Inter‐finger Variability and Other Influencing Factors for Using Reflectance Spectroscopy for Skin Carotenoid Detection
Authors: Whigham Leah D , Redelfs Alisha H 
Published: November 2023

Background

Carotenoids have long been accepted as the best biomarker for fruit and vegetable intake. Recently, resonance Raman spectroscopy (RRS) and reflectance spectroscopy (RS) have been shown as valid methods of measuring skin carotenoids non‐invasively. Researchers using either method need to know what conditions influence the variability of the measurements. Carotenoid levels are known to vary widely across tissue sites, mostly because of the varying thickness of each tissue site. Skin chromophores, such as melanin, oxy and deoxy‐hemoglobin, influence carotenoid measurements. In addition, stained or unclean skin will cause variability in the measurement. Our objective was to assess inter‐finger variability and potential surface staining from foods commonly eaten in our region while using RS to measure skin carotenoids.

Methods

RS was used to measure carotenoid Optical Density (OD) across all fingers under clean conditions (alcohol wipe used to clean the fingers prior to measurement) (n=10), as well as after handling chips coated with a powder containing chile peppers (which contain carotenoids), then washing hands and cleaning the finger with an alcohol wipe (n=9). Following the initial post‐chip cleaning measurement, an additional cleaning step was done by rubbing the finger with canola oil (to solubilize the carotenoids) followed by thorough cleaning with an alcohol wipe.

Results

OD varied across fingers, with the thumb and index finger both typically showing higher mean OD values than the ring and pinky finger (a 15% difference, p<0.001). Participants had as high as a 49% difference in OD across fingers on one hand. On average, right hands had a slightly higher OD than left hands when controlling for which finger was used. Finally, there was a 31% increase in mean OD on right index fingers after handling chips (from 0.13 ± 0.01 SE; to 0.19 ± 0.04 SE). Treatment condition explained 98% of this increase. Following the oil plus alcohol wipe cleaning, measurements returned to within 2%–34% of pre‐chip levels.

Conclusions

As expected, a high level of OD variability was found across fingers. Additionally, the coating from chips resulted in temporary staining of the fingers (even after initial hand washing) that influenced the carotenoid measurement of a finger, introducing non‐systematic error to data. As with any optical method, these effects are not surprising since additional absorbers were present besides the carotenoids of interest. Additional cleaning steps further reduced the staining of the skin, but more research is needed to determine best cleaning approaches. Next steps could include the development and integration of a contaminant detection function to the RS instruments to strengthen confidence in OD values and identify cases where more extensive cleaning is needed. In the meantime, use of RS (and other optical methods) requires a standardized approach that uses the same tissue site (e.g. same finger) across multiple time points within an individual and minimizes the influence of potential factors that could stain or otherwise impact the OD measurements of the skin.

Support or Funding Information

Support for this research was provided by the Paso del Norte Health Foundation.