Bidirectional Conversion Models to Harmonize Total Phenolic and Flavonoid Content Analysis in Medicinal Plant Extracts

Accurate quantification of total phenolic content (TPC) and total flavonoid content (TFC) is essential for evaluating the quality and biological potential of medicinal plant extracts. However, variations in reference standards and assay conditions often lead to inconsistent data across laboratories. Therefore, this study aimed to develop bidirectional conversion models to harmonize TPC and TFC values obtained using different calibration standards and compare with previous reports. A total of 15 Indonesian medicinal plant extracts were analyzed using UV–Visible spectrophotometry, with seven standards for TPC (gallic acid, catechin, quercetin, rutin, apigenin, kaempferol, and myricetin) and five TFC (quercetin, rutin, apigenin, kaempferol, and myricetin). Linear regression models (R2 > 0.995) were established to interconvert values among these standards. The results showed that there was a strong positive correlation (r = 0.89) between TPC and TFC, while a weak negative relationship was observed with antioxidant activity (IC50). This indicated that only total content did not reliably predict antioxidant capacity. The bidirectional conversion models enabled reliable transformation of data obtained using different reference compounds, facilitating data harmonization across laboratories and studies. This method provided a practical tool for standardizing spectrophotometric assays in natural products and supported the development of consistent phytochemical databases as well as quality control systems for herbal materials.