The increasing accessibility of genetic testing has led to questions about the two most common sample types: blood and saliva. Both are used to analyze a person’s DNA, but they are collected differently and have distinct characteristics. The best sample type often depends on the specific goals of the analysis.
The Source of DNA in Blood and Saliva
The DNA for genetic testing originates from cells. In a blood sample, genetic material is extracted from the nuclei of white blood cells (leukocytes). These cells are a rich source of high-quality DNA. Red blood cells are not used because they lack a nucleus and do not contain the body’s complete genetic blueprint.
The DNA in a saliva sample comes from two primary sources. A significant portion is from epithelial cells, which are cheek cells continuously shed into the mouth. The other source is white blood cells, also present in saliva, which can contribute up to 74% of the DNA and help ensure a robust sample.
Comparing Sample Quality and Accuracy
Blood is often considered the gold standard for genetic testing because it yields a large quantity of pure, human DNA. This high yield and purity make it a reliable source for demanding genetic analyses. The concentration of human DNA is high, with minimal genetic material from other organisms.
Saliva samples can present more variability and yield a lower amount of DNA than a blood draw. Saliva is also more prone to contamination from bacteria and other microbes that live in the mouth. This means a percentage of the extracted genetic material can be non-human.
Despite these differences, modern laboratory techniques have made saliva a reliable source for most genetic tests. Advanced methods effectively isolate and amplify human DNA, filtering out microbial contaminants. For most consumer-facing tests, like those for ancestry or wellness, the accuracy of results from a saliva sample is comparable to that of blood. For clinical diagnostics, however, blood is often preferred for its inherent purity.
Practical Differences in Sample Collection
The primary advantage of saliva collection is its convenience and non-invasive nature. Individuals can collect their own sample at home by spitting into a specialized tube, a painless process. This ease of collection makes it accessible to many people, including those apprehensive about needles.
At-home saliva collection comes with a higher risk of user error. A common issue is providing an insufficient amount of saliva. Eating, drinking, or smoking shortly before collection can contaminate the sample. Improperly sealing the collection tube is another step where errors can occur, compromising the sample’s integrity.
Obtaining a blood sample is an invasive procedure requiring a trained phlebotomist to perform a venipuncture. This occurs in a clinical setting, making it less convenient than a home kit. While the process involves a needle and can cause mild discomfort, the likelihood of a collection error is extremely low when performed by a skilled professional.
Which Sample Type is Right for Your Test?
For direct-to-consumer testing for ancestry, wellness traits, or general genetic exploration, a saliva sample is adequate. Companies specializing in these services have optimized their laboratory processes for saliva, ensuring reliable results. The convenience of this method makes it the standard for the consumer market.
When a genetic test is ordered by a healthcare provider for clinical reasons, blood is the required sample type. These tests are used for diagnosing genetic conditions, identifying disease carriers, or guiding medical treatment. The higher yield and purity of DNA from blood provide the confidence needed for health decisions, so a doctor will require a blood draw for precision.