Color-resolved third harmonic generation microscopy for single-RBC HbA1c measurement and glycemic history assessment

The spectral absorption of biomolecules holds immense potential for revealing molecular identity, biological functions, and clinical diagnostics. Here, we present color-resolved third harmonic generation microscopy (cTHGM), a noninvasive method for molecular imaging based on absorption-enhanced THG response. Leveraging one single broadband femtosecond laser beam, cTHGM captures subtle absorption variations, including ~2-nanometer shifts in the Soret band, enabling precise distinction of glycated hemoglobin (HbA1c) from hemoglobin with negligible phototoxicity. We demonstrate that cTHGM measures HbA1c fractions in single red blood cells (RBCs) in vivo and ex vivo, providing noninvasive HbA1c measurement and insights into HbA1c distribution at the cellular level. In addition, cTHGM reconstructs historical glycemic trajectories by decoding single-RBC HbA1c distributions, offering a retrospective view of glycemic variability over months. This innovative method combines label-free imaging, high spatial and spectral resolution, and noninvasive manners, making it a promising tool for diabetes management, glycemic variability monitoring, and broader applications in precision medicine.