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Test Catalog

Test ID: WBSEQ    
Beta Globin Gene Sequencing, Blood

Useful For Suggests clinical disorders or settings where the test may be helpful

Diagnosis of beta thalassemia intermedia or major

 

Identification of a specific beta thalassemia mutation (ie, unusually severe beta thalassemia trait)

 

Evaluation of an abnormal hemoglobin electrophoresis identifying a rare beta globin variant

 

Evaluation of chronic hemolytic anemia of unknown etiology

 

Evaluation of hereditary erythrocytosis with left-shifted p50 oxygen dissociation results

 

Preconception screening when there is a concern for a beta hemoglobin disorder based on family history

Genetics Test Information Provides information that may help with selection of the correct genetic test or proper submission of the test request

A hemoglobin electrophoresis evaluation (HBELC / Hemoglobin Electrophoresis Cascade, Blood) is always indicated prior to beta globin gene sequencing because these conditions can be complex and protein data allows accurate and rapid classification of the patient phenotype.

 

Beta globin gene (HBB) sequencing can be used to identify hemoglobin variants and the most common beta thalassemia mutations, including beta plus and beta zero thalassemias. It also identifies hyperunstable hemoglobin variants and dominant beta thalassemia mutations, as well as other hemoglobin variants that cannot be identified by protein methods. Some hemoglobin disorders will not be detected by beta globin gene sequencing, such as large deletional mutations and crossover events. As such, the results of this test should always be interpreted within the context of the protein studies and RBC indices.

Testing Algorithm Delineates situations when tests are added to the initial order. This includes reflex and additional tests.

This is a second-tier evaluation of beta thalassemia minor, intermedia, and major, as well as beta globin variant identification.

 

First-tier testing for beta thalassemia or beta globin variant detection is THEVP / Thalassemia and Hemoglobinopathy Evaluation or HBELC / Hemoglobin Electrophoresis Cascade, Blood.

Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test

Beta globin gene sequencing is useful in the evaluation of beta globin chain variants and beta thalassemia. It detects almost all beta globin variants and the most common beta thalassemia mutations, although prevalence is ethnicity-dependent. Because these conditions are often complex, this test should always be interpreted in the context of protein studies, such as hemoglobin electrophoresis and RBC indices.

 

The majority of beta globin chain variants are clinically and hematologically benign; however, some have important clinical consequences, such as erythrocytosis, cyanosis/hypoxia, chronic hemolysis, or unexplained microcytosis. Most of the common clinically significant hemoglobin (Hb) variants (ie, Hb S, Hb C, Hb E, and others) are easily distinguished by hemoglobin electrophoresis and do not require molecular analysis. In addition, they are frequently found in complex hemoglobin disorders due to multiple mutations, and accurate classification requires sequencing data within the context of protein data. In some instances, beta globin sequencing is necessary to identify or confirm the identity of rare variants, especially those associated with erythrocytosis and chronic hemolytic anemia. Rare hyperunstable variants (also termed dominant beta thalassemia mutations) result in hemolytic anemia and do not create protein stable enough to be detectable by protein methods, including stability studies. They are not always associated with elevated Hb A2 or microcytosis and, therefore, can be electrophoretically silent. These require a high degree of clinical suspicion as all electrophoretic testing as well as stability studies cannot exclude this condition.

 

Beta thalassemia is an autosomal recessive condition characterized by decreased or absent synthesis of beta globin chains due to mutations in the beta globin gene (HBB). No abnormal protein is present and diagnosis by electrophoresis relies on hemoglobin fraction percentage alterations (ie, Hb A2 or Hb F elevations).

 

Beta thalassemia can be split into 3 broad classes (categorized by clinical features)

1. Beta thalassemia trait (also called beta thalassemia minor and beta thalassemia carrier) (B[A]B[+] or B[A]B[0]).

2. Beta thalassemia intermedia (B[+]B[+] or B[+]B[0])

3. Beta thalassemia major (B[+]B[0] or B[0]B[0])

 

Beta thalassemia trait is typically a harmless condition with varying degrees of microcytosis and hypochromia and sometimes mild anemia. Transfusions are not required. Beta thalassemia intermedia is a clinical distinction and is characterized by a more severe degree of anemia than beta thalassemia trait with few or intermittent transfusions required. Later in life, these individuals are at risk for iron overload even in the absence of chronic transfusion due to increased intestinal absorption of iron. Beta thalassemia major typically comes to medical attention early in life due to severe anemia, hepatosplenomegaly, and failure to thrive. Skeletal changes are also common due to expansion of the bone marrow. Without appropriate treatment these patients have a shortened lifespan.

 

The majority of beta thalassemia mutations (>90%) are point mutations, small deletions, or insertions, which are detected by beta globin gene sequencing. The remaining beta thalassemia mutations are either due to large genomic deletions of HBB or, very rarely, trans-acting beta thalassemia mutations located outside of the beta globin gene cluster. Some rare beta chain variants can be clinically or electrophoretically indistinguishable from beta thalassemia and cannot be confirmed without molecular analysis.

Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.

An interpretive report will be provided.

Interpretation Provides information to assist in interpretation of the test results

The alteration will be provided with the classification, if known. Further interpretation requires correlation with protein studies and RBC indices.

Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances

This assay will not detect large deletions or duplications within the beta globin gene. In addition, hybrid beta globin variants (ie, Hb Lepore) will not be detected by this method. This method cannot distinguish between homozygous mutations and compound heterozygous mutations associated with large deletions. This method cannot distinguish between double substitution on single chromosome and a compound heterozygous state. Beta globin sequencing alone is not able to distinguish between mutations that are found in the same copy of the HBB gene (ie, mutations that are "linked" or "in cis") and mutations found on different HBB gene copies (ie, are "in trans"). This limitation of sequencing may complicate diagnosis and has implications for inheritance and proper genetic counseling. To resolve these cases, molecular results must be correlated with electrophoretic and protein data, other laboratory findings, clinical findings, and family studies. Misinterpretation of results may occur if the information provided is inaccurate or incomplete.

 

Rare polymorphisms exist that could lead to false-negative or false-positive results. If results obtained do not match the clinical findings, additional testing should be considered.

Clinical Reference Recommendations for in-depth reading of a clinical nature

1. Hoyer JD, Hoffman DR: The thalassemia and hemoglobinopathy syndromes. In Clinical Laboratory Medicine. Second edition. Edited by KD McClatchey. Philadelphia, Lippincott Williams and Wilkins, 2002, pp 866-895

2. Thein SL: The Molecular Basis of Beta-Thalassemia. Cold Spring Harb Persepct Med 2013;1;3(5):a011700

3. Hoyer JD, Kroft, SH: Color Atlas of Hemoglobin Disorders: A Compendium Based on Proficiency Testing. Northfield, IL. CAP, 2003

4. Merchant S, Oliveira JL, Hoyer JD, Viswanatha DS: Molecular diagnosis in hematopathology. In Hematopathology: A Volume in Foundations in Diagnostic Pathology Series. Second edition Edited by J Goldblum.  Volume Editor E Hsi. Churchill Livingstone, 2012

Special Instructions Library of PDFs including pertinent information and forms related to the test