Standardizing the 13C-Methacetin Breath Test: A Call for Clinical Integration in Liver Function Testing

Background/Objectives: The 13C-Methacetin Breath Test (MBT) is a non-invasive tool to evaluate hepatic microsomal function via exhaled 13CO2, reflecting cytochrome P450 1A2 (CYP1A2)-mediated metabolism. Despite decades of evidence demonstrating its utility in diagnosing cirrhosis, stratifying liver disease severity, and predicting outcomes, MBT adoption remains limited due to methodological inconsistencies and variable diagnostic thresholds. This review aimed to summarize MBT data in adults and assess its diagnostic and prognostic performance. Methods: A literature review was conducted using PubMed, Web of Science, and Scopus. Eligible studies included those applying oral or intravenous methacetin with defined reference values or diagnostic cutoffs. Outcomes of interest were percent dose recovery (PDR), cumulative PDR (cPDR), and LiMAx® values. Due to heterogeneity in protocols, units, and endpoints, results were synthesized narratively. Results: Healthy individuals typically demonstrated rapid metabolism (e.g., cPDR30 10–15%), whereas cirrhotic patients showed significantly reduced values (e.g., cPDR30 ≈ 1%). Diagnostic cutoffs varied widely (<0.35% to <8%), reflecting methodological and population differences. MBT reliably identified advanced liver disease but showed inconsistent sensitivity for early-stage fibrosis and metabolic dysfunction-associated steatotic liver disease. Variability in dosing, timing, measurement duration, and analytic technique limited cross-study comparability. Conclusions: MBT is a validated, dynamic marker of liver function with both diagnostic and prognostic relevance. However, inconsistent protocols and thresholds hinder its clinical implementation. Standardization of MBT procedures, reference ranges, and reporting metrics is essential. A harmonized protocol (“MBT-60”), supported by multicenter validation, demographic stratification, and direct comparison with structural and serologic liver tests, is recommended to facilitate MBT integration into routine hepatology practice.