Spit is mostly water, yet a teaspoon of it carries enough of your DNA to read your genetic makeup. That’s why ancestry and health-testing companies ask for a tube of saliva instead of a blood draw.1PubMed Central. Direct-to-consumer genetic testing: advantages and pitfalls. Not all of the DNA in the tube is yours, though, because your mouth is full of microbes whose genetic material ends up in the sample too.2DNA Genotek. Human genomic DNA content of saliva samples collected with the Oragene self-collection kit The same easy-to-collect sample is now used well beyond family trees, from choosing medicines to catching criminals.
Where the DNA in Your Saliva Comes From
The fluid made by your salivary glands carries no DNA of its own. The DNA comes from whole cells mixed into it. Those cells come from two places: the inner lining of your cheeks and mouth, and white blood cells.3Salimetrics. Collecting and Handling Saliva for DNA Analysis Lining cells are flat, skin-like cells that your mouth sheds every day. White blood cells leak into the mouth through the gums. Both carry the same full copy of your DNA in their nucleus.
White blood cells, not the lining cells, appear to supply most of the human DNA in saliva.4Revista Romana de Medicina de Laborator. Saliva leukocytes rather than saliva epithelial cells represent the main source of DNA Lining cells are shed constantly, but much of their DNA is already breaking down as they die off.
Your mouth’s microbes add their own DNA. In typical spit samples, about 12% of the DNA is bacterial, ranging from 2% to about 40%.5DNA Genotek. Human genomic DNA content of saliva samples collected with the Oragene self-collection kit Saliva holds more bacterial genomes than human ones, though each human genome is far longer.6PubMed Central. Evaluation of Bacteria and Fungi DNA Abundance in Human Tissues Brushing your teeth before giving a sample raised the human share of DNA from about half to about three-quarters in one test.7PubMed Central. Pre-sampling conditions affect salivary genomic DNA yield and human DNA fraction: a single-donor pilot study
Bacterial DNA doesn’t scramble your human results. Human and bacterial DNA have very different sequences, so tests built for human DNA simply ignore the microbe DNA. The real cost of bacteria is less usable human DNA. A sample with very high bacterial content may need to be redone, which is one reason kits tell you not to eat or drink beforehand.
The human DNA in saliva is overwhelmingly your own, but a sample can pick up someone else’s if you recently kissed, shared a drink or used another person’s utensil. Waiting before you spit lets your mouth clear out stray material.
How to Give a Good Saliva Sample and How Long It Keeps
Spitting about 2 ml into a collection tube gave over ten times more DNA than a cheek swab or brush.8PubMed Central. New Saliva DNA Collection Method Compared to Buccal Cell Collection Techniques for Epidemiological Studies A mouth-rinse sample came second, and both spit and rinse gave cleaner DNA than swabs or brushes.9PubMed Central. New Saliva DNA Collection Method Compared to Buccal Cell Collection Techniques for Epidemiological Studies
Before you spit, skip eating, drinking, smoking and gum for about 30 minutes, as kits usually instruct. If it’s allowed, rinse or brush earlier, since brushing raised the human DNA share. If your mouth is dry, rub your cheeks gently or think of something sour to build up saliva. Fill to the line with liquid spit, not bubbles. Then close the cap fully so the preserving liquid mixes in, and shake the tube.
Most kits have you mix your spit with a preserving liquid in the cap. Without such a liquid, DNA in saliva measurably degraded within 10 weeks at room temperature, while treated samples stayed stable.10PubMed Central. A universal pre-analytic solution for concurrent stabilization of salivary proteins, RNA and DNA at ambient temperature The liquid breaks open the cells and shields the DNA from the enzymes that would otherwise chop it up. It also stops bacteria in your spit from multiplying while the tube travels, so the human share of DNA doesn’t shrink as the sample sits. That is why you shouldn’t open the tube again or pour out the liquid.
