Transfusion Service
Claudia S. Cohn, MD, PhD (she/her/hers)
University of Minnesota
Saint Paul, Minnesota, United States
Antibodies against the Rh(D) antigen can cause severe hemolytic disease of the fetus and newborn. Titer levels are used to monitor pregnant patients with anti-D. Most transfusion services use the traditional ‘tube’ titer method, which is manual, labor-intensive, and known for imprecision. Obstetricians commonly increase fetal monitoring when the anti-D tube titer reaches 8 or 16. Titering with the gel method offers advantages over tube as it can be automated and has greater precision. However, the gel method is more sensitive than tube and studies comparing tube and gel anti-D titers yielded a wide range of results. This makes it difficult to identify gel titer values at which fetal monitoring should be increased. The comparison studies used different standard operating procedures (SOP), and red cells with different Rh phenotypes; these factors likely contributed to the wide variation in reported results. The DATA study is a well-controlled effort in which clinical samples were titered in parallel using tube and gel methods. The methods and reagents were consistent across all study sites. The results of this study should allow for a better understanding of tube to gel equivalence.
Study
Design/Methods: A total of 410 samples from 6 study sites were tested using manual tube versus automated gel technologies. A uniform SOP was used with serial dilutions up to 1:1024 and RBC with R2R2 phenotype. Samples could be tested when fresh or after a single freeze/thaw cycle, but each sample had to be treated in a similar manner so that tube and gel titer results were comparable. Each site received IRB approval or exemption for this study.
Results/Findings:
A total of 410 tube/gel titer sets from 6 sites were available for analysis. Overall, the gel method yielded mean titers that were 2.5 dilutions greater than tube titers [ (TT), see Table 1]. Of the 113 samples that were negative in tube, 110 had detectable titers in gel, ranging from 1– 64. The difference in gel titers was greater at lower tube dilutions. For TT < 64, the mean gel titer was 2-3 dilutions higher. For TT >64, the mean difference in gel titers decreased, with no difference in gel:tube titers seen at dilutions >512. The range of tube to gel ratios was similar across the 6 study sites.
Conclusions: This study provided a well-controlled direct comparison of tube to gel titering techniques. Overall, the gel method was more sensitive than tube, but this difference began to decline at higher titers; this may be due to the final dilution of 1024 used. Several samples considered negative by tube method may have been false negatives based on the gel titer test. These results indicate that the 8 – 16 tube titer thresholds used by obstetricians is approximately equivalent to gel titers of 32 – 128. Transfusion services should use internal validation tests to confirm these findings.