High Melanin Levels in Skin May Lead to Inaccurate Oxygen Measurements

The hemoglobin concentration and oxygen levels within the body are measured using Near-infrared spectroscopy (NIRS), a noninvasive technique. This method utilizes near-infrared light, mainly within the wavelength range of 650 to 900 nm, which penetrates biological tissues. The absorption characteristics of this light differ between oxygen-saturated hemoglobin (oxyhemoglobin) and oxygen-depleted hemoglobin (deoxyhemoglobin). By analyzing variations in the intensity of light that is either reflected or transmitted, NIRS facilitates the continuous monitoring of changes in the concentrations of these two hemoglobin forms, as well as the proportion of hemoglobin molecules that are oxygenated, thereby providing insights into oxygen saturation levels ().

However, NIRS measurements may demonstrate reduced accuracy in individuals with darker skin tones due to the absorption of near-infrared light by melanin, the pigment responsible for darker skin coloration. This absorption can diminish the intensity of light that penetrates the tissues beneath the skin, resulting in lower signal-to-noise ratios (SNRs) and potentially leading to erroneous assessments of hemoglobin concentration and oxygen saturation.

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Melanin in Skin: Cause of potential inaccurate NIRS measurements

In a study published in the Journal of Biomedical Optics (JBO), researchers led by Assistant Professor Sossena Wood from Carnegie Mellon University explored how varying levels of melanin affect the accuracy and reliability of NIRS measurements. “The study emphasizes the importance of considering the optical properties of melanin in NIRS technology and similar light-based technology development and application, advocating for adjustments in device operation to ensure accurate, inclusive outcomes for all skin types,” says Dr. Wood.

The researchers measured blood and tissue oxygen levels in the brains of 35 healthy adults aged 18 to 30 using a frequency-domain NIRS (FD-NIRS) system that operated at a modulation frequency of 110 MHz, employing near-infrared light with wavelengths of 690 nm and 830 nm. The participants represented a diverse range of skin tones, which were quantified using a DSM-III colorimeter—a noninvasive device that assesses melanin concentration via reflective spectroscopy.

The team then examined how melanin levels related to four key measurements derived from the NIRS data: signal quality-indicating SNR; arterial oxygen saturation (SpO2), which indicates the percentage of oxygen-carrying hemoglobin in the blood; tissue oxygen saturation (StO2), which measures oxygen-carrying hemoglobin in the tissues; and the optical properties represented by the absorption coefficient and reduced scattering coefficient, which quantify light absorption and scattering, respectively.

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Why Near-Infra Red (NIRS) measurements Need to Account for Differences in Skin Type?

The researchers found that individuals with higher melanin levels—particularly those with a melanin index above 56—experienced decreased SNR at 690 nm. This reduction occurs because melanin absorbs more light at this wavelength than at 830 nm. Higher melanin levels also led to lower SpO2 readings. However, melanin was not found to affect StO2 or the absorption or reduced scattering coefficients at either wavelength.

The findings indicate that as melanin levels increase, the accuracy of NIRS measurements decreases. This underscores the necessity of considering melanin when adjusting or calibrating NIRS data. Currently, most NIRS measurements account for differences in melanin using race, ethnicity, or the Fitzpatrick scale, which classifies skin types from Type I (very fair skin) to Type VI (very dark skin).

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However, these methods oversimplify skin tones since there are many different melanin levels within and among these groups. To enhance the reliability of NIRS data—particularly for individuals with darker skin tones—the researchers advocate for incorporating colorimeter measurements that accurately quantify a person’s melanin levels and using this device to recruit such skin tones in research studies. Furthermore, conducting NIRS measurements using specific wavelengths that are less absorbed by melanin can also improve measurement accuracy. Addressing these factors will ensure accurate assessments of blood oxygen levels and hemoglobin concentrations across all skin tones, leading to better health outcomes and equity in medical care for everyone.

Reference:

Exploring the impact and influence of melanin on frequency-domain near-infrared spectroscopy measurements - (https://www.spiedigitallibrary.org/journals/journal-of-biomedical-optics/volume-29/issue-S3/S33310/Exploring-the-impact-and-influence-of-melanin-on-frequency-domain/10.1117/1.JBO.29.S3.S33310.full#_=_)

Source-Eurekalert

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