Stabilized saliva stayed usable for genetic analysis after 18 months even at body temperature.11PubMed. Evaluation of the long-term storage stability of saliva as a source of human DNA DNA in collection tubes has stayed intact at room temperature for more than 5 years.12PubMed Central. Contaminating DNA in human saliva alters the detection of variants from whole genome sequencing
Age can lower how much DNA you give. Older people tend to yield slightly less DNA in saliva, though the effect is small.13PubMed Central. Saliva DNA quality and genotyping efficiency in a predominantly elderly population
Is Saliva as Accurate as Blood for DNA Testing
Most consumer kits use genotyping, which means reading hundreds of thousands of single-letter spots where people commonly differ. On such a test, saliva and blood from the same people agreed more than 99% of the time.14PubMed Central. The utility of DNA extracted from saliva for genome-wide molecular research platforms On a test of drug-processing genes, saliva and blood agreed 98.7% of the time, though saliva gave slightly fewer readings.15PubMed Central. Genotyping Performance between Saliva and Blood-Derived Genomic DNAs on the DMET Array: A Comparison Saliva also found all 13 medically important missing or extra chunks of chromosome that blood found, regardless of bacteria in the sample.16PubMed Central. Chromosomal Microarray Detection of Constitutional Copy Number Variation Using Saliva DNA Tiny mismatches usually fall on spots that weren’t read, not spots that were read wrongly.
Genotyping checks a chosen list of common spots, so it can be very accurate on those spots and still say nothing about the rest of your genome. A match between saliva and blood tells you the two sample types give the same answer on that list, not that the list is complete.
Reading the entire genome demands more and cleaner DNA. In heart patients, almost half of saliva samples didn’t meet the strict quality bar for whole-genome sequencing.17PubMed Central. Quality of whole genome sequencing from blood versus saliva derived DNA in cardiac patients Samples that passed matched blood on 90 to 95% of single-letter differences.18PubMed Central. Quality of whole genome sequencing from blood versus saliva derived DNA in cardiac patients Agreement fell below 80% for larger duplicated or missing stretches of DNA.19PubMed Central. Quality of whole genome sequencing from blood versus saliva derived DNA in cardiac patients
If a doctor orders full sequencing, a blood draw may still be preferred. Blood is also worth asking for after a bone marrow transplant, covered later, or if a saliva sample has already failed twice.
Ancestry and Health Kits Built on Saliva
Direct-to-consumer kits commonly use saliva as their DNA source.20PubMed Central. Direct-to-consumer genetic testing: advantages and pitfalls. They report genealogical ancestry, wellness traits, carrier status and health predispositions.21PubMed Central. Direct-to-consumer genetic testing: advantages and pitfalls. These results reach you without a healthcare professional necessarily involved.22PubMed Central. Direct-to-consumer genetic testing: advantages and pitfalls. So have any worrying health result confirmed with a doctor before acting on it.
After the tube arrives, the lab extracts DNA from the cells in your sample and applies it to a chip that reads a fixed set of variants. The readings are compared with reference databases of people from different regions to estimate ancestry, and with published research linking variants to traits and conditions. The same raw readings can be reinterpreted later, so a report may change as the company updates its databases.
A raised risk is not a diagnosis, and since kits read only selected variants, a clear result doesn’t rule out a condition. A genetic counselor can explain what a result means for you.
Before sending a sample, check whether the company keeps your sample or destroys it, whether your data can be shared with researchers or police, and whether you can delete your data later.
Saliva DNA for Choosing Medicines
Some gene versions make people slow or fast at clearing common anti-inflammatory painkillers such as ibuprofen.23PubMed. Pharmacogenetic and Pharmacokinetic Assays from Saliva Samples Can Guarantee Personalized Drug Prescription Knowing this from a saliva test helps a doctor dose to avoid side effects while still easing pain.24PubMed. Pharmacogenetic and Pharmacokinetic Assays from Saliva Samples Can Guarantee Personalized Drug Prescription In a small group of people with epilepsy, saliva gene testing flagged current seizure drugs with a raised side-effect risk in a third of them.25PubMed Central. A real‐life pilot study of the clinical application of pharmacogenomics testing on saliva in epilepsy It also explained poor past response in a third and identified drugs to avoid in about a quarter.26PubMed Central. A real‐life pilot study of the clinical application of pharmacogenomics testing on saliva in epilepsy
Your liver uses enzymes to break down many medicines, and the gene versions you inherit change how active those enzymes are. A slow processor can build up too much of a drug, which raises the chance of side effects at a standard dose. A fast processor may clear a drug so quickly that a normal dose doesn’t work well.
You might be offered this when a doctor or pharmacist orders a spit kit instead of a blood test. The results go into your record to guide future prescriptions, and it helps most when you’ve had side effects or drugs that didn’t work. Never change a dose on your own based on a result. Your gene versions don’t change over your lifetime, so a single result can inform later prescriptions as well as the current one.
