Genetic testing is the analysis of a person's DNA, chromosomes or related genetic material to identify specific genetic variants, chromosome abnormalities or patterns associated with inherited conditions, disease risk or other biological characteristics. Depending on the test, laboratories may examine a single gene, a group of genes, selected DNA variants, an entire exome, an entire genome or chromosome structure.
Genetic testing can be used for diagnosis, carrier identification, reproductive decision making, newborn screening, prediction of disease risk, pharmacogenomic assessment and investigation of unexplained medical conditions. It can also be used in nonmedical contexts such as biological relationship testing and some forms of identification. The meaning of a result depends heavily on the purpose of the test, the condition being investigated and the strength of the evidence connecting a genetic finding with a particular outcome.
Diagnostic genetic testing is performed when a person has signs or symptoms suggesting a genetic disorder and the aim is to determine whether a relevant genetic cause can be identified. A test may focus on one gene when the clinical presentation strongly suggests a particular disorder, or use a broader panel when several genes can produce similar symptoms. Whole exome sequencing examines the protein coding regions of many genes, while whole genome sequencing examines coding and noncoding genomic regions. A positive result can sometimes establish or strongly support a diagnosis, while a negative result does not necessarily exclude a genetic disorder because current technology may fail to detect certain variants or because the relevant gene or mechanism may not yet be known.
Carrier testing is used to determine whether a person carries a genetic variant associated with a recessive or X linked condition that may not cause symptoms in the carrier. If both members of a couple carry pathogenic variants for the same autosomal recessive condition, their biological children can have an increased probability of inheriting the condition. Carrier testing can therefore be relevant to reproductive planning.
Newborn screening is a different application in which newborns are tested for selected conditions where early detection and treatment can improve outcomes. Prenatal testing can analyse genetic material associated with a pregnancy, while preimplantation genetic testing can examine cells from embryos created through assisted reproduction for selected genetic or chromosomal findings. Each approach has distinct indications, limitations and ethical considerations.
Genetic tests can produce several categories of results. A pathogenic or likely pathogenic variant may provide evidence that a genetic change contributes to a condition, while a benign or likely benign variant is generally not considered clinically significant for that purpose. A variant of uncertain significance means that available evidence is insufficient to determine whether the variant affects health. Such a finding should not automatically be treated as a diagnosis.
Test reports may also identify incidental or secondary findings unrelated to the original reason for testing. Interpretation depends on databases, population frequency, laboratory evidence, functional studies, family information and clinical context. Because genetic knowledge changes over time, some results may be reinterpreted as new evidence becomes available.
Genetic testing usually involves obtaining a biological sample such as blood, saliva, cheek cells or tissue. DNA is extracted from the sample and analysed using methods appropriate to the test. Techniques can include polymerase chain reaction, targeted sequencing, microarray analysis, chromosome analysis and high throughput sequencing. Bioinformatic pipelines may identify variants and compare them with reference sequences or clinical databases. Laboratory quality control is essential because sample contamination, sequencing errors and analytical mistakes can produce incorrect results. The laboratory method must also be capable of detecting the relevant class of genetic alteration. A sequencing test may not reliably detect every large structural rearrangement, repeat expansion or epigenetic abnormality, so test selection must correspond to the suspected biological mechanism.
Genetic testing differs from genetic screening. A diagnostic test is generally performed because a person has symptoms or a particular clinical indication, whereas screening is applied to people who may not have symptoms in order to identify those at increased probability of a condition. A screening result is usually not equivalent to a diagnosis and may require confirmatory testing.
Predictive genetic testing similarly estimates the probability of developing certain conditions based on genetic information, but the strength of prediction varies considerably among diseases. Some single gene disorders have highly informative genetic variants, while common conditions such as cardiovascular disease and diabetes usually involve many genetic and environmental factors. A genetic result therefore does not always provide a deterministic prediction of future health.
Genetic testing has important ethical, legal and privacy dimensions. Genetic information can concern not only the tested individual but also biological relatives because relatives share portions of their genomes. Issues include informed consent, data storage, secondary use of samples, possible discrimination, psychological effects and the interpretation of unexpected family relationships. Professional genetic counselling can help people understand what a test can and cannot establish, what different results might mean and what choices may follow.
Direct to consumer genetic testing can provide ancestry or selected health information without the same clinical context as medical testing, and consumers should understand that different companies test different variants and apply different interpretation methods. Clinical decisions should therefore rely on appropriately validated testing and professional interpretation when medical consequences are involved.
Genetic testing has transformed diagnosis and research but has important limitations. A negative test can reflect genuine absence of a detectable variant, limitations of the technology, incomplete knowledge or an incorrect initial hypothesis. A positive genetic finding may not predict disease with certainty because penetrance and variable expressivity can differ among individuals. Environmental factors and other genes can also modify outcomes.
The quality of a result depends on analytical validity, clinical validity and clinical utility, which are related but distinct concepts. Genetic testing is most informative when the test is appropriately selected, the laboratory method is validated, the result is interpreted using current evidence and the finding is integrated with the person's clinical and family history.
