Chromosome Analysis, Amniotic Fluid
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Chromosomal abnormalities are the cause of a wide range of disorders associated with birth defects and congenital diseases. Many of these disorders can be diagnosed prenatally by analysis of amniocytes. Amniotic fluid can be safely collected after 12 weeks of gestation, although for optimal cell growth, collection from 14 to 18 weeks of gestation is preferred. This method permits diagnosis of chromosome abnormalities during the second trimester of pregnancy or later.
The most common reasons for cytogenetic studies for prenatal diagnosis include advanced maternal age, abnormal maternal serum screen, a previous child with a chromosome abnormality, abnormal fetal ultrasound, or a family history of a chromosome abnormality.
Prenatal diagnosis of chromosome abnormalities (trisomies, deletions, translocations, etc)
Cytogenetic studies on amniotic fluid are considered nearly 100% accurate for the detection of large fetal chromosome abnormalities. However, subtle or cryptic abnormalities involving microdeletions usually can be detected only with the use of targeted FISH testing.
Approximately 3% of amniotic fluid specimens analyzed are found to have chromosome abnormalities. Some of these chromosome abnormalities are balanced and may not be associated with birth defects.
A normal karyotype does not rule out the possibility of birth defects, such as those caused by submicroscopic cytogenetic abnormalities, molecular mutations, and other environmental factors (ie, teratogen exposure). For these reasons, clinicians should inform their patients of the technical limitations of chromosome analysis prior to performing the amniocentesis.
It is recommended that a qualified professional in Medical Genetics communicate all results to the patient.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
-Improper syringes or transport vessels may be unsuitable to amniotic cells. Amniotic fluid should not be exposed to the syringe plunger tip for longer than a few seconds and fluid should be transferred to a transport (centrifuge) tube as soon as possible following collection.
-Transport time should not exceed 2 days.
-A bloody specimen may interfere with attempts to culture cells.
-Inadequate amount of fluid (we recommend 25 mL of fluid) may not permit adequate analysis.
-Improper packaging may result in broken, leaky, and contaminated specimen during transport.
-Exposure of the specimen to temperature extremes may kill cells and severely interferes with attempts to culture cells.
-Contamination of the amniotic fluid by maternal cells can cause minor interpretive problems.
-False chromosome mosaicism may occur due to an artifact of culture.
-True mosaicism is rare and low levels may be missed due to statistical sampling error.
-Subtle structural chromosome abnormalities can occasionally be missed.
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.
46,XX or 46,XY. No apparent chromosome abnormality.
An interpretative report will be provided.
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Van Dyke DL, Roberson JR, Wiktor A: Chapter 31: Prenatal cytogenetic diagnosis. In Clinical Laboratory Medicine. Edited by KD McClatchey. Second edition, Philadelphia, Lippincott, Williams and Wilkins, 2002, pp 636-657