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

Decision-ready map

• Before trusting a number: check site for polish, tattoo ink, thick hair, scar tissue

• If confounder present: move sensor (earlobe/toe/alternate site) and re-check

• Use quality indicators; don’t use unstable PR as arrhythmia evidence

• In OR: anticipate dye interference (patent blue/isosulfan/methylene blue)

• Document confounders and confirm with reference tests when stakes are high

(1) What it is

Hair, tattoos, scars, and cosmetics are ‘surface confounders’—they sit in or on the optical path and can distort light delivery/collection. In pulse oximetry and wearable PPG/SpO₂, confounders change how red/IR/green light reaches pulsatile blood, reducing signal-to-noise or shifting the red/IR ratio. In tissue spectroscopy (DRS/fluorescence/Raman), surface layers alter baseline reflectance/fluorescence and can masquerade as physiologic change. In NIRS/tissue oximetry, altered scattering and absorption in scar tissue or pigment can bias regional saturation (rSO₂) baselines and trends. Theme 6 is about translating this into practical clinical workflow: recognize confounders, relocate the measurement, interpret quality flags, and document/label limitations so ‘numbers’ do not become false reassurance or false alarms.

(2) Who it helps

This brief helps clinicians who use optical signals as decision gates: oxygen escalation, discharge/triage, intraoperative monitoring, tissue perfusion checks, or monitoring cerebral/tissue oxygenation. It is especially relevant in perioperative settings (where surgical dyes can cause factitious desaturation), and in outpatient/remote monitoring where cosmetics and tattoos are common and staff may not inspect the measurement site.

(3) What evidence exists

Evidence shows multiple confounder pathways. Fingernail polish can marginally reduce or sometimes prevent SpO₂ measurement; a 2023 systematic review found small color-dependent effects and occasional failed readings for dark colors. Nail polish can also destabilize the pulse waveform and create misleading pulse-rate displays. Surgical dyes used for sentinel node mapping (e.g., patent blue V, isosulfan blue, methylene blue) can interfere with pulse oximetry and co-oximetry, causing apparent desaturation without a true drop in arterial oxygenation. Tattoos introduce pigments with specific absorption and Raman/fluorescence signatures; Raman studies demonstrate that ink composition varies and can be identified in situ, implying that spectral baselines can be pigment-driven. Scar tissue measurably changes optical properties: DRS studies in keloid scars report differences in collagen/water content and scattering/oxygenation estimates compared with normal skin, and DRS can track response over time. Cosmetics such as foundation measurably change reflectance/transmittance/absorbance of a surface layer; methods exist to estimate foundation-layer optical properties, and reflectance spectroscopy is widely used to quantify skin response to topical products—highlighting why cosmetics can change measured spectra even when physiology is unchanged.

(4) Translation barriers

Clinical barriers are usually workflow and documentation: (1) confounders are not systematically checked (clear polish/gel can be hard to notice); (2) staff may not know that ‘pulse rate’ on a pulse oximeter is derived from PPG and can be erratic with optical interference; (3) devices differ in robustness, so a workaround that works in one unit may fail in another; (4) in spectroscopy, baseline shifts from cosmetics or tattoos can be misread as inflammation, oxygenation change, or biochemical differences unless the site is cleaned and documented; (5) in NIRS, scar tissue or pigment at/near the probe can shift baseline rSO₂, making cross-patient comparisons unsafe without a reference site.

(5) Equity/safety checks

Equity/safety checks should treat confounders as ‘measurement conditions’ rather than patient ‘noncompliance.’ If a patient cannot remove cosmetics (cultural, occupational, or personal reasons), protocols should offer alternative sites (earlobe/toe/forehead) rather than excluding the patient or accepting low-quality data. Labeling and patient instructions should be clear and non-stigmatizing: ‘If tattoos/polish/scars are present at the site, move the sensor.’ For high-stakes decisions (ICU escalation, OR events), use confirmatory tests when optical signals are unstable or discordant with the clinical picture.

(6) Decision questions

• Do our protocols require site inspection (polish/gel, tattoos, scars, heavy hair) before relying on SpO₂/PPG/NIRS?

• Do we have an approved alternative-site workflow (earlobe/toe/forehead) and staff training for it?

• Are we distinguishing ‘true desaturation’ from dye-related factitious desaturation in the OR?

• Do we document confounders and device model/firmware for quality improvement audits?

• When signals are low-quality or discordant, what confirmatory pathway is triggered (ABG/co-oximetry, clinical reassessment, repeat measurement)?

(7) Practical next steps

1) Add a one-line site-check to SOPs: inspect/remove cosmetics if feasible; note tattoos/scars/hair and relocate if needed.

2) Standardize alternative-site practice (earlobe/toe/forehead) and train staff on correct placement and quality flags.

3) In surgery/sentinel node procedures, brief teams that blue dyes can cause factitious desaturation; confirm with ABG if clinically concerning.

4) For spectroscopy/NIRS, clean the site when possible and document tattoos/scars; use a contralateral reference site for trends.

5) Audit cases where confounders triggered escalation or missed events; feed results into procurement (request interference testing and labeling from vendors).

(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