Genetic testing can detect hundreds of diseases, ranging from single-gene disorders like cystic fibrosis and sickle cell anemia to inherited cancer syndromes, heart conditions, neurological diseases, and metabolic disorders. The specific conditions a test can identify depend on the type of test ordered and the reason for testing. Some tests look for a single known mutation in a family, while others scan thousands of genes at once.
How Different Types of Tests Work
Not all genetic tests serve the same purpose, and the type your doctor orders determines what diseases it can find. Diagnostic testing is used when you already have symptoms and your doctor suspects a genetic cause. It can confirm conditions like cystic fibrosis, Huntington’s disease, or muscular dystrophy. Predictive testing, on the other hand, is done before symptoms appear. If a close relative has been diagnosed with a hereditary condition, predictive testing can reveal whether you carry the same mutation and face an elevated risk.
Carrier testing is designed for people planning to have children. You may carry a gene change for a condition like sickle cell disease or Tay-Sachs without ever developing symptoms yourself, but if your partner carries the same change, your child could be affected. This type of testing identifies those hidden risks before pregnancy.
Results typically take anywhere from a few days to several weeks, depending on the complexity of the test. Prenatal results tend to come back faster because timing matters for pregnancy decisions.
Hereditary Cancer Syndromes
Cancer is one of the most common reasons people seek genetic testing. Mutations in the BRCA1 and BRCA2 genes are the best-known examples. More than 60% of women who inherit a harmful change in either gene will develop breast cancer during their lifetime. The ovarian cancer risk is also dramatically elevated: 39% to 58% for BRCA1 carriers and 13% to 29% for BRCA2 carriers. Men are affected too. BRCA2 carriers face a prostate cancer risk of 19% to 61% by age 80, and male breast cancer, while rare in the general population, occurs in up to 7% of men with BRCA2 changes.
Beyond BRCA, genetic testing can identify mutations linked to Lynch syndrome (which raises the risk of colorectal, endometrial, and other cancers), Li-Fraumeni syndrome, and hereditary diffuse gastric cancer, among others. Knowing you carry one of these mutations doesn’t mean cancer is inevitable, but it opens the door to earlier and more frequent screening, preventive medications, or risk-reducing surgery.
Neurological and Movement Disorders
Huntington’s disease is one of the clearest examples of a genetic test delivering a definitive answer. The test measures a specific repeating DNA sequence in the gene responsible for the disease. If you have 26 or fewer repeats, you will not develop Huntington’s and cannot pass it on. At 40 or more repeats, the disease will develop assuming a normal lifespan, and each child has a 50% chance of inheriting it. A gray zone exists between 36 and 39 repeats, where symptoms may or may not appear. Repeat counts above 60 are associated with juvenile onset, though the exact age symptoms begin cannot be predicted from the number alone.
Other neurological conditions detectable through genetic testing include spinal muscular atrophy (SMA), which is now part of routine newborn screening, Fragile X syndrome, certain forms of epilepsy, early-onset Alzheimer’s disease linked to specific gene variants, and Charcot-Marie-Tooth disease, a group of inherited nerve disorders.
Heart Conditions
Several inherited heart conditions can be identified through genetic testing, often before they cause any symptoms. Hypertrophic cardiomyopathy, a condition where the heart muscle thickens abnormally, is one of the leading causes of sudden cardiac death in young athletes. Genetic testing can identify the responsible mutations in family members who haven’t yet shown signs on an echocardiogram.
Long QT syndrome, a disorder of the heart’s electrical system that can trigger dangerous arrhythmias, is another key target. About one in eight patients with hypertrophic cardiomyopathy also has long QT, making genetic testing valuable for uncovering overlapping risks. Familial hypercholesterolemia, which causes dangerously high cholesterol from birth and dramatically raises heart attack risk, is also detectable and is one of the most underdiagnosed genetic conditions.
