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Decision-ready map
• Light-based readings are estimates (not the same as a lab/blood test)
• Biology (Hb, pregnancy, circulation) can shift readings and baselines
• Ask whether the device was tested in women and men
• Know the mismatch plan: symptoms override a single number
• Ask how your results will be reported and protected
(1) What it is
Biophotonic technologies use light to estimate things like oxygen levels (SpO₂), blood flow patterns (PPG), tissue oxygenation (NIRS), or biochemical composition (spectroscopy including fluorescence and Raman). These are estimates, not direct measurements like a blood test. Biological sex and life stages can shift the readings because hemoglobin levels, skin structure, and circulation can differ. The goal of validation is to ensure devices work safely and fairly for everyone who will use them.
(2) Who it helps
This brief is for patients and community members participating in research studies or pilot programs that use wearable PPG/SpO₂, tissue spectroscopy (DRS/fluorescence/Raman), or NIRS/tissue oximetry. It is also useful if you use a home monitor where a reading might influence when to seek care.
(3) What evidence exists
Studies show that optical signals can differ between women and men. PPG research has found sex-related differences in certain waveform features (Dehghanojamahalleh & Kaya 2019). Large studies that estimate skin optical properties from diffuse reflectance show that absorption and scattering can differ across sex, which matters because devices interpret light using these properties (Jonasson et al. 2023). Studies of skin microcirculation show differences in blood volume and perfusion between sexes, which can affect signal strength and reliability (Samils et al. 2023). In brain monitoring, time-resolved NIRS has reported lower baseline oxygenated hemoglobin levels in women compared with men (Asahara & Matsukawa 2023). Skin fluorescence studies also report sex differences in autofluorescence intensity (Morvová et al. 2018). WHO guidance notes that hemoglobin ‘normal’ ranges differ by sex and pregnancy status (WHO 2024), which can affect light absorption.
(4) Translation barriers
Many other factors can change readings for anyone: movement, cold hands, poor sensor contact, swelling, nail products, and different device models. Some studies record sex but do not measure the reasons a reading might differ (like hemoglobin), so results can be unclear.
(5) Equity/safety checks
You can help protect fairness and safety by asking how the device was tested. Ask whether the study includes women and men, whether results will be reported by sex, and what happens for pregnancy or anemia. Your privacy matters: the study should explain what personal data it collects and why. And for safety, you should know the mismatch plan—what to do if you feel worse even when the number looks ‘okay’.
(6) Decision questions
• Was this device tested in women and men, and will results be reported by sex?
• If I’m pregnant, anemic, or on hormone therapy, what evidence exists and what extra safeguards apply?
• What should I do if symptoms worsen but the reading looks normal (the mismatch plan)?
• How does the study handle poor-quality signals (movement, cold extremities, poor contact)?
• Who do I contact if readings seem wrong or if I have questions about my results?
(7) Practical next steps
1) Request a plain-language information sheet describing what the device estimates and its limits.
2) Learn correct use (stay still, warm hands, good contact) and repeat a reading if it seems inconsistent.
3) Keep track of symptoms and context and report mismatches to the study team promptly.
4) Ask how results will be reported (including by sex) and how privacy is protected.
5) Make sure there is a clear escalation plan and access to confirmatory assessment when needed.
(8) References
Dehghanojamahalleh S, Kaya M. Sex-Related Differences in Photoplethysmography Signals Measured From Finger and Toe. IEEE J Transl Eng Health Med. 2019;7:1900607.
https://doi.org/10.1109/JTEHM.2019.2938506
Charlton PH, Pilt K, Kyriacou PA. Establishing best practices in photoplethysmography signal acquisition and processing. Physiol Meas. 2022;43(5):050301.
https://doi.org/10.1088/1361-6579/ac6cc4
Jonasson H, Fredriksson I, Bergstrand S, et al. Absorption and reduced scattering coefficients in epidermis and dermis from a Swedish cohort study. J Biomed Opt. 2023;28(11):115001.
https://doi.org/10.1117/1.JBO.28.11.115001
Samils L, Henricson J, Strömberg T, Fredriksson I, Iredahl F. Workload and sex effects in comprehensive assessment of cutaneous microcirculation. Microvasc Res. 2023;148:104547.
https://doi.org/10.1016/j.mvr.2023.104547
Asahara R, Matsukawa K. Prefrontal oxygenation is quantified with time-resolved NIRS: effect of sex on baseline oxygenation and response during exercise. Am J Physiol Regul Integr Comp Physiol. 2023;325:R31–R44.
https://doi.org/10.1152/ajpregu.00048.2023
Morvová M Jr, Jeczko P, Šikurová L. Gender differences in the fluorescence of human skin in young healthy adults. Skin Res Technol. 2018;24(4):599–605.
https://doi.org/10.1111/srt.12471
Hung C-H, Chou T-C, Hsu C-K, Tseng S-H. Broadband absorption and reduced scattering spectra of in-vivo skin using δ-P1 approximation. Biomed Opt Express. 2015;6(2):443–456.
https://doi.org/10.1364/BOE.6.000443
Staritzbichler R, Hunold P, Estrela-Lopis I, et al. Raman spectroscopy on blood serum samples of patients with end-stage liver disease. PLoS One. 2021;16(9):e0256045.
https://doi.org/10.1371/journal.pone.0256045
WHO. Guideline on haemoglobin cutoffs to define anaemia in individuals and populations. 2024.
https://www.who.int/publications/i/item/9789240088542
NIH Office of Research on Women’s Health. Sex as a Biological Variable (SABV).
https://orwh.od.nih.gov/sex-as-biological-variable
FDA. Evaluation of Sex-Specific Data in Medical Device Clinical Studies (final guidance). March 2025.
https://www.fda.gov/regulatory-information/search-fda-guidance-documents/evaluation-sex-specific-data-medical-device-clinical-studies-guidance-industry-and-food-and-drug