187-193.

Brad S. Karon, MD, PhD, is associate professor of laboratory medicine and pathology, and director of the Hospital Clinical Laboratories, point-of-care testing, and phlebotomy services at Mayo Clinic in Rochester, MN.