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What is a Deoxyribonucleic Acid (DNA) test?

A DNA test or a genetic test can identify genetic disorders that could arise due to a change in an individual’s genes, chromosomes or proteins. Each cell in the human body contains DNA strands within its nucleus. A particular sequence on a DNA strand is called a gene, which controls heredity, be it external physical features or internal factors, such as blood group or presence or absence of disease. DNA strands are tightly packed together to form chromosomes. Each human cell contains 23 pairs of chromosomes. DNA testing can be done at three levels:

  • Molecular: At the molecular level, short-length DNA strands, ie single genes, are studied to identify mutations that could lead to genetic disorders such as sickle cell anaemia.
  • Chromosomal: At the chromosomal level, chromosomal pairs are tested to identify chromosomal abnormalities such as Down syndrome.
  • Biochemical: Biochemical tests for genetic disorders at the protein level are also performed. Many genes encode for enzymes and proteins in the body, which control the metabolic activity. Thus, biochemical tests help to detect inherent defects in metabolism.
  1. Why is a DNA test performed?
  2. How do you prepare for a DNA test?
  3. How is a DNA test performed?
  4. What do DNA test results indicate?

A DNA test is performed for the following reasons:

  • Presymptomatic testing: It is done to identify whether faulty genes that could cause diseases are present in an individual prior to the manifestation of disease symptoms.
  • Diagnostic testing: It is done to confirm whether the signs and symptoms exhibited by a patient are due to genetic abnormalities.
  • Carrier testing: It is done to identify whether a couple is a carrier of recessive genes for predicting serious congenital disorders in their subsequent generation.
  • Prenatal genetic testing: Prospective parents are advised this test if any abnormalities are found in the foetus in an ultrasound scan.

No special physical preparation is required for DNA testing. However, mental preparation is required, and the individual undergoing this test must be informed about its benefits, limitations and risks. They must also be educated regarding the possible implications the test results could have on their life.

A DNA test is performed by taking a sample of saliva, blood, urine, hair, body tissue or bone marrow from an individual. A simple way to perform this test is by taking a buccal smear, ie a smear of the inside of the mouth using a cotton swab. The sample is then sent to a  laboratory for testing any genetic, chromosomal or protein abnormalities.

In case of prenatal genetic testing, a procedure called amniocentesis or chorionic villus sampling is performed. Amniocentesis is a procedure of testing the amniotic fluid to detect any foetal abnormalities. Amniotic fluid is present in the womb and nourishes the foetus. For the procedure, the fluid is collected by inserting a needle with the assistance of ultrasonography into the amniotic sac. It is usually done in the 15th to 20th week of pregnancy. Anaesthesia is given around the abdomen before the withdrawal of the fluid, and the entire procedure lasts for no more than 30 minutes.

Chorionic villus sampling can be done by passing a thin plastic tube through the vagina and cervix to reach the placenta. It can also be done by passing a needle through the abdomen into the uterus and finally the placenta.

Normal results: If the diagnostic DNA testing done for a particular disorder is negative, it implies that the particular gene, chromosomal or protein abnormality is not present in the individual.

If a carrier test is done and the results turn out to be negative, it implies that the individual does not carry the gene for the disease.

Negative prenatal test results indicate that the foetus does not have congenital abnormalities. However, it is possible that the test done was not exhaustive of all genetic disorders and that it missed out certain genetic mutations that could cause disorders. Further testing would be required for ruling out genetic disorders with certainty.

Abnormal results: A positive result for any of the DNA tests suggests that a particular gene, chromosome or protein of interest has been identified.

In case of a diagnostic test, a positive result would allow the doctor to diagnose the disease and provide treatment accordingly. For example, a woman who is tested positive for the Human Epidermal growth factor Receptor-2 (HER-2) gene, which is expressed more than normal in certain patients suffering from breast cancer can be effectively treated with a drug called trastuzumab that specifically targets the HER-2 gene. This drug cannot be used effectively in patients who show a negative test result for this gene.

A positive result in a predictive/presymptomatic genetic testing need not imply that the individual will suffer from that disorder; however, it is a good way of preparing oneself for future problems and taking necessary health precautions.

A prenatal diagnostic test that shows a positive result for specific genetic abnormalities indicates that the baby will most likely suffer from that disorder at birth. Disorders, such as Down syndrome, Edward’s syndrome and X-linked recessive disorders, can be detected through a prenatal diagnostic DNA test. Parents must undergo counselling for the procedure to make an informed decision based upon the family’s financial capabilities and personal beliefs.

Disclaimer: All results must be clinically correlated with the patient’s complaints to make a complete and accurate diagnosis. The above information is provided from a purely educational perspective and is in no way a substitute for medical advice from a qualified doctor. 

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References

  1. National Institute of Health. US National Library of Medicine [internet]: Bethesda (MA), US. US Department of Health and Human Services What is genetic testing?
  2. Lab Tests Online-Au; Australasian Association of Clinical Biochemist; Clinical genetic testing
  3. National Health Service [internet]. UK; What happens - Amniocentesis
  4. Cheng EY. Prenatal diagnosis. In: Gleason CA, Juul SE, eds. Avery's Diseases of the Newborn. 10th ed. Philadelphia, PA: Elsevier; 2018:chap 18.
  5. Driscoll DA, Simpson JL, Holzgreve W, Otano L. Genetic screening and prenatal genetic diagnosis. In: Gabbe SG, Niebyl JR, Simpson JL, et al, eds. Obstetrics: Normal and Problem Pregnancies. 7th ed. Philadelphia, PA: Elsevier; 2017:chap 10.
  6. Wapner RJ, Dugoff L. Prenatal diagnosis of congenital disorders. In: Resnik R, Lockwood CJ, Moore TR, Greene MF, Copel JA, Silver RM, eds. Creasy and Resnik's Maternal-Fetal Medicine: Principles and Practice. 8th ed. Philadelphia, PA: Elsevier; 2019:chap 32
  7. National Institute of Health. US National Library of Medicine [internet]: Bethesda (MA), US. US Department of Health and Human Services What do the results of genetic tests mean?
  8. Mary Anne Koda-Kimble. Applied Therapeutics. 9th edition. 2009. Neoplastic Disorders-Solid tumours. 91:2-3.