The invisible bridge: how harmonization closes the gap between impossible standardization and clinical necessity.

Clinical laboratory testing plays a pivotal role in modern medicine, yet result variability across methods remains a major challenge. This review examines current approaches for achieving standardized and harmonized test results, with particular focus on ISO 17511's calibration hierarchies and ISO 21151 harmonization protocols. While standardization through metrological traceability to higher-order references remains ideal, many analytes require harmonization when reference materials or methods are unavailable. Key international organizations including IFCC, JCTLM, and ICHCLR have developed infrastructure supporting these efforts. Successful applications demonstrate significant improvements in inter-method comparability for various clinical assays. However, substantial challenges remain in achieving widespread harmonization, particularly in markets with high platform diversity. Addressing these limitations requires expanded development of commutable reference materials, advanced calibration algorithms, and robust quality monitoring systems. Future efforts focus on scalable solutions that maintain global comparability while addressing regional testing variations.