While the ability to repair defective genes remains far in the future, genetic testing can help patients determine the likelihood of passing on or inheriting certain disorders today. Genetic testing usually refers to the analysis of DNA to identify changes in gene sequence (deletions, additions, or misspellings) or expression levels. Genetic testing can also refer to biochemical tests for gene products (proteins) and for microscopic analysis of stained chromosomes. Genetic testing still is in its early stages, so both patients and experienced physicians may need guidance when it comes to navigating this new and complex territory.
How is genetic testing used clinically?
Diagnostic medicine: identify whether an individual has a certain genetic disease. This type of test commonly detects a specific gene alteration but is often not able to determine disease severity or age of onset. It is estimated that there are >4000 diseases caused by a mutation in a single gene. Examples of diseases that can be diagnosed by genetic testing includes cystic fibrosis and Huntington’s disease.
Predictive medicine: determine whether an individual has an increased risk for a particular disease. Results from this type of test are usually expressed in terms of probability and are therefore less definitive since disease susceptibility may also be influenced by other genetic and nongenetic (e.g. environmental, lifestyle) factors. Examples of diseases that use genetic testing to identify individuals with increased risk include certain forms of breast cancer (BRCA) and colorectal cancer.
Pharmacogenomics: classifies subtle variations in an individual’s genetic makeup to determine whether a drug is suitable for a particular patient, and if so, what would be the safest and most effective dose.
Whole-genome and whole-exome sequencing: examines the entire genome or exome to discover genetic alterations that may be the cause of disease. Currently, this type of test is most often used in complex diagnostic cases, but it is being explored for use in asymptomatic individuals to predict future disease.