Genetic testing is a medical or biological test that examines DNA, chromosomes or sometimes proteins to identify genetic changes associated with health, disease, inheritance or other biological characteristics. MedlinePlus defines it as testing for changes, often called variants or mutations, in DNA. Clinical genetic testing can help diagnose an inherited disorder, identify whether someone carries a disease-associated variant, estimate susceptibility to certain diseases, investigate unexplained illness, guide treatment and determine whether particular genetic changes are present in a tumour. Testing therefore encompasses many different laboratory methods rather than one universal test. The appropriate method depends on the clinical question, suspected condition and type of genetic change being sought.
Samples for genetic testing can come from blood, saliva, cells collected from inside the cheek, skin, hair, amniotic fluid or other tissues. A laboratory extracts genetic material and applies an appropriate analytical method. Targeted testing may look for one known variant, whereas a gene panel can examine multiple genes associated with a particular disorder or group of disorders. Exome sequencing examines the protein-coding portions of the genome, while whole-genome sequencing examines essentially the entire genomic DNA sequence. Chromosomal tests can identify large-scale gains, losses or rearrangements that would not necessarily be detected by a test designed to inspect individual DNA bases.
Genetic testing is used for several distinct purposes. Diagnostic testing investigates whether a person with symptoms or clinical findings has a particular genetic condition. Predictive or presymptomatic testing can identify a pathogenic variant associated with a future disease risk in someone who currently has no symptoms. Carrier testing determines whether someone carries a recessive or X-linked disease-associated variant that could be passed to children. Prenatal testing can assess a fetus, while newborn screening identifies selected conditions soon after birth. Pharmacogenomic testing examines genetic differences that influence medication response. Tumour genomic testing can identify acquired cancer-cell variants that may help guide therapy.
A crucial distinction is between inherited germline variants and acquired somatic variants. Germline variants are present in reproductive cells and consequently can be transmitted to offspring; depending on the condition, they are generally present throughout the person's body. Somatic variants arise in individual cells during life and are not ordinarily passed to children.
Cancer testing frequently analyses tumour DNA to identify somatic changes that drive or influence cancer behaviour and treatment response, although some findings may suggest an inherited predisposition and require separate germline assessment. Therefore, finding a genetic change in a tumour does not automatically mean that the person's children have inherited it. The biological source of the sample and the purpose of testing are essential for interpretation.
Genetic test results require careful interpretation because a DNA change is not automatically a disease-causing mutation. Many variants are benign or have no known effect, while some are classified as pathogenic, likely pathogenic, variants of uncertain significance, likely benign or benign. A variant of uncertain significance means that available evidence is insufficient to determine whether it contributes to disease; it should not automatically be treated as a diagnosis. Variant classifications can change as research accumulates. The presence of a pathogenic variant may also indicate increased risk rather than certainty that disease will occur, particularly for multifactorial conditions. Thus, genetic testing produces information whose clinical meaning depends on evidence, inheritance pattern, penetrance and the person's clinical context.
Genetic testing has substantial benefits but also limitations. A confirmed diagnosis can end a prolonged diagnostic search, allow appropriate surveillance or treatment and provide information about relatives. Testing can clarify reproductive risks and identify relatives who may benefit from targeted testing. However, testing may produce uncertain or unexpected findings, may not detect every disease-causing variant and may not predict exactly whether symptoms will occur or how severe they will be. Whole-genome and whole-exome sequencing can generate incidental findings unrelated to the original reason for testing. There can also be emotional, social, financial and privacy considerations. MedlinePlus therefore emphasises understanding benefits, limitations and possible consequences before testing and using informed consent.
Direct-to-consumer genetic testing differs from clinical genetic testing. Commercial tests may provide ancestry information or examine selected health-related variants, but they may analyse only a fraction of relevant genetic information and may use different interpretation standards from clinical laboratories. A consumer result suggesting increased disease risk does not necessarily establish a diagnosis, while a reassuring result does not necessarily exclude disease.
Positive or unexpected findings may warrant confirmation through an appropriate clinical test and discussion with a healthcare professional or genetic counsellor. Clinical testing is generally selected in relation to a defined medical question and interpreted using the person's medical and family history. The same DNA variant can therefore have different significance depending on the test context.
Genetic testing is increasingly important in precision medicine because genetic information can influence diagnosis, prevention, surveillance and treatment. In oncology, molecular testing can identify alterations that make a tumour susceptible to targeted medicines or provide information relevant to immunotherapy. In pharmacogenomics, inherited variants can influence how rapidly medicines are metabolised or how patients respond to particular drugs. In rare disease medicine, genome or exome sequencing can identify a molecular explanation after conventional testing has failed. Nevertheless, genetic testing should be chosen because it answers a meaningful clinical or personal question, not merely because sequencing is technically possible. Appropriate counselling, consent, laboratory quality and interpretation remain as important as the sequencing technology itself.
