KnowYourPrime Evidence Registry · Finding KYP-ER-0019

Blood Oxygen Readings With Poor Circulation

Question assessedDoes poor circulation make a pulse oximeter or smartwatch blood oxygen reading unreliable?

KnowYourPrime evidence ratingUncertainEvidence is conflicting, inconsistent, or difficult to interpret.

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.

Finding KYP-ER-0019Version 1.2Reviewed October 24, 2026Rating rule 1.6CC BY 4.0

Why this is rated Uncertain

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.

How the rating rule reads this evidence

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.

Key results

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. (Poorzargar et al., Journal of Clinical Monitoring and Computing, 2022)
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. (Gudelunas et al., Anesthesia and Analgesia, 2024)
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. (Smatlak et al., American Journal of Critical Care, 1998)

What this finding covers

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 base

7 studies scored · 1 anchor · 3 conflicting · 3 supporting · Evidence last evaluated Oct 25, 2026

Anchor studies

The studies the rating rests on.

Accuracy of pulse oximeters in measuring oxygen saturation in patients with poor peripheral perfusion: a systematic review.Poorzargar et al., Journal of Clinical Monitoring and Computing, 2022. doi.org/10.1007/s10877-021-00797-8 [funding listed from a ResMed-endowed research chair]

Conflicting studies

Studies whose findings run against the claim.

Low Perfusion and Missed Diagnosis of Hypoxemia by Pulse Oximetry in Darkly Pigmented Skin: A Prospective Study.Gudelunas et al., Anesthesia and Analgesia, 2024. doi.org/10.1213/ANE.0000000000006755
Pulse oximetry: principles and limitations.Sinex et al., The American Journal of Emergency Medicine, 1999. doi.org/10.1016/s0735-6757(99)90019-0
Clinical evaluation of noninvasive monitoring of oxygen saturation in critically ill patients.Smatlak et al., American Journal of Critical Care, 1998. pubmed.ncbi.nlm.nih.gov/9740887/

Supporting studies

Studies consistent with the claim that do not carry the rating on their own.

Determining factors of pulse oximetry accuracy: a literature review.León-Valladares et al., Revista Clinica Espanola, 2024. doi.org/10.1016/j.rceng.2024.04.005
Accuracy of pulse oximetry in the intensive care unit.Louw et al., Intensive Care Medicine, 2002. doi.org/10.1007/s001340101064
Pulse oximetry and postoperative hypothermia. An evaluation of the Nellcor N-100 in a cardiac surgical intensive care unit.Gabrielczyk et al., Anaesthesia, 1988. doi.org/10.1111/j.1365-2044.1988.tb09025.x

Version history

Version	Date	Rating	Change
1.0	October 24, 2026	Uncertain	First published.
1.1	October 24, 2026	Uncertain	Opening restated in plain words; the size of the skin-tone difference stays in the key results with its study context.
1.2	October 24, 2026	Uncertain	Opening restores the skin-tone figures with their study and sample, under the revised opening rule.

Rating history on the research desk: Uncertain (September 29, 2026).

Cite this finding

KnowYourPrime Evidence Registry, Finding KYP-ER-0019, version 1.2, reviewed 2026-10-24. https://knowyourprime.com/evidence-blood-oxygen-poor-circulation

Machine-readable: JSON · Markdown · All findings

Research desk: the blood oxygen research desk entry for this question, with every scored study and its role
Related finding: Pulse Oximeter Accuracy and Skin Tone (KYP-ER-0002)
Related finding: Smartwatch Blood Oxygen (SpO2) Accuracy Against Arterial Blood (SaO2) (KYP-ER-0001)
On this site: Do Cold Hands or Poor Circulation Throw Off Blood Oxygen Readings?
On this site: Does Wearable SpO2 Work the Same for Everyone?

This is KnowYourPrime's own assessment of the published evidence, rated under the KnowYourPrime evidence methodology. The rating is set by a fixed rule from the roles of the studies above and moves only when the evidence or the rule changes. Every change to the rating or the text is published as a new dated version. To report an error, write to corrections@knowyourprime.com.