{
  "registry": "KnowYourPrime Evidence Registry",
  "license": {
    "name": "CC BY 4.0",
    "url": "https://creativecommons.org/licenses/by/4.0/",
    "attribution": "Credit KnowYourPrime and link to the finding URL."
  },
  "fields": {
    "devices": "Devices this finding names, including models it says it does not cover. What was actually tested is in scope.applies_to; exclusions are in scope.does_not_cover.",
    "evidence": "Studies by role: anchor (the rating rests on it), conflicting, null_result, supporting.",
    "industry_ties": "Device-maker ties listed in Europe PMC metadata: author_affiliations names makers an author lists as an affiliation, funders names makers listed as funding. stated_ties holds ties a paper states only in its own conflict, funding or acknowledgement text, each read from that statement and worded to say only what it says. Sources are Europe PMC metadata, funding printed in the abstract, and those reviewed statements. An empty list means no tie was found there, not that the study is independent: many papers have no open full text to read. Metadata, not a judgment of the study. checked false means no Europe PMC metadata was available."
  },
  "id": "KYP-ER-0019",
  "version": "1.2",
  "reviewed": "2026-10-24",
  "url": "https://knowyourprime.com/evidence-blood-oxygen-poor-circulation",
  "title": "Blood Oxygen Readings With Poor Circulation",
  "question": "Does poor circulation make a pulse oximeter or smartwatch blood oxygen reading unreliable?",
  "measurement": "Blood oxygen (SpO2)",
  "devices": [
    "Pulse oximeter",
    "Smartwatch"
  ],
  "rating": {
    "label": "Uncertain",
    "code": "uncertain",
    "definition": "Evidence is conflicting, inconsistent, or difficult to interpret.",
    "rule_version": "1.6",
    "rule_reasons": [
      "Systematic-review or meta-analytic evidence affirms the core relationship.",
      "Multiple supporting studies are consistent with the core relationship.",
      "The claim rests on a single anchor whose finding affirms it; a second independent affirming anchor is needed to reach Established.",
      "A clean anchor supports this claim, but an unresolved conflicting study contradicts it on the core relationship, so the evidence is treated as in genuine dispute.",
      "Some evidence conflicts on secondary points that do not change the core direction."
    ]
  },
  "conclusion": "On its own, poor circulation is handled reasonably well by modern clinical oximeters, but combined with darker skin the readings miss low oxygen far more often: in a laboratory study of 146 people under low perfusion, low oxygen went undetected in 21.1 percent of readings with dark skin against 1.1 percent with light skin.",
  "reasoning": [
    "A systematic review of patients with poor perfusion found oximeters accurate, especially newer models and earlobe sensors, which is the clean anchor. It also noted that only one of its studies controlled for skin pigmentation.",
    "A prospective study built to fill that gap contradicts the claim on its core: low perfusion and darker skin together produced clinically significant high readings. With both present, the rule rates the claim Uncertain."
  ],
  "key_results": [
    {
      "text": "A systematic review of 22 studies in adults with poor peripheral perfusion, each compared with arterial blood gas, concluded oximeters were accurate, particularly newer models and earlobe placement.",
      "source": "doi:10.1007/s10877-021-00797-8",
      "source_url": "https://doi.org/10.1007/s10877-021-00797-8",
      "source_citation": "Poorzargar et al., Journal of Clinical Monitoring and Computing, 2022"
    },
    {
      "text": "In 146 healthy subjects stratified by skin type, missed hypoxemia under low perfusion occurred 1.1 percent of the time for light skin, 8.2 percent for medium and 21.1 percent for dark.",
      "source": "doi:10.1213/ane.0000000000006755",
      "source_url": "https://doi.org/10.1213/ANE.0000000000006755",
      "source_citation": "Gudelunas et al., Anesthesia and Analgesia, 2024"
    },
    {
      "text": "In 46 critically ill patients over 48 hours, oximetry bias against co-oximetry was -2.29 percent, and with abnormal cardiac index the oximeter overestimated saturation by up to 7 points.",
      "source": "pmid:9740887",
      "source_url": "https://pubmed.ncbi.nlm.nih.gov/9740887/",
      "source_citation": "Smatlak et al., American Journal of Critical Care, 1998"
    }
  ],
  "scope": {
    "applies_to": "Clinical pulse oximeters in adults with poor peripheral circulation, compared with arterial blood.",
    "does_not_cover": "Direct tests of consumer smartwatches with poor circulation, which are scarce; cold hands during everyday use, except as a cause manufacturers name; children."
  },
  "limitations": [
    "Most evidence is from hospital and laboratory oximeters, not wrist wearables.",
    "Perfusion was defined and measured differently across studies."
