Groups urge phase-in of RHD genotyping

Anne Paxton

October 2015—It may not be quite like boxing frogs or herding cats. But gaining broad consensus on a laboratory medicine practice can be difficult, especially where multiple organizations must agree. A new joint statement on RHD genotyping by the CAP and the AABB, plus four other organizations, shows that such consensus is possible, however, even where it involves a laboratory medicine practice in place for more than 50 years—especially when advances in molecular testing are offering a solution to a problem.

But gaining broad consensus on a laboratory medicine practice can be difficult, especially where multiple organizations must agree. A new joint statement on RHD genotyping by the CAP and the AABB, plus four other organizations, shows that such consensus is possible, however, even where it involves a laboratory medicine practice in place for more than 50 years—especially when advances in molecular testing are offering a solution to a problem.

The problem, in this case, is a lack of standard practice in the U.S. for laboratory testing and interpreting of the Rh blood type of patients, including pregnant women, with a weak D phenotype. Blood typing of persons with a weak D phenotype is “really unlike any other test we do in the laboratory,” says Anne Eder, MD, PhD, vice president of national medical affairs for the American Red Cross.

As a 2014 CAP Survey revealed, some laboratories test blood samples from patients, including pregnant women, by methods intended to avoid detecting a weak D phenotype and thereby to interpret the result as Rh-negative. This practice, recommended by AABB standards since 1958, has been intended to protect Rh-negative persons, particularly Rh-negative women of childbearing potential, from inadvertent exposure and alloimmunization to Rh-positive red blood cells.

But RHD genotyping methods are now available that would make the practice obsolete. The CAP, the AABB, America’s Blood Centers, the American Red Cross, the American College of Obstetricians and Gynecologists, and the Armed Services Blood Program agree on the need to bring more precise decision-making to obstetrical practice and transfusion medicine, and a workgroup consisting of representatives from these six organizations was organized by the CAP and the AABB to explore how RHD genotyping might help.

This year the workgroup announced that high-quality evidence from observational studies backed its main recommendation: RHD genotyping should be performed whenever a weak D phenotype is detected by routine Rh blood typing of pregnant women and other women of childbearing potential. Such genotyping should be phased in “as a feasible and appropriate first step to delivering specific benefits of molecular science to a well-defined and relatively limited number of patients,” says a joint statement by the six groups. The data and arguments backing up the statement were published in the March 2015 issue of Transfusion (Sandler SG, et al. 2015;55:680–689).

The interdisciplinary effort leading to the joint statement represented a unique level of cooperation, says workgroup chair S. Gerald Sandler, MD, director of transfusion medicine and professor of medicine and pathology at MedStar Georgetown University Hospital. “I have no recollection of six national organizations endorsing a transfusion practice before.”

Approximately 0.2–1.0 percent of Caucasians inherit a weak D phenotype, and under conventional laboratory practices of long standing, patients have been Rh-typed using methods that interpret weak D phenotypes as Rh-negative. But, as Dr. Eder explains, current practice is inconsistent. In most clinical laboratories, pregnant women are not tested for weak D.

“This is a conservative approach to avoid inadvertent exposure to D positive cells and the potential of alloimmunization and complications in pregnancy,” she says. “For blood donors, on the other hand, we use a test to detect weak D and call them ‘D positive.’ So the method selection is based on the circumstances, not on getting a real answer for the patient.” In fact, the CAP’s Survey showed that some labs conduct the test for weak D serological testing for women when they’re patients, but then, no matter what the result is, report it as Rh-negative, Dr. Eder notes.

“Unlike any other test in the laboratory, we’re saying we’re not even going to test you for weak D if you’re a patient; we’re going to manage you conservatively and just assume you’re Rh-negative when the woman is in the hospital, and we’re going to give you Rh immune globulin [RhIG] when you might not need it. It goes completely against the grain of a clinical mindset,” Dr. Eder says. “The upshot is a woman can be told she’s D positive when she’s a blood donor, and when she’s pregnant be told she’s D negative and needs Rh immune globulin.”

