Genetic testing for cancer typically starts with a simple sample, most often a blood draw, saliva kit, or cheek swab. From there, a laboratory extracts your DNA and analyzes it for changes in specific genes linked to cancer risk or tumor growth. The entire process, from sample collection to results, usually takes a few days to several weeks depending on the type of test and its complexity.
There are two fundamentally different kinds of cancer genetic tests, and understanding which one you’re getting clarifies the entire process.
Inherited Risk vs. Tumor-Specific Testing
Germline testing looks for mutations you were born with, passed down through egg or sperm cells at conception. These are the tests people think of when a family history of breast, ovarian, or colon cancer prompts a doctor to recommend screening. A positive result means every cell in your body carries the mutation, and you may have passed it to your children. The goal is to quantify your cancer risk before cancer develops, or to explain why it already has.
Somatic testing is different. It analyzes mutations that developed inside a tumor after conception, changes that exist only in cancer cells and aren’t inherited. Doctors order somatic testing on patients who already have cancer, usually to identify which drugs are most likely to work against that specific tumor’s genetic profile. The sample for somatic testing comes from a tissue biopsy of the tumor itself, or increasingly, from a blood draw.
How Samples Are Collected
For inherited (germline) testing, the National Cancer Institute notes that testing is done on a small sample of bodily fluid or tissue, usually blood but sometimes saliva or cells from inside the cheek. Saliva kits and mouth swabs can often be collected at home and mailed to the lab. In rare cases where a result comes back inconclusive, a second sample from a different tissue type (like a skin biopsy) may be needed to confirm the finding.
For tumor testing, the sample is typically a piece of tissue removed during a biopsy or surgery. The lab needs actual tumor cells to identify the somatic mutations driving that particular cancer. If a tissue biopsy isn’t feasible, a newer option called a liquid biopsy can sometimes substitute.
What Happens in the Lab
Most cancer genetic tests today use a technology called next-generation sequencing, which can read thousands of genes simultaneously rather than checking them one at a time. The laboratory workflow involves several stages.
First, DNA is extracted from your sample and broken into smaller fragments. The ends of these fragments are repaired and tagged with tiny molecular adapters so the sequencing machine can recognize and process them. This prepared collection of tagged DNA fragments is called a library.
Unless the lab is sequencing your entire genome, the library is then filtered to focus on the genes that matter. Two common approaches exist: one uses specially designed probes that latch onto the target gene sequences and physically pull them out of the mix using magnetic beads, while the other copies and amplifies only the gene regions of interest. Either way, the result is a concentrated sample of the DNA the lab actually wants to read.
The sequencing step itself works by reading one DNA base at a time. Each base is paired with a fluorescent marker that flashes a specific color when it’s incorporated into a growing DNA strand. A high-resolution camera captures these flashes in sequence, and software translates the color pattern into a readable DNA sequence. This produces millions of short “reads” that are then digitally stitched together and compared against a reference human genome to spot differences.
A bioinformatics team then filters and interprets the variants found. Not every DNA difference is meaningful. Specialists determine which variants are known to cause disease, which are harmless, and which fall into an uncertain gray area.
Liquid Biopsies: Testing From a Blood Draw
When cancer cells die, they release small fragments of their DNA into the bloodstream. These fragments, called circulating tumor DNA, tend to be shorter than normal cell-free DNA floating in blood, which is one way labs distinguish between the two. A liquid biopsy captures these tumor DNA fragments from a standard blood draw and sequences them to identify gene mutations.
Liquid biopsies are already used to guide treatment decisions for certain cancers, particularly lung cancer. They can identify mutations that make a patient eligible for targeted therapies without requiring an invasive tissue biopsy. They’re also useful for monitoring how a tumor responds to treatment over time, since a blood draw is far easier to repeat than a surgical biopsy.
Genetic Counseling Before and After
For inherited cancer testing, most providers recommend meeting with a genetic counselor both before and after your test. The pre-test session covers more ground than people expect. A counselor will walk through your family history, explain what the test can and cannot detect, discuss how results might affect other family members, and outline the possible outcomes, including the possibility of an uncertain result. You’ll also discuss confidentiality protections and the limits of those protections.
One practical detail: for inherited testing, the most informative approach often starts with a family member who already has cancer. Testing an affected relative first makes it easier to interpret results for the rest of the family. If no affected relative is available or willing, testing can still proceed, but interpretation may be less definitive.
Understanding Your Results
Results generally fall into three categories. A positive result means a known disease-causing mutation was found. A negative result means no harmful mutations were detected in the genes tested. The third category is the one that catches people off guard: a variant of uncertain significance, or VUS.
A VUS means the lab found a DNA change, but there isn’t enough evidence yet to say whether it actually increases cancer risk. The National Human Genome Research Institute notes that the great majority of new variants identified in a person’s genome fall into this uncertain category. These variants are often so rare in the population that researchers simply haven’t gathered enough data to classify them. Over time, as more people are tested and more research accumulates, a VUS may be reclassified as either harmful or harmless. In the meantime, a VUS generally does not change your medical management.
Additional information can help resolve a VUS, including population studies, laboratory experiments on the variant’s function, and testing other family members to see whether the variant tracks with cancer in your family tree.
How Long Results Take
Turnaround time varies by test type. Simple single-gene tests may return results in a few days, while larger multi-gene panels typically take two to four weeks. Some complex analyses take longer. Your ordering provider or genetic counselor can usually give you a specific estimate based on the lab being used.
Legal Protections for Test Results
The Genetic Information Nondiscrimination Act (GINA), which took effect in 2009, makes it illegal for employers to use genetic information in hiring, firing, pay, promotions, or any other employment decision. Employers are also restricted from requesting or requiring genetic information in the first place, with only narrow exceptions like inadvertent disclosure or voluntary wellness programs.
GINA also addresses health insurance, prohibiting insurers from using genetic information to deny coverage or set premiums. However, the law has a notable gap: it does not cover life insurance, disability insurance, or long-term care insurance. This means a positive genetic test result could potentially affect your ability to obtain these policies, something worth considering before testing and worth discussing with a genetic counselor.