Chromosome Analysis, Chorionic Villus Sampling
Prenatal diagnosis of chromosome abnormalities (trisomies, deletions, translocations, etc) at about 9 to 12 weeks of gestation
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Although not used as widely as amniocentesis, the use of chorionic villus sampling (CVS) for chromosome analysis is an important procedure for the prenatal diagnosis of chromosome abnormalities.
CVS can be collected by either transcervical or transabdominal techniques. It can be safely performed at an earlier gestational age (ie, 9-12 weeks) than amniocentesis (usually performed between 15-18 weeks of gestation).
The medical indications for performing chromosome studies on CVS are similar to amniocentesis, and may include advanced maternal age, abnormal first trimester screen, history of a previous child with congenital anomalies, abnormal fetal ultrasound, and family history of a chromosome abnormality.
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 interpretive report will be provided.
Cytogenetic studies on chorionic villus sampling (CVS) are considered more than 99% reliable for the detection of most 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 chorionic villi 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 environmental factors (ie, teratogen exposure). For these reasons, clinicians should inform their patients of the technical limitations of chromosome analysis before the procedure is performed, so that patients may make an informed decision about pursuing the procedure.
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
-Inadequate amount of specimen (we recommend 20-30 mg of chorionic villus sampling: CVS) may not permit adequate analysis
-Exposure of the specimen to temperature extremes may kill cells and severely interfere with attempts to culture cells
-Improper packaging may result in broken, leaky, and contaminated specimens during transport
-Extended transport time
-Contamination of the CVS by maternal cells can cause minor interpretive problems
-False chromosome mosaicism may occur due to artifact of culture.
-True mosaicism may be missed due to statistical sampling error
-Presence of chromosome abnormalities in placental cells that do not occur in cells of the fetus (confined placental mosaicism)
-Subtle structural chromosome abnormalities can occasionally be missed
This test should be performed for prenatal diagnostic purposes only. Chromosome analysis on CVS collected from products of conception (ie, stillbirth or miscarriage tissue) should be ordered as POC / Chromosome Analysis, Autopsy, Products of Conception, or Stillbirth.
Clinical Reference Provides recommendations for further in-depth reading of a clinical nature
Van Dyke DL, Roberson JR, Wiktor A: Chapter 31: Prenatal cytogenetic diagnosis. In KD McClatchey's Clinical Laboratory Medicine. Second edition. Philadelphia, Lippincott, Williams and Wilkins, 2002, pp 636-657