A microbiome test tells you what is in a stool sample. Everything past that — good, bad, balanced, deficient — is interpretation, and the quality of the interpretation varies far more than the sequencing does.
What a test genuinely does
It identifies organisms present in the sample. That is real information, and it is the necessary starting point for any personalized approach — Johnson and colleagues' daily-sampling work found diet-microbiome associations are individual, meaning your response genuinely cannot be read off a population average.[1] If responses are personal, measurement is the only alternative to guessing.
What it cannot do
It cannot return a validated verdict that your microbiome is healthy or dysbiotic. Healthy microbiomes vary enormously between people,[2] so there is no threshold to score you against. And host variables — age, BMI, diet, medication — confound disease associations badly enough that controlling for them weakens or erases apparent signals.[3] A report that hands you a single "gut health score" is expressing confidence the field does not have.
Method matters more than people expect
The Microbiome Quality Control project consortium assessed variation in microbial community amplicon sequencing and found substantial differences in results across labs and protocols.[4] Costea and colleagues showed that fecal sample processing alone — how the sample is collected, stored and prepared — shifts what you find.[5] Two tests on the same stool can disagree, and the disagreement is often methodological rather than biological.
Sequencing depth is a real, non-marketing distinction. Amplicon (16S) surveys read one marker gene and typically resolve to genus; whole-genome shotgun sequencing reads everything and can resolve species and strains — which matters because, as the Lactobacillus reclassification made obvious, genus is often too coarse to be actionable.
How to read a report honestly
Treat it as a description of a moment, not a diagnosis. Ask which method was used and at what depth. Be skeptical of any score, grade, or "imbalance" verdict. And be most skeptical when the test's findings map neatly onto products the same company sells — the useful test tells you what is there, then lets the evidence decide what, if anything, follows. Persistent symptoms are a clinical matter, not a testing one.
Gut microbiome tests and microbiome supplements: reading the pairing
Searches for gut microbiome tests, the best microbiome testing, and microbiome supplements usually arrive together, and the pairing deserves scrutiny. A test describes what is present. Whether a gut microbiome supplement follows from that description depends on strain-level evidence,[6] not on the test having found something. Be most careful where the report's conclusions align neatly with the same company's catalogue.