Can the Firmicutes-to-Bacteroidetes ratio hide abnormal bacterial abundance and intestinal inflammation?

The claim is largely correct: the Firmicutes:Bacteroidetes (F:B) ratio is a proportional microbiome summary, not a measure of absolute bacterial quantity or intestinal inflammatory activity.

Abundance and ratio interpretation

• A normal F:B ratio can coexist with elevated Firmicutes if Bacteroidetes increases proportionally; the same is mechanistically plausible for Bacteroidetes. The ratio therefore cannot establish whether either phylum—or total bacterial biomass—is increased, decreased, or unchanged.
• This limitation follows from compositional data: relative abundances must sum to a fixed total, so an apparent shift in one group can arise from change elsewhere in the community or from altered overall microbial load.
• Quantitative profiling is required to assess absolute abundance, using measures such as qPCR, flow-cytometric cell counts, or spike-in standards. In quantitative fecal microbiome work, microbial load varied by up to tenfold even among healthy individuals, and incorporating load changed inferred ecological and disease-associated patterns.
• “Normal” and “elevated” remain assay-dependent terms; a ratio result alone does not provide validated reference-based evidence of bacterial expansion.

Inflammation

• The F:B ratio does not measure, exclude, or reliably monitor intestinal inflammation. Phylum-level associations with inflammatory bowel disease are inconsistent and are influenced by disease phenotype, treatment, sample site, sequencing method, and analytic approach.
• There are no validated prospective thresholds or diagnostic-performance data linking an F:B ratio to fecal calprotectin, endoscopic activity, or histologic inflammation.
• Fecal calprotectin is a sensitive noninvasive marker that tracks endoscopic activity, although it is not specific to IBD. Endoscopy with histology remains the reference assessment when mucosal or microscopic inflammation must be established.

Bottom line

• A normal F:B ratio can mask abnormal absolute microbial abundance and should not be used to infer intestinal inflammatory status. It is an exploratory relative-abundance metric, not a stand-alone clinical test.