Newborn Screening
Every baby born in the United States is screened for a panel of genetic and metabolic conditions within the first few days of life. The federal Recommended Uniform Screening Panel includes 38 core conditions. Many of these are metabolic disorders where the body cannot properly break down certain proteins, fats, or sugars. Left untreated, they can cause intellectual disability, organ damage, or death. Caught early, many are manageable with dietary changes or medications.
The panel includes sickle cell disease and related blood disorders, cystic fibrosis, severe combined immunodeficiency (sometimes called “bubble boy” disease), phenylketonuria (PKU), congenital hypothyroidism, Pompe disease, spinal muscular atrophy, and galactosemia, among others. Newborn screening is one of the most successful public health programs in genetics because it catches conditions during the narrow window when treatment can prevent irreversible harm.
Prenatal Genetic Testing
During pregnancy, genetic testing can screen for chromosomal conditions in the fetus. Non-invasive prenatal testing (NIPT), which uses a simple blood draw from the mother, analyzes fragments of fetal DNA circulating in the mother’s bloodstream. For Down syndrome (trisomy 21), NIPT has a sensitivity around 93% and specificity around 96%. For Edwards syndrome (trisomy 18), sensitivity is roughly 89% with similar specificity. Patau syndrome (trisomy 13) detection is somewhat lower, around 74% sensitivity, though specificity reaches 100% in some studies.
Because NIPT is a screening test rather than a diagnostic one, a positive result is typically confirmed with amniocentesis or chorionic villus sampling. These invasive tests carry a small risk of miscarriage but provide a definitive answer. Prenatal testing can also detect conditions like Turner syndrome, Klinefelter syndrome, and certain inherited disorders when a family history is known.
Pharmacogenomic Testing
Genetic testing isn’t only about disease detection. It can also reveal how your body processes medications. The FDA currently lists over 670 medications with pharmacogenomic information on their labels. Some of these carry boxed warnings, the most serious safety alerts, tied to specific genetic variants.
For example, genetic differences affect how quickly your body activates the blood thinner clopidogrel. People who process it slowly may not get adequate protection against blood clots. Codeine poses the opposite problem: some people convert it to its active form so rapidly that normal doses become dangerously strong. The blood thinner warfarin requires careful dosing that genetic testing can help fine-tune. Certain seizure medications can trigger life-threatening skin reactions in people with specific immune system gene variants, making pre-prescription testing critical in some populations.
This type of testing is becoming more routine in oncology, psychiatry, pain management, and cardiology, where choosing the right drug at the right dose can prevent serious adverse reactions.
Direct-to-Consumer Tests vs. Clinical Tests
Consumer genetic testing kits available online are not equivalent to the tests your doctor orders. Clinical-grade tests go through rigorous quality control and are interpreted by trained geneticists or genetic counselors. Direct-to-consumer raw data comes with disclaimers that it is not validated for accuracy or intended for medical use.
The gap in reliability is significant. Research from Stanford Medicine found that 40% of genetic variants flagged in direct-to-consumer raw data turned out to be false positives when sent to a clinical lab for confirmation. That means nearly half of the “concerning” results weren’t real. If a consumer test flags something potentially meaningful, the recommendation is always to have it confirmed through clinical-grade testing before making any medical decisions.
Polygenic Risk and Complex Diseases
Conditions like type 2 diabetes, coronary artery disease, and most common cancers aren’t caused by a single gene. They result from the combined influence of hundreds or thousands of small genetic variations, layered on top of lifestyle and environmental factors. Researchers have developed polygenic risk scores that attempt to quantify this cumulative genetic risk.
The concept is promising, but the clinical reality hasn’t caught up. A systematic review of 591 studies on polygenic risk scores found that while 22 showed strong evidence that the scores could identify higher-risk individuals, not a single study demonstrated that using the scores actually improved patient outcomes. The CDC has noted that we still don’t know how best to integrate these scores into healthcare, and cost-effectiveness analyses for conditions like heart disease and diabetes are still needed. For now, polygenic risk scores remain a research tool rather than a standard part of clinical care.