How Does Genetic Testing Work?

How is a DNA sample collected?

Almost every cell in your body carries a full copy of your DNA, so a test only needs a small number of cells. The NHS notes that genetic tests usually use a sample of blood or saliva.

For home tests, saliva is the most common choice. You spit into a tube or use a cheek swab, seal it with a stabilising liquid, and post it back. It is painless, and a well-collected saliva sample gives enough good-quality DNA for most testing.

A few tips help the sample work first time:

• Avoid eating, drinking, smoking or chewing gum for around 30 minutes beforehand, or as your kit instructs
• Fill to the line shown on the tube
• Register your kit and send it back promptly

What happens to your DNA in the lab?

Once the sample arrives, the lab extracts the DNA from your cells and checks that there is enough of it, and that it is good enough quality, to test. The DNA is then read using one of two main technologies.

Genotyping arrays (DNA chips)

A genotyping array, often called a SNP chip, checks hundreds of thousands of pre-chosen positions in your DNA. It is fast and affordable, and many consumer tests use it.

Arrays are very accurate for common variants. They are much less reliable for rare ones. A large study in the BMJ compared chip results with sequencing in nearly 50,000 UK Biobank participants. For very rare variants, only 16% of the chip results were confirmed. For harmful BRCA1 and BRCA2 variants, most positive chip results turned out to be false positives.

Sequencing

Sequencing reads the DNA letters directly, one after another. Whole exome sequencing reads the protein-coding parts of genes, which make up about 1% of the genome. Whole genome sequencing reads almost all of your DNA, so it can detect variants in any part of it, including rare ones that chips miss.

The NHS itself is introducing whole genome sequencing for some patients. Its trade-off is that it finds far more variants, and the health meaning of many of them is not yet known.

What is actually analysed?

Reading the DNA is only half the work. The raw data is compared with a reference genome to find where yours differs, and each difference is then checked against scientific databases and published research.

Depending on the test, the analysis can cover:

• Single-gene variants linked to inherited conditions, such as BRCA1 and BRCA2
• Carrier status for recessive conditions you could pass to children
• Pharmacogenomic variants that affect how you process certain medicines
• Polygenic risk scores, which add up many small effects for common conditions
• Trait variants linked to nutrition, fitness and metabolism

Labs grade how confident they are that each variant affects health, from harmful, through uncertain, to harmless. Only the more confident findings should guide decisions.

→ What Can a DNA Test Reveal About Your Health?

Why does lab accreditation matter?

Accreditation tells you a lab has been independently checked. In the UK, the United Kingdom Accreditation Service (UKAS) assesses medical laboratories against ISO 17025, an international standard covering quality management and technical competence, and genetics is one of the disciplines it covers.

The NHS Genomics Education Programme advises that results from consumer tests should be confirmed in a clinical diagnostic laboratory before any change to medical care. So when choosing a test, it is worth asking which lab analyses the samples and what accreditation it holds.

How is your genetic data protected?

Your genome is about as personal as data gets, so privacy deserves real attention. Under UK data protection law, genetic data is classed as special category data, which means organisations need extra legal grounds and safeguards to process it.

In the UK, there is also a voluntary agreement between the government and the Association of British Insurers. Under this Code on Genetic Testing and Insurance, insurers who have signed up cannot ask for or use the results of predictive genetic tests, with one narrow exception: a predictive test for Huntington's disease for life insurance over £500,000.

Before you test, it is sensible to check:

• Where your data is stored and who can access it
• Whether it is ever shared for research, and whether you can opt in or out
• Whether you can ask for your data and sample to be deleted

Where does genetic counselling fit in?

Genetic counsellors are trained to explain what results mean for you and your family. The NHS describes their role as helping people understand the risks, benefits and possible results of testing, and its implications for relatives, both before and after a test.

This matters because some findings are sensitive. A result can carry implications for relatives, and occasionally reveal unexpected family information. Talking it through with an expert turns a long list of variants into something you can understand and act on calmly.

What are the limits of genetic testing?

Even the best test has limits. Results can be:

• Positive, meaning a known variant was found, though this usually cannot give your exact risk
• Negative, meaning no known variant was found, though a relevant change can occasionally be missed
• Uncertain, meaning a variant of uncertain significance was found and its effect is not yet known

Polygenic scores also carry an ancestry gap. Most genetic research has involved people of European ancestry, so scores tend to be less accurate for other groups. And genes are only part of the story, as lifestyle and environment shape most common conditions.

→ Genetic Risk vs Destiny: Are Your Genes Your Fate?

What can you do next?

If you want the fullest picture, look for whole genome sequencing from an accredited lab, clear privacy terms and access to a genetic counsellor. Phenome's Comprehensive Genetic Test uses whole genome sequencing, with genetic counsellors available to help you interpret your results. If you are planning a family, carrier screening answers a more focused question.

→ Explore the Comprehensive Genetic Test
→ Explore the Carrier Screening Test
→ How Inherited Conditions Are Passed On
→ What Is Nutrigenomics? How Your Genes Affect Diet