Spotting Cancer and Infections in Saliva
Tumors in the mouth shed cells and DNA changes into saliva. Tumor-specific gene changes were found in the saliva of 82% of people with mouth cancer, whatever the tumor’s stage or location.27PubMed Central. Mutation detection in saliva from oral cancer patients Tests of DNA chemical tags in saliva and mouth swabs caught about 86% of mouth cancers and correctly cleared about 91% of people without it.28PubMed. Efficacy of hypermethylated DNA biomarkers in saliva and oral swabs for oral cancer diagnosis: Systematic review and meta-analysis Roughly 1 in 7 mouth cancers could be missed, so these tests support rather than replace an exam.
A positive result points to a need for a biopsy, not a diagnosis, and a negative doesn’t replace regular dental checks. See a dentist for any mouth sore lasting over two to three weeks.
Saliva also holds cell-free DNA, loose fragments that have escaped from dying cells.29PubMed Central. Isolation of salivary cell-free DNA for cancer detection In a small early study, loose saliva fragments differed between people with stomach cancer and healthy people.30PubMed Central. Multi-faceted attributes of salivary cell-free DNA as liquid biopsy biomarkers for gastric cancer detection For cancers farther away, any signal has to travel and is likely to be fainter, which is why those tests remain at an earlier stage. They are still research, not something you can order.
Many viruses can be detected in saliva, including herpes viruses, HPV, the COVID-19 virus and mpox.31PubMed Central. The Emergence of Saliva as a Diagnostic and Prognostic Tool for Viral Infections Saliva is easy and safe to self-collect, and can work about as well as blood or urine for some infections.32PubMed Central. The Emergence of Saliva as a Diagnostic and Prognostic Tool for Viral Infections In malaria, 80% of positive saliva samples still tested positive after a year at room temperature.33PubMed Central. Detection of Plasmodium falciparum DNA in saliva samples stored at room temperature: potential for a non-invasive saliva-based diagnostic test for malaria
How Police Use DNA From Saliva
Forensic labs build a DNA profile by matching 13 to 17 standard DNA markers.34PubMed Central. Saliva as a forensic tool The lab reads those markers from the trace and records the pattern of variants at each. It then compares that pattern with a reference sample from a suspect or with profiles stored in a database.
In one comparison, 75% of saliva samples gave a usable result, all matching the same person’s blood.35PubMed Central. Quantitative and qualitative assessment of DNA extracted from saliva for its use in forensic identification Cups, bottles, cigarette butts and utensils can carry your saliva, as can envelope flaps and stamps you lick. Dried saliva can still hold traces weeks later.
Police in Los Angeles solved decades-old murders using saliva from a suspect’s discarded coffee cup.36Northwestern University Law Review. Reclaiming “Abandoned” DNA: The Fourth Amendment and Genetic Privacy Seattle police got a suspect’s DNA by mailing a fake legal letter he sealed by licking the envelope.37Northwestern University Law Review. Reclaiming “Abandoned” DNA: The Fourth Amendment and Genetic Privacy In the US, DNA in saliva you leave behind has been treated as abandoned, which raises privacy concerns.38Northwestern University Law Review. Reclaiming “Abandoned” DNA: The Fourth Amendment and Genetic Privacy
A trace sometimes fails. Too few cells may be left behind, heat, sun and moisture break DNA down, and another person’s DNA can mix in.
When Saliva Holds Someone Else’s DNA
A stem cell transplant replaces your blood-forming cells, so your white blood cells carry the donor’s DNA. White blood cells are the main DNA source in saliva, so the donor’s DNA appears there too.39Revista Romana de Medicina de Laborator. Saliva leukocytes rather than saliva epithelial cells represent the main source of DNA Hair roots and fingernails grow from skin-type cells, which the transplant did not replace, so they keep the patient’s own DNA. That is why saliva can give the donor’s or mixed results while hair gives the patient’s own.
Saliva can show a mix of two people’s DNA, with the donor making up about a third in one case.40Forensic Science International: Genetics Supplement Series. The extent of STR chimerism in different biological samples following bone marrow transplantation: A case report Of 20 transplant patients’ saliva samples, 9 kept the patient’s own profile, 6 showed the donor’s and 4 a mix.41PubMed Central. Analysis of HLA genotype in the recipients’ different tissues after haploidentical hematopoietic stem cell transplantation Hair roots and fingernails kept only the patient’s DNA.42Forensic Science International: Genetics Supplement Series. The extent of STR chimerism in different biological samples following bone marrow transplantation: A case report
If you’ve had a transplant, tell any DNA testing service, as a spit kit may report donor results. An ancestry result may partly reflect the donor. Doctors use pre-transplant samples or hair, not saliva, to track the transplant. Ask the company whether another sample type is better, since a cheek swab can also pick up donor white blood cells.