Even though only a small percentage of the population has a weak D phenotype, about 2.5 million women are screened for RhD every year. “So it’s not a small number of women.” And many of them will be given RhIG when they don’t need to receive it because they’re not at risk. “They’re weak D type 1, 2, or 3 so they can be managed safely as Rh-positive.”

“A lot of people say there really is no harm in giving women Rh immune globulin because reactions are few,” Dr. Eder adds. “But the workgroup estimated if the recommendations were followed, then more than 13,000 women could be managed safely as D positive. That would avoid 24,700 doses of RhIG annually.”

Testing and interpretation are all over the board when it comes to RhD typing, agrees workgroup member Louis M. Katz, MD, chief medical officer of America’s Blood Centers. And as a result of erring on the side of safety, supplies of Rh-negative red blood cells can be hard hit. “I think it’s true that, when in doubt, clinicians will do more rather than less, but Rh-negative red blood cells are in very tight supply right now because of things like massive transfusion protocols in the emergency room. We’re at the point where we’re asking for more Rh-negative cells than are available.” Although the number of women affected by the genotyping recommendations appears relatively small, “our Rh-negative donor base is being stretched really thin, so every little bit helps.”

Dr. Sandler points to two important aspects of the Rh-negative supply problem. “One, there’s a seasonal shortage, in that a lot of elective surgery occurs over vacation periods, but fewer donors are donating at that time. The other thing is a lot of people are getting Rh-negative blood who don’t need it. The logistics of getting a blood sample collected, transported, identified as urgent, and confirmed as to type, and then getting an answer back to the trauma center are such that, in a lot of cases, it’s easier to just get a unit of O negative and move on.”

Performance characteristics of different laboratories’ testing methods are also an issue, says workgroup member Susan T. Johnson, MSTM, MT(ASCP)SBB, director of clinical education at BloodCenter of Wisconsin and a board member of the AABB. “Serologic testing is becoming automated, but there are still many labs that are doing manual testing, with its inherent subjectivity. We have many different anti-D reagents available, and each manufacturer meets the FDA requirements. But the requirements are based on serologic observations. The tests cannot differentiate variants identified by genotyping, so it’s not enough in this day and age, with everything we know.”

This interorganizational project on RHD genotyping arose in large part from CAP proficiency testing, explains Jerome L. Gottschall, MD, senior medical director of the BloodCenter of Wisconsin and chair of the CAP Transfusion Medicine Resource Committee. “The Survey asked how much Rh immune globulin would you give based on what was detected. Adequate Rh immune globulin is a separate issue from weak D phenotype policy, but the Survey generated a lot of interest, and as red cell genotyping has become available, it has allowed us to look at ways of determining whether a mother who has a weak D blood type truly needs Rh immune globulin,” Dr. Gottschall says.

Progress in technology for genotyping is certainly part of what made the interorganizational effort possible, he adds. “We’ve been detecting weak D in mothers for a long time, and with the advent of red cell genotyping and particularly of the RHD, we are able to determine variants of weak D that do not elicit an immune response and therefore the mother doesn’t need RhIG and can be treated as Rh-positive.”

“Giving a person RhIG when they don’t need it may seem like a somewhat minor issue, but there’s no person who would want any type of injection if they didn’t have to have it,” Dr. Gottschall says.

In a 2014 paper, the committee compared answers of 2012 Survey participants to answers that 1999 Survey participants gave to the same questions, which related to weak D phenotype testing policies and procedures. The comparison showed a decrease in the percentage of transfusion services performing a serological weak D test on patients as a strategy to manage those with a weak D as Rh-negative—from 58.2 percent to 19.8 percent. That led the committee to conclude that “selective integration” of RHD genotyping policies and practices might be advisable (Sandler SG, et al. Arch Pathol Lab Med. 2014; 138:620–625).

“That was the first step—a recognition by the committee that there wasn’t a standard way of technically typing for Rh at the bench and for interpreting the result for the serological weak D,” Dr. Sandler says. “The CAP’s observation, together with the AABB’s support for the group and joint appointment of the representatives, resulted in the AABB/CAP-sponsored interorganizational workgroup.”