Because we perform ABO/Rh testing for pregnant patients, we want to ensure that any mother who can become sensitized to the Rh(D) antigen by a Rh(D)-positive fetus is given the opportunity to receive RhIG. Some mothers who are serologically weakly positive for D phenotype (weak D) may still form antibodies to the D antigen. Our test system is designed to identify these mothers with Rh(D) typing discrepancies as D negative, to ensure that they have the opportunity to receive RhIG to prevent future HDFN.1,2
There are 2 categories of altered Rh(D) expression that can cause typing discrepancies: weak D antigen and partial D antigen. Weak D antigen is observed in individuals who have RHD gene alleles that cause diminished expression (rather than full or no expression) of the D antigen on their red blood cells. Testing of these individuals may variably result in a D+ (Rh positive) or D- (Rh negative) report, depending upon the sensitivity and specificity of commercial Rh(D) typing reagents used in different serological testing methods. Thus, these D variants are often called “weak D+” based on their serological reactivity to anti-D reagents.3
In contrast, up to 4% of patients who have inherited an altered RHD gene possess a partial D antigen or D variant antigen, in which only a portion of the normal D antigen is expressed. Unlike most patients with the weak D phenotype, patients with these partial D phenotypes or D variant phenotypes may form anti-D antibodies if exposed to fetal red blood cells expressing the D antigen. Because routine serologic testing does not differentiate these altered RHD gene subtypes, some women who type as “weak D+” are at risk for developing antibodies that could cause HDFN. Therefore, RhIG may be appropriate for patients with suspected “weak D+” during pregnancy.3-5
For patients with suspected D variants who wish to avoid the use of RhIg, the Weak RHD Analysis Workup is available from Versiti (Blood Center of Wisconsin) using the Versiti order code 3040. Results from this Workup can be used to classify a patient’s Rh type and determine if RhIg is needed.3-5