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Decision-ready map
• Forward model: Hb + scattering (µa/µs′) vary with sex and tissue structure
• Inverse model: calibration/learning can embed subgroup priors
• Metrics: report worst-group + tail-risk near thresholds (not only RMSE)
• Uncertainty: quality gating + safe failure modes when coupling is poor
• Change control: revalidate after updates; monitor drift by subgroup
(1) What it is
Sex-linked biology perturbs both forward (µa/µs′, blood volume fraction) and inverse models (calibration/learning). Hb, scattering, and perfusion differences can shift feature distributions and SNR regimes in PPG/SpO₂, spectroscopy, and NIRS. Validate subgroup priors, tail-risk metrics near thresholds, uncertainty gating, and post-update drift monitoring.
(2) Who it helps
Engineers building sensors, signal processing, ML models, or decision systems consuming optical biosignals across devices and populations.
(3) What evidence exists
Population DRS shows sex-associated absorption/scattering (Jonasson et al. 2023; Hung et al. 2015). Microcirculation differences affect amplitude/dynamics (Samils et al. 2023). Sex-dependent PPG features (Dehghanojamahalleh & Kaya 2019) and best practices for PPG acquisition/processing (Charlton et al. 2022). Time-resolved NIRS shows sex differences in baseline oxygenated hemoglobin (Asahara & Matsukawa 2023).
(4) Translation barriers
Missing labels/mediators, confounding, device heterogeneity, mean-error metrics hiding tail risk, and lack of change control causing drift after updates.
(5) Equity/safety checks
Collect minimal necessary metadata; distinguish sex from gender; publish stratified distributions and worst-group metrics; implement uncertainty/quality gating and safe failure modes; monitor drift by subgroup post-deployment.
(6) Decision questions
• Which forward-model parameters are sex-sensitive and modeled?
• Do metrics capture tail-risk near thresholds and worst-group performance?
• Is metadata sufficient to diagnose domain shift (sex + Hb + device version)?
• How is uncertainty quantified and used to gate outputs?
• What triggers revalidation and how is drift monitored?
(7) Practical next steps
1) Simulate/bench-test Hb and scattering variation.
2) Evaluate worst-group + tail-risk metrics near thresholds; publish stratified results.
3) Separate coupling artifacts from physiology with quality flags.
4) Add observability + uncertainty gating.
5) Define change-control triggers and revalidate after updates.
(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