Genetic Testing for the Diagnosis of Hemoglobinopathies Has Very High Clinical Utility

Background: The hemoglobinopathies are a genetically complex group of blood disorders that includes sickle cell disease (SCD), thalassemia, and other structural and/or functional hemoglobin (Hb) variants. For decades, the mainstay of diagnostic testing for hemoglobinopathies has been Hb separation techniques (e.g., electrophoresis, chromatography), sometimes paired with functional assays of Hb (e.g., solubility, instability, oxygen affinity). Such protein-based testing cannot detect or adequately differentiate a number of clinically significant hemoglobinopathies. Nevertheless, most clinicians continue to rely primarily on protein-based diagnostic methods despite the availability of genetic testing. The reasons for this are not fully understood, but include concerns about the cost of genetic testing and the perception that protein-based methods are usually sufficient for clinical care.

Objective: To determine the clinical utility of genetic testing for the diagnosis of hemoglobinopathies by quantifying how often a suspected clinical diagnosis is changed, clarified or excluded by genetic testing.

Methods: We reviewed the results of 500 sequential orders for clinical genetic testing for known or suspected hemoglobinopathies that were performed and interpreted at Cincinnati Children's Hospital Medical Center between 1/2013 and 5/2015. This comprehensive genetic testing service is a collaboration between the divisions of Hematology and Human Genetics. For this analysis we reviewed orders for copy number variation analysis (CNV) of the α-globin gene cluster, sequencing of the α-globin genes (HBA1, HBA2), CNV of the β-globin gene cluster, and/or sequencing of the β-globin genes (HBB). An order for testing could include any combination of these 4 tests. We compared the stated indications and suspected diagnoses for testing provided by the ordering physician with the final results of the genetic testing. For each order, the results of this comparison (suspected diagnosis vs. genetic diagnosis) were classified into 1 of 4 mutually exclusive groups: (1) suspected diagnosis confirmed, (2) suspected diagnosis excluded, (3) suspected diagnosis clarified, or (4) new or unexpected diagnosis. The category "suspected diagnosis clarified" includes cases where the overall suspected diagnosis was confirmed, but additional clinically meaningful genetic data was also uncovered (e.g., homozygous sickle cell anemia was confirmed, but co-inherited α-globin gene deletions were also identified).

Results: 500 serial genetic testing panels were performed on 475 unique individuals (7 specimens were from an embryo or fetus). These 500 panels included 1-4 individual tests: HBB sequencing in 423, α-globin CNV in 475, β-globin CNV in 345, and HBA1/2 sequencing in 164. The final diagnosis was a specific genotype of SCD (± α-thalassemia and/or gene-deletion HPFH) in 234; thalassemia (α, β, or δβ) in 139; Hb S trait (± α-thalassemia) in 18; Hb C trait or disease (± α-thalassemia) in 13; Hb E trait or disease (± α-thalassemia) in 9; another named or novel Hb variant in 22; and normal Hb in 65. Overall, the suspected diagnosis was confirmed in 246 (49.2%), clarified in 156 (31.2%), excluded in 77 (15.4%), and in 21 (4.2%) a new or unsuspected diagnosis was made. For patients with a suspected diagnosis of SCD, the diagnosis was mostly confirmed (66.7%) and never excluded by genetic testing, but it was clarified in 31.6%, and an unsuspected genotype of SCD was identified in 1.8%. For patients with a suspected diagnosis other than SCD, the diagnosis was confirmed in 34%, clarified in 30.6%, excluded in 29.1%, and an unsuspected genotype was identified in 6.4%. Not considering the clinical utility of the "suspected diagnosis clarified" category, a suspected diagnosis was excluded or determined to be a new or unsuspected diagnosis in 98/500 (19.6%) of all cases. Additionally, follow-up genetic testing was recommended for 31/500 (6.2%).

Conclusion: In half (50.8%) of all cases, genetic testing provided new or additional diagnostic information that was not apparent on protein-based diagnostic methods. Genetic testing excluded or substantially changed a suspected diagnosis in one-quarter (19.6%) of all cases. Therefore, genetic testing for hemoglobinopathies has very high clinical utility, and we propose that it should be considered a standard of care for all patients with known or suspected hemoglobinopathies.