Carrier Screening Test

General Description

The carrier screening test, or genetic compatibility test (GCT), is a study performed on couples who wish to have children in order to assess the risk of passing on a genetic disorder to their offspring.

Most individuals carry a genetic alteration in their DNA that does not affect their health or that of their descendants. However, if both parents have a mutation in the same gene, there is a 25% probability that their children will develop the disorder.

The carrier screening test analyzes approximately 420 monogenic diseases, i.e., conditions caused by the malfunction of a single gene, which are associated with either an autosomal recessive inheritance pattern (both genes in a pair must be altered) or X-linked inheritance (a single abnormal copy on the X chromosome is sufficient for the disease to manifest).

Knowing the likelihood of passing on genetic disorders allows couples to develop a reproductive plan and take the necessary precautions to prevent transmission.

When is it indicated?

Genetic compatibility testing is indicated for:

• Couples planning to conceive.
• Gamete donors (oocytes or sperm).
• Individuals interested in knowing whether they are carriers of a genetic disorder.
• Individuals undergoing assisted reproduction procedures.
• Couples with a family history of genetic anomalies.
• Couples who have previously had a child with a hereditary disorder.
• Consanguineous couples (biological relatives).

How is it performed?

A blood sample, and occasionally saliva, is sufficient to obtain the DNA for analysis. The procedure is straightforward:

• The patient sits with the arm extended.
• After disinfecting the area, a needle is inserted into a vein, typically in the arm. A tourniquet is often applied above the elbow to facilitate blood pooling.
• A small amount of blood is collected in a sterile purple-top tube containing EDTA, an anticoagulant that prevents clotting before the sample reaches the laboratory.

In general, DNA is extracted from the proteins in the blood, requiring the addition of ions or sodium sulfate to disrupt cell membranes and molecular structures. Subsequently, an alcohol (ethanol or isopropanol) is added to precipitate the DNA, which is then isolated via centrifugation.

DNA sample analysis can be performed using different methods:

• PCR Genotyping: used when analyzing a small number of genes. Polymerase chain reaction (PCR) uses synthetic DNA fragments as references to bind to the target segment and amplify it.
• Exome Sequencing: allows analysis of more genes but is slower. The desired sequence is selected and compared with a reference sample using specialized equipment.
• Next-Generation Sequencing (NGS): enables simultaneous analysis of multiple chromosomal sequences, providing faster results. DNA is divided into small fragments, which are analyzed and compared to a standardized sequence using specialized machinery.

Risks

Genetic compatibility testing does not pose health risks.

What to Expect from a Carrier Screening Test

On the day of blood collection, informed consent must be signed. During the procedure, the patient experiences a brief pinch lasting a few seconds. Afterward, it is recommended to apply pressure to the puncture site for a few minutes to prevent bruising. Once completed, normal activities can be resumed immediately.

Results from a genetic compatibility study typically take approximately 20 days. The report, which must be interpreted by a specialist, provides individual information for each member of the couple and compares their genetic profiles to assess the risk of transmitting a genetic disorder.

A specialist must interpret and explain the findings and, based on the report, recommend the next steps.

Specialties Requesting Carrier Screening Tests

Carrier screening is conducted by specialists in Genetics. These tests are commonly included in studies requested by Gynecology and Obstetrics and Assisted reproduction specialists.