Test Catalog

Test ID: BLOOM    
Chromosome Analysis, Sister Chromatid Exchange (SCE) for Bloom Syndrome, Blood

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

Establishing a diagnosis of Bloom syndrome

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

Diagnostic testing for Bloom syndrome

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

This test includes a charge for cell culture of fresh specimens and professional interpretation of results. Analysis charges will be incurred for total work performed, and generally include 2 banded karyograms and the analysis of 20 metaphase cells. If no metaphase cells are available for analysis, no analysis charges will be incurred. If additional analysis work is required, additional charges may be incurred.

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

Sister chromatid exchange analysis is appropriate in individuals with clinical features suggestive of Bloom syndrome.


Bloom syndrome is a genetic disorder associated with various congenital defects and predisposition to acute leukemia, pulmonary fibrosis, and Hodgkin lymphomas. Carcinoma also is commonly seen in these patients. Approximately one-fourth to one-half of patients develop some type of cancer with a mean age of 25 years at onset. The severity and age of onset of cancer varies among patients.


These patients often have prenatal or postnatal growth retardation, short stature, malar hypoplasia, telangiectatic erythema of the face and other regions, hypo- and hyperpigmentation, immune deficiencies, occasional mild mental retardation, infertility, and high-pitched voices.


Bloom syndrome is an autosomal recessive disorder caused by mutations in the BLM gene located at 15q26.1. While multiple mutations have been detected, the use of molecular testing to diagnose Bloom syndrome is limited in many ethnic groups. Patients with Bloom syndrome demonstrate a high frequency of chromosome abnormalities when their cells are cultured. Thus, cytogenetic studies can be helpful to establish a diagnosis.


Bloom syndrome results in 2 characteristic cytogenetic abnormalities. First, the cells are at increased risk for random breaks leading to fragments or exchanges between nonhomologous chromosomes. Second, cells in these patients have an increased frequency of sister chromatid exchanges (SCE: exchange of material between homologous chromosomes) of approximately 10-fold to 20-fold higher than average.


This test is diagnostic for Bloom syndrome. This test cannot be used to identify heterozygote carriers for Bloom syndrome and is not appropriate as part of a prenatal screening panel.


A normal result does not rule out the possibility of birth defects, such as those caused by chromosomal abnormalities, molecular mutations, and environmental factors (ie, teratogen exposure). The test does not rule out other numeric or structural abnormalities. If a constitutional chromosome abnormality is suspected, a separate conventional cytogenetic study, CHRCB / Chromosome Analysis, for Congenital Disorders, Blood should be requested.

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

A frequency of sister chromatid exchange comparable to a control specimen and historical reference values will be reported as normal.


A 10-fold or more increase in sister chromatid exchange relative to a control specimen and historical reference values will be reported as abnormal. This is consistent with a diagnosis of Bloom syndrome.

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

Interfering factors

-Cell lysis caused by forcing the blood quickly through the needle

-Use of an improper anticoagulant or improperly mixing the blood with the anticoagulant

-Excessive transport time

-Inadequate amount of blood may not permit adequate analysis

-T lymphocytes that do not respond to mitogens used to stimulate T cells to undergo mitosis (rare)

-Improper packaging may result in broken, leaky, and contaminated specimen during transport.

-Exposure of the specimen to temperature extremes (freezing or > 30 degrees C) may kill cells and interfere with attempts to culture cells

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

1. Dicken CH, Dewald G, Gordon H: Sister chromatid exchanges in Bloom's syndrome. Arch Dermatol 1978:114;755-760

2. Sanz MM, German J. Bloom's Syndrome. In GeneReviews. Edited by RA Pagon, MP Adam, HH Ardinger, et al: University of Washington, Seattle; 1993-2014. 2006 Mar 22 (Updated 2013 Mar 28). Accessed 05/22/2013 Available at: http://www.ncbi.nlm.nih.gov/books/NBK1398/

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