Weak Rh positivity—a condition in which blood tests sometimes show a person as Rh positive and other times as Rh negative—can complicate HDFN prevention, according to a recent case report published in the journal Clinical Practice and Cases in Emergency Medicine.
This matters because Rh status guides whether a pregnant woman should receive Rh immunoglobulin (RhIG), which can prevent the mother’s immune system from mistakenly attacking her baby’s red blood cells. Without treatment, this immune reaction can cause hemolytic disease of the fetus and newborn (HDFN), a serious condition in which a baby’s red blood cells break down faster than they can be replaced.
In the report, doctors described a 28-year-old woman who came to the emergency department after an early miscarriage. Her blood test showed she was O positive, but in past pregnancies, she had been told she was O negative and had received RhIG. Because of the conflicting results, doctors gave her RhIG again, just to be safe.
Later testing showed she had weak Rh positivity, meaning her blood carried very low levels of the Rh factor. This explained why her test results weren’t always the same.
Read more about treatment and care for HDFN
Weak Rh positivity is rare, and most women with this condition are not at risk for complications and don’t actually need RhIG. But for a small number, there is a real risk of developing HDFN, so doctors often give RhIG as a precaution.
The downside of this is that it uses up doses of a medicine that’s already in short supply. A study published in March 2020 found that in the U.S., about 13,360 pregnant women each year receive RhIG unnecessarily.
“Identifying weakly Rh+ individuals holds potential to reallocate scarce RhIG resources to those who require them,” the authors wrote.
