Pathology Associates Of Lexington, P.A.
Pathology Associates Of Lexington, P.A.
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        Rhesus blood group system

In addition to the ABO red blood cell group system, we also have the important rhesus (Rh) blood group system discovered in 1939. It suppliments the ABO system so that a person's blood type is determined as either Rh positive (+) or negative (-). The most common blood type is A+. Classically, the Rh+ status has included "weak D", also known as Du. In the USA, a movement began about 2005 to quit identifying Du. Our lab finally ceased the weak Du designation in about June 2010; but, note some of the following thinking.

In 2008, our lab was challenged to cease reporting weak D variant as some type of “Rh positive”.  But, we determined that we would keep on reporting it because (1) there was lack of unity of opinion as to this issue among national societies; and, (2) we were concerned about being party to over-use of Rhogam without such a consensus.  Also, (3) we were concerned that intelligent patients would have their confidence in the laboratory shaken when they were retyped and were left to think that their blood type had changed from their old “Rh positive” designation to a new designation of “Rh negative”.  Since the invention in 1968, D negative pregnant women get Rhogam during their pregnancy (Rhogam is a dose of antibodies given to cleanse a Rh negative mother’s blood stream of any of the baby’s D positive red blood cells (RBCs) leaking through the placenta into mother's blood before those leaked cells can trigger mom’s system into an all-out attack against the baby's possibly Rh positive RBCs through the placenta blood barrier staright into the baby's blood stream before the baby is born. If that happens, the baby's life is threatened by "hemolytic disease of the newborn" (HDN). Trouble is, it is difficult to know what the baby's blood type is before birth. So, if mother's RBCs are interpreted as Rh negative, the Rhogam is likely given in a Pre-emptive strike to thwart HDN from ever getting a toe-hold.  

Patients perspective, confusion:

“The doctor told me that my blood type changed and that I would have to take Rhogam! I’m not stupid; I know that blood types don’t change! The lab must have made an error; don’t those people know what the hell they’re doing!?”

This was the response of one patient who brought to our attention that the whole issue of a person’s Rh type continues to be problematic, and now the problems are surfacing in the patient community as patients personally have blood typing by different labs.

Although blood typing can be done on a genetic basis, that is a very uncommon method of typing world-wide.  Your “blood type” tells 2 facts: (1) your ABO status and (2) your D (Rh) positive or negative status.  Most laboratories still perform “serological” blood typing.  This means that lab-purchased reagents containing antibodies are mixed under very precise conditions with a patient’s red blood cells (RBCs are covered with surface antigens).  Reagent antibodies attach immediately or after incubation to their “twin” antigens and, thereby, cause a “positive” test reaction. 

To have an idea of how this works, imagine antibodies suspended in liquid being like puzzle pieces.  Then imagine the red blood cell surface as containing thousands of dimples (antigen sites) the exact shape of a certain antibody “puzzle piece”.  Blood type A has a dimple shape different from blood type B, and O is the absence of dimples of A or B.  Imagine D (the Rh factor) as having dimples which can vary in depth and shape a little bit so that D puzzle pieces may fit well or may fit very loosely.  Rarely, it can be a “partial” dimple.  Also, imagine that the RBC surface can be highly populated with D dimples or scantily populated with D dimples.  If the puzzle pieces fit loosely or very loosely, the antibody may lack “avidity”, and the lab test appears “negative” for the D antigen…this is “weak D”. 

The Rh factor, D, is especially prone to have variation in it’s antigen sites such that the fast-testing, “immediate-spin” blood typing test seems negative.  For over fifty years, laboratories have additionally performed a second test with incubation (indirect anti-globulin test…indirect Coombs test) which allowed detection of D antigen/dimples when they were either less numerous or less avid.  When the rapid immediate spin was negative but the indirect Coombs test was positive for the D antigen, then the patient was considered “weak” D positive.  Therefore, if you were one with type A blood and “weak D” positive, you would be known as A+ (along with all other A types who were either strong or weak D positive).  Technically, in that laboratory or blood bank records, you would be known as type “A, Du positive” or “A, weak D positive”.

Current, modern anti-D reagents have been found to now react directly in the “immediate spin” test with most old-time-test weak D red cells that carry a lower number of D antigen sites so that they are “D positive”.  People who are modern-day classified as weak D positive only very rarely make the Anti-D antibody.

Finally, there are small numbers of people whose RBCs have only part of the D antigen; and they are classified as partial D and can make anti-D antibodies against their baby’s blood.  These folks are problematic! 

For Physicians:

Rh positive and Rh negative refer to the presence or absence of the D antigen on RBCs, respectively. 

Some people have reduced expression of the D antigen and in the past have been called “Du positive”, now referred to as “weak D positive” and interpreted as an Rh positive with respect to concerns about HDN & possible need to get Rhogam.  There are over 50 weak D types now known (as of late 2009).  The most common weak D types (1, 2, 3) are unlikely to make anti-D; but very rarely this does occur.  Hence, there is a potential roll for Rhogam, even in these folks.

Other people may have only part of the D antigen present and can make anti-D.  They are called “partial D”.  Hence, there is a potential roll for Rhogam, even in these folks.

Most D typing is performed by serological methods which use reagents that have improved greatly over the years and now can pick up weak D types by direct (“immediate”) methods.  In the past, a longer (incubated), indirect method was used to detect weak D.  Because of this, some people may see a change in how their D type is interpreted.  This does not mean that their blood type has changed; it just means that more has been learned about this very complicated antigen system and that the terminology used has morphed/evolved. 

Medical technologist's perspective:

  • Indiana Blood Bank info sheet HERE.
  • Concerning the 2000 College of American Pathologists (CAP) survey of lab Blood Banks HERE.
  • About Du variants, 2005 HERE.


  1. See Wikipedia HERE about blood typing.
  2. Kumar H, et. al., "Difficulties in Immunohaematology : The Weak D Antigen", MJAFI, Vol. 61, No. 4, 2005.
  3. Dr. Carter's memo 1/16/2008 HERE.
  4. See Wikipedia HERE about Rhogam.

(posted 22 October 2009; 23 October 2011)

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