Quantitation of multiple markers by a single marker (QAMS) compared with the external standard method for quality control of Lactuca indica: a multi-level agreement assessment

This study evaluated a quantitative analysis of multi-components by single marker (QAMS) approach against the external standard method (ESM), using luteolin (LUT) as the single reference marker for the simultaneous quantification of chlorogenic acid (CGA), luteolin-7-O-glucuronide (L7G), and LUT in 68 sample units of Lactuca indica L. by high-performance liquid chromatography with diode-array detection (HPLC–DAD). The QAMS model was assessed for RCF repeatability and robustness and for marker-level agreement with ESM. The analyses also examined whether the observed deviations propagated to the multivariate structure or outlier-screening decisions; the paired analyses used 60 of the 68 units in which all three markers were quantifiable. The nominal RCFs were 0.9506 ± 0.0082 for CGA/LUT and 0.4747 ± 0.0070 for L7G/LUT. Back-calculation deviations for CGA and L7G did not exceed 2.07% under all investigated conditions. Compared with ESM, QAMS showed high marker-level agreement (Bland–Altman limits of agreement within ±7.1%), preserved the multivariate data structure of the two RCF-derived markers (m 2 = 0.00056; Procrustes correlation = 0.99972), and reproduced the primary three-marker ESM outlier-screening decisions at both thresholds, with one discordant unit emerging in a stricter two-marker sensitivity analysis. Within the evaluable set and under the investigated conditions, QAMS reproduced not only the marker contents but also the multivariate sample structure and the outlier-screening decisions of the ESM, supporting its use as a resource-efficient alternative to the ESM for multi-marker quality control of L. indica .