Why the Same Biomarker Gives Different Numbers

Once animal experiments make a TMAO pathway look causal, it is tempting to expect the human evidence to settle into one clean risk estimate. The updated clinical synthesis shows why that expectation fails. The key issue is not whether TMAO is associated with atrial fibrillation at all, but how much the association changes with the population, endpoint, and physiological setting. This chapter reads the meta-analysis as a map of that variation rather than as a universal TMAO score.

What the pooled estimate actually combines

The 2026 systematic review and meta-analysis retained 15 studies for qualitative synthesis. Nine supplied effect estimates suitable for quantitative analysis: eight contributed to the primary odds-ratio synthesis, and two contributed hazard ratios, with one study appearing in both analyses. The studies differed in design, population, disease background, TMAO contrast, and AF-related endpoint. Some followed people for years to observe incident AF; others examined postoperative AF, recurrence after ablation, AF progression, or outcomes in people who already had cardiovascular disease.

That study mix matters before any number is calculated. A cohort asking whether a baseline TMAO level predicts a first AF diagnosis is not asking the same question as a surgical study asking whether a preoperative level predicts arrhythmia during hospitalization. A recurrence study begins with an atrium that has already demonstrated an arrhythmic tendency. A prognostic study in established AF may be measuring disease progression or mortality rather than the initiation of AF. These questions can be related through biology, but they do not have the same baseline risk, time scale, or meaning of “event.”

The primary odds-ratio analysis nevertheless found a consistent overall direction. Across eight prospective estimates, higher versus lower circulating TMAO was associated with AF with a random-effects pooled odds ratio of 1.35 and a 95% confidence interval from 1.15 to 1.59. In plain language, the exposure categories with higher TMAO had higher odds of the relevant AF outcome on average across the included estimates. The phrase “on average” does essential work. It does not describe the probability that a particular person will develop AF, because that probability also depends on baseline risk, follow-up, endpoint definition, and the population from which the estimate came.

Nor is an odds ratio a treatment effect. If a study reports 1.35 for higher versus lower TMAO, it has not shown that lowering TMAO would reduce AF odds by 35%. The comparison is between naturally occurring exposure categories, usually with observational adjustment. It does not randomize the pathway, specify a safe way to lower it, or demonstrate that the lower category was produced by an intervention. The pooled result supports a relationship; it does not reverse itself into a prescription.

The review used a random-effects model because the studies were not assumed to share one identical underlying effect. That choice acknowledges that the true association may vary across settings. The resulting I² was 68.3%, indicating substantial between-study heterogeneity. Heterogeneity is not just a technical property of the forest plot. It means that the pooled estimate may be a summary across different biological and clinical situations rather than a description of a typical patient.

Several sources of variation are visible in the study set.