Hair, tattoos, scars, and cosmetics as optical confounders – investors brief

Decision-ready map

• Risk: confounders drive false alarms or false reassurance and complaints

• Ask for evidence: interference testing matrix + labeling strategy

• Regulatory: standards compliance + human factors + claims discipline

• Durability: postmarket monitoring of confounder-related failures

• Upside: better labeling + robustness can win procurement trust

(1) What it is

For investors, Theme 6 is about hidden product risk and differentiation. Hair, tattoos, scars, and cosmetics can push optical devices into failure modes that create false alarms, missed deterioration, user distrust, and support burden. Because many optical products are deployed at scale (RPM programs, consumer wearables, perioperative monitoring), small confounder rates become large absolute incident counts. The diligence question is: does the team treat confounders as first-class requirements (design + labeling + validation), or as afterthoughts handled by ‘user education’?

(2) Who it helps

This brief helps investors and strategic partners evaluating companies in wearables (PPG/SpO₂), spectroscopy diagnostics, and NIRS/tissue oximetry—particularly those making thresholded decision-support or clinical claims.

(3) What evidence exists

Published evidence shows confounders are real and device-relevant. Nail polish interference is quantified across many studies and summarized in a 2023 systematic review. Surgical dyes (patent blue V and related dyes) cause factitious desaturation and interfere with oximetry/co-oximetry, demonstrating that exogenous pigments can break optical inference. Scar tissue measurably changes optical parameters and can be tracked by DRS in keloids—implying that ‘normal skin’ calibration is insufficient. Tattoo pigments are chemically diverse and identifiable in situ by Raman; phantom studies show spectra depend on scattering matrices, which supports the need for confounder-aware spectroscopy validation. Cosmetics measurably alter surface optical properties; foundation-layer absorbance/transmittance can be estimated, and reflectance spectroscopy is used to assess topical product effects—reinforcing that topical layers can masquerade as physiologic change.

(4) Translation barriers

Companies often underinvest in confounder validation because it is ‘unsexy’ and expensive: creating test matrices, recruiting tattooed participants, and performing human factors studies. Another barrier is claims creep: teams market ‘clinical-grade’ performance while validation excludes confounders. Finally, update risk is significant in algorithmic products: an ML update can unintentionally worsen robustness to cosmetics or tattoos unless there is strict change control.

(5) Equity/safety checks

Confounder robustness can become an equity and market-access issue: if devices fail more often on tattooed skin, scarred sites, or with common cosmetics, the product can exclude segments or create uneven safety. Regulators and purchasers increasingly expect transparent limitations and evidence; confounder-aware labeling can reduce liability and improve procurement trust.

(6) Decision questions

• Does the company have an interference test plan (polish, dyes, tattoos, scars, cosmetics, hair/contact) tied to intended use?

• Are failure rates (‘unable to measure’) and tail-risk near thresholds reported, not just mean accuracy?

• Is labeling/UI explicit about confounders and alternative sites/workflows?

• Does the QMS define revalidation triggers after updates and monitor field complaints by confounder type?

• Could robust confounder handling be a differentiator in procurement, reimbursement, or partnerships?

(7) Practical next steps

1) Add a confounder diligence section: interference matrix, human factors, labeling text, and postmarket monitoring.

2) Make confounder-inclusive validation a milestone before scale-up contracts or reimbursement applications.

3) Require evidence that commercial BOM/firmware matches validation configuration.

4) Evaluate downside: support burden, complaints, and recalls from confounder-driven errors.

5) Evaluate upside: vendors who quantify and label confounders can win trust and reduce liability.

(8) References

Aggarwal AN, Agarwal R, Dhooria S, et al. Impact of Fingernail Polish on Pulse Oximetry Measurements: A Systematic Review. Respiratory Care. 2023.

https://doi.org/10.4187/respcare.10399

Yeganehkhah M, Dadkhahtehrani T, Bagheri AR, Kachoie A. Effect of Glittered Nail Polish on Pulse Oximetry Measurements in Healthy Subjects. Iran J Nurs Midwifery Res. 2019.

https://doi.org/10.4103/ijnmr.IJNMR_176_17

Hueter L, Schwarzkopf K, Karzai W. Interference of patent blue V dye with pulse oximetry and co-oximetry. Eur J Anaesthesiol. 2005.

https://doi.org/10.1017/S0265021505230818

Howard JD, Moo V, Sivalingam P. Anaphylaxis and other adverse reactions to blue dyes: a case series. Anaesth Intensive Care. 2011.

https://doi.org/10.1177/0310057X1103900221

Piñero A, Illana J, García-Palenciano C, et al. Effect on Oximetry of Dyes Used for Sentinel Lymph Node Biopsy. Arch Surg. 2004.

https://doi.org/10.1001/archsurg.139.11.1204

Poon KWC, Dadour IR, McKinley AJ. In situ chemical analysis of modern organic tattooing inks by micro-Raman spectroscopy. J Raman Spectrosc. 2008.

https://doi.org/10.1002/jrs.1973

Sadura F, Wróbel MS, Karpienko K. Colored Tattoo Ink Screening Method with Optical Tissue Phantoms and Raman Spectroscopy. Materials (Basel). 2021.

https://doi.org/10.3390/ma14123147

Hsu C-K, Tzeng S-Y, Yang C-C, et al. Non-invasive evaluation of therapeutic response in keloid scar using diffuse reflectance spectroscopy. Biomed Opt Express. 2015.

https://doi.org/10.1364/BOE.6.000390

Tseng S-H, Hsu C-K, Lee JY-Y, et al. Noninvasive evaluation of collagen and hemoglobin in keloid scars using DRS. J Biomed Opt. 2012.

https://doi.org/10.1117/1.JBO.17.7.077005

Yoshida K, Okiyama N. Estimation of reflectance/transmittance/absorbance of cosmetic foundation layer on skin. Opt Express. 2021.

https://doi.org/10.1364/oe.442219

Mancuso A, d’Avanzo ND, Cristiano MC, Paolino D. Reflectance spectroscopy to explore skin reactions to topical products. Front Chem. 2024.

https://doi.org/10.3389/fchem.2024.1422616

Kim KB, Baek HJ. Photoplethysmography in Wearable Devices: A Comprehensive Review. Electronics. 2023.

https://doi.org/10.3390/electronics12132923

Cooksey CC, Allen DW, Tsai BK. Reference Data Set of Human Skin Reflectance. J Res Natl Inst Stan. 2017.

https://doi.org/10.6028/jres.122.026

Cooksey CC, Allen DW, Tsai BK. Reference Data Set of Human Skin Reflectance (data). NIST. 2017.

https://doi.org/10.18434/M38597

IEC. ISO 80601-2-61:2026 Pulse oximeter equipment — safety and essential performance.

https://webstore.iec.ch/en/publication/74527