Why your gut microbiome test results could be completely wrong, according to a new 2026 NIST study
Swab, scoop or smear: at-home gut microbiome test kits promise to read the ecosystem inside your gut and tell you what to eat or which supplements to buy.
But can you trust the numbers that come back?
A new study put that question to the test. Researchers sent identical stool samples to seven different companies and expected matching answers.
They did not get them. The companies disagreed wildly. Some reported a few dozen microbe types, others more than 900, and one company even contradicted itself on the same sample.
Here is everything to know about the research, why gut microbiome testing is so inconsistent and what it means before you act on a result.
The gut microbiome test study and who ran it
The research was published February 26, 2026 in Communications Biology, a Nature journal.
It came from the National Institute of Standards and Technology (NIST) in Gaithersburg, Md., and the University of Maryland School of Medicine in Baltimore.
Stephanie Servetas, acting group leader of NIST’s Complex Microbial Systems Group, was the study’s first author. Scott Jackson, a former NIST molecular geneticist who now runs a biological consulting firm called NEST Consulting and consults for microbe-related companies, was the corresponding author.
How the at-home microbiome test experiment worked
The team tested seven direct-to-consumer microbiome companies. None of the companies were named in the study.
Researchers created a standardized fecal reference material by blending bowel movements from multiple people into a single homogenized sample. That ensured the biology was identical across every portion sent out.
Each company received three identical samples, tested in triplicate. This measured two things at once: consistency between companies and consistency within a single company’s own process.
“What this material is meant to do is really to say how reproducible are the results, either between companies or within a company, but it’s not going to be able to tell us who was closest to the correct answer,” Servetas said in a news briefing reported by Science News.
For comparison, the researchers also ran samples from seven additional biologically distinct donors through a single NIST workflow.
What the gut microbiome test results showed
The companies gave very different answers about which microbes were present, even though every sample was identical.
The number of microbe types reported ranged from 34 to more than 900. Only three types showed up in every single test.
One company tested the same sample three times and called two results “healthy” and one “unhealthy.” That showed how a single company can hand the same person very different conclusions.
The bottom line: the differences between companies were as large as the differences you would expect between totally different people.
Why gut microbiome testing methods differ so much
Companies collect and analyze samples in completely different ways, with no agreed-upon standard.
Some have you sample the whole stool. Others have you swab toilet paper. Some preserve the sample and some do not. They also use different lab technologies, which produce different results.
To make matters worse, microbiome testing has gotten cheaper and faster, but not more accurate.
Companies also grade your microbes as good or bad by comparing you to a reference group of other people. But each company uses a different reference group, so the same person can get different scores.
Why inconsistent gut microbiome test results matter
Based on inaccurate results, consumers may take probiotics they do not need, make harmful dietary changes or even attempt fecal transplants.
Diane Hoffman, a professor of health law at the University of Maryland, previously found clinicians reporting patients who sought medical help based on at-home microbiome test results.
One concern is parents of autistic children, who often have gastrointestinal issues, though the science linking those issues to autism is unsettled.
“They are kind of desperate to find anything that might help their child,” Hoffman said, per the UM. “If parents are restricting the intake of certain nutrients [based on these recommendations], this can be harmful for the child.”
Hoffman documented at least one case of someone performing a fecal microbiota transplant at home based on test results, which carries real infection risk without proper screening and physician oversight.
There are also financial conflicts. Some companies sell gut health supplements alongside the test, and the products they sell happen to “solve” the issues the tests raise.
What experts recommend before you buy gut health supplements
NIST now sells a standard reference sample that companies can use to check and improve their accuracy. Experts want basic industry-wide quality rules. The goal is not to shut companies down, but rather to make results more trustworthy.
Jackson’s advice to consumers is “buyer beware,” especially when a company is trying to sell you something.
“Some of these companies also offer probiotics and other dietary supplements that they say will correct your microbiome,” Jackson said. “It just so happens that the thing that is wrong with your microbiome happens to complement the product that they also sell.”
Jacques Ravel, a professor of medicine at the University of Maryland School of Medicine and one of the study’s authors, urged the same caution.
“You should be very careful in interpreting what companies are telling you,” Ravel said. “Especially be very cautious when they start recommending any kind of treatment or any kind of supplement or anything they [want to] sell you.”
This article was created by content specialists using various tools, including AI.
