Noninvasive In Vivo Estimation of HbA1c Based on the Beer–Lambert Model from Photoplethysmogram Using Only Two Wavelengths

Glycated hemoglobin (HbA1c) is the most important factor in diabetes control. Since HbA1c reflects the average blood glucose level over the preceding three months, it is unaffected by a patient’s activity level or diet before a test. Noninvasive HbA1c measurement reduces both the pain and complications associated with fingertip piercing to collect blood. Photoplethysmography is helpful for measuring HbA1c without blood samples. Herein, only two wavelengths (615 and 525 nm) were used to estimate HbA1c noninvasively, where two different ratio calibrations were applied and their performances were compared to a work that used three wavelengths. For the fingertip type, the Pearson’s r values for HbA1c estimates were 0.896 and 0.905, considering the ratio calibrations for the blood vessel and whole finger models, respectively. Using another value (HbA1c) calibration in addition to the ratio calibrations, we could improve this performance such that the Pearson’s r values of the HbA1c levels were 0.929 and 0.930 for the blood vessel and whole finger models, respectively. In a previous study, using three wavelengths, the Pearson’s r values were 0.916 and 0.959 for the blood vessel and whole finger models, respectively. Here, the RCF of the SpO2 estimation was 0.986 when the SpO2 ratio calibration was applied, while in a previous study, the RCF values of the SpO2 estimation were 0.983 and 0.986 for the blood vessel and whole finger models, respectively. Thus, we have shown that HbA1c estimation using only two wavelengths has a comparable performance to previous studies.