  ],
  "evidence": {
    "anchor": [
      {
        "key": "doi:10.1007/s10877-021-00797-8",
        "title": "Accuracy of pulse oximeters in measuring oxygen saturation in patients with poor peripheral perfusion: a systematic review.",
        "first_author": "Poorzargar",
        "journal": "Journal of clinical monitoring and computing",
        "year": 2022,
        "url": "https://doi.org/10.1007/s10877-021-00797-8",
        "industry_ties": {
          "checked": true,
          "author_affiliations": [],
          "funders": [
            "a ResMed-endowed research chair"
          ],
          "stated_ties": []
        }
      }
    ],
    "conflicting": [
      {
        "key": "doi:10.1213/ane.0000000000006755",
        "title": "Low Perfusion and Missed Diagnosis of Hypoxemia by Pulse Oximetry in Darkly Pigmented Skin: A Prospective Study.",
        "first_author": "Gudelunas",
        "journal": "Anesthesia and analgesia",
        "year": 2024,
        "url": "https://doi.org/10.1213/ANE.0000000000006755",
        "industry_ties": {
          "checked": true,
          "author_affiliations": [],
          "funders": [],
          "stated_ties": []
        }
      },
      {
        "key": "doi:10.1016/s0735-6757(99)90019-0",
        "title": "Pulse oximetry: principles and limitations.",
        "first_author": "Sinex",
        "journal": "The American journal of emergency medicine",
        "year": 1999,
        "url": "https://doi.org/10.1016/s0735-6757(99)90019-0",
        "industry_ties": {
          "checked": true,
          "author_affiliations": [],
          "funders": [],
          "stated_ties": []
        }
      },
      {
        "key": "pmid:9740887",
        "title": "Clinical evaluation of noninvasive monitoring of oxygen saturation in critically ill patients.",
        "first_author": "Smatlak",
        "journal": "American journal of critical care : an official publication, American Association of Critical-Care Nurses",
        "year": 1998,
        "url": "https://pubmed.ncbi.nlm.nih.gov/9740887/",
        "industry_ties": {
          "checked": false
        }
      }
    ],
    "supporting": [
      {
        "key": "doi:10.1016/j.rceng.2024.04.005",
        "title": "Determining factors of pulse oximetry accuracy: a literature review.",
        "first_author": "León-Valladares",
        "journal": "Revista clinica espanola",
        "year": 2024,
        "url": "https://doi.org/10.1016/j.rceng.2024.04.005",
        "industry_ties": {
          "checked": true,
          "author_affiliations": [],
          "funders": [],
          "stated_ties": []
        }
      },
      {
        "key": "doi:10.1007/s001340101064",
        "title": "Accuracy of pulse oximetry in the intensive care unit.",
        "first_author": "Van de Louw",
        "journal": "Intensive care medicine",
        "year": 2002,
        "url": "https://doi.org/10.1007/s001340101064",
        "industry_ties": {
          "checked": true,
          "author_affiliations": [],
          "funders": [],
          "stated_ties": []
        }
      },
      {
        "key": "doi:10.1111/j.1365-2044.1988.tb09025.x",
        "title": "Pulse oximetry and postoperative hypothermia. An evaluation of the Nellcor N-100 in a cardiac surgical intensive care unit.",
        "first_author": "Gabrielczyk",
        "journal": "Anaesthesia",
        "year": 1988,
        "url": "https://doi.org/10.1111/j.1365-2044.1988.tb09025.x",
        "industry_ties": {
          "checked": true,
          "author_affiliations": [],
          "funders": [],
          "stated_ties": []
        }
      }
    ]
  },
  "evidence_last_evaluated": "Oct 25, 2026",
  "rating_history": [
    {
      "date": "2026-09-29",
      "rating": "Uncertain",
      "from": null,
      "rule_version": "1.6"
    }
  ],
  "versions": [
    {
      "version": "1.0",
      "date": "2026-10-24",
      "rating": "Uncertain",
      "change": "First published."
    },
    {
      "version": "1.1",
      "date": "2026-10-24",
      "rating": "Uncertain",
      "change": "Opening restated in plain words; the size of the skin-tone difference stays in the key results with its study context."
    },
    {
      "version": "1.2",
      "date": "2026-10-24",
      "rating": "Uncertain",
      "change": "Opening restores the skin-tone figures with their study and sample, under the revised opening rule."
    }
  ],
  "research_desk": "https://knowyourprime.com/research-desk-spo2#poor_perfusion",
  "related_findings": [
    "https://knowyourprime.com/evidence-pulse-oximeter-skin-tone-bias",
    "https://knowyourprime.com/evidence-wearable-blood-oxygen-accuracy"
  ],
  "related_articles": [
    "https://knowyourprime.com/does-poor-circulation-affect-blood-oxygen-sensor-accuracy",
    "https://knowyourprime.com/does-your-wearables-blood-oxygen-reading-work-the-same-for-everyone"
  ],
  "methodology": "https://knowyourprime.com/methodology",
  "citation": "KnowYourPrime Evidence Registry, Finding KYP-ER-0019, version 1.2, reviewed 2026-10-24. https://knowyourprime.com/evidence-blood-oxygen-poor-circulation"
}