Guide — Clinical Skills
Pulse Oximetry & Co-oximetry
Pulse oximetry is the most trusted bedside number and one of the easiest to misread. This guide explains the two-wavelength principle, its accuracy envelope, the conditions that make it lie, and exactly when only co-oximetry gives the truth.
8 min read · Clinical Skills
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Pulse oximetry (SpO₂) is a noninvasive estimate of arterial oxygen saturation. It shines two wavelengths of light — red (660 nm) and infrared (940 nm) — through a pulsatile vascular bed and computes the ratio absorbed by oxygenated versus deoxygenated hemoglobin. The result is reported as SpO₂, a percentage that most clinicians treat as a real-time window into the patient’s oxygen status.
Co-oximetry is the laboratory gold standard. It uses multiple wavelengths on an arterial blood sample to measure the actual fractions of all hemoglobin species, including carboxyhemoglobin (COHb) and methemoglobin (MetHb). Where pulse oximetry offers speed and convenience, co-oximetry offers completeness — and the two are not interchangeable when a dyshemoglobinemia is in play.
Key Concepts
- Two wavelengths, two hemoglobins.SpO₂ is built on the assumption that only oxyhemoglobin and deoxyhemoglobin are present. Any additional hemoglobin species the device cannot distinguish is invisible — and that invisibility is clinically dangerous.
- Carbon monoxide poisoning. COHb absorbs red light (660 nm) almost identically to oxyhemoglobin. The pulse oximeter cannot tell them apart, so SpO₂ reads falsely normal or high while the patient may be profoundly hypoxic. Co-oximetry is required to diagnose and quantify COHb.
- Methemoglobinemia.MetHb causes the SpO₂ reading to drift toward approximately 85% and pin there, regardless of the true saturation. Whether the patient’s true SaO₂ is 98% or 60%, the monitor will display near 85%.
- Accuracy envelope.In the 70–100% saturation range, SpO₂ is typically within ±2–3% of the true SaO₂. Accuracy decreases below 70–80%, where most devices are no longer calibrated against reference co-oximetry data.
- Causes of inaccuracy. Poor perfusion (hypotension, vasoconstriction, hypothermia), motion artifact, nail polish (especially blue, green, or black), bright ambient light, severe anemia, intravenous dyes such as methylene blue, and darker skin pigmentation can all degrade or falsely elevate the SpO₂ reading.
- Occult hypoxemia. Research has shown that pulse oximetry can overestimate SaO₂ in patients with darker skin pigmentation, particularly at saturations below 90%. This can mask clinically significant hypoxemia that would be detected by co-oximetry.
Accuracy at a Glance
| Condition | SpO₂ Accuracy | Notes |
|---|---|---|
| SpO₂ 70–100% | Within ±2–3% of true SaO₂ | Clinically acceptable; standard operating range |
| SpO₂ below 70–80% | Less reliable | Calibration range of most devices does not extend here |
| Carbon monoxide poisoning | Falsely normal or high | COHb reads like oxyhemoglobin at 660 nm |
| Methemoglobinemia | Plateaus near 85% | MetHb pins the reading regardless of true saturation |
| Poor peripheral perfusion | Unreliable or absent signal | Probe requires pulsatile arterial flow |
Assessment & Findings
Always correlate SpO₂ with the clinical picture and the plethysmographic waveform. A strong, consistent waveform increases confidence in the reading; a weak, irregular, or absent waveform is itself a finding that demands attention.
- Waveform quality. Assess the pleth waveform before trusting the number. A dampened or erratic waveform signals poor signal quality and unreliable readings.
- Clinical discordance. When SpO₂ appears normal but the patient has altered mental status, cyanosis, tachycardia, or a plausible exposure history (fire, confined space, certain medications), do not accept the reading at face value.
- When to obtain co-oximetry.An arterial blood gas with hemoximetry is indicated whenever a dyshemoglobinemia is suspected — carbon monoxide poisoning, methemoglobinemia, or any situation where the SpO₂–SaO₂ gap cannot be excluded clinically.
- SpO₂–SaO₂ gap. If a co-oximetry result shows SaO₂ meaningfully lower than the SpO₂ was reporting, the gap is accounted for by dyshemoglobin species. The clinical response (oxygen therapy, specific antidotes) should target the co-oximetry result, not the pulse oximeter.
Safety note. A patient rescued from a house fire or running generator may present with SpO₂ of 98–100% and simultaneously have COHb of 30–40%. Relying on SpO₂ alone in this scenario is a recognized cause of delayed diagnosis and adverse outcomes. Always obtain co-oximetry.
RT Priorities & Interventions
- Confirm probe placement and waveform. Position the probe on a well-perfused site (fingertip, earlobe). Verify a consistent plethysmographic waveform before documenting or acting on a SpO₂ value.
- Rotate the site. Prevent pressure injury by changing the probe location at least every 4 hours on monitoring patients.
- Recognize when SpO₂ is unreliable. Poor perfusion, hypothermia, motion artifact, nail polish, bright ambient light, and IV dyes all degrade signal quality. Address the cause or move to a better site before treating the number.
- Order co-oximetry for suspected dyshemoglobinemia. When CO poisoning or methemoglobinemia is possible, an ABG with hemoximetry is the minimum acceptable diagnostic step. Communicate the indication clearly when ordering so co-oximetry is not omitted from the panel.
- Account for the SpO₂–SaO₂ gap when titrating O₂. When co-oximetry confirms a gap, titrate supplemental oxygen to the co-oximetry result, not the pulse oximeter reading.
Common Pitfalls
- Trusting a normal SpO₂ in CO poisoning. This is the single most dangerous misuse of pulse oximetry. A reading of 98% does not exclude severe carboxyhemoglobinemia.
- Acting on SpO₂ during poor perfusion or motion. A waveform that is dampened, irregular, or absent signals that the reading cannot be trusted. Treat the waveform as part of the assessment, not just the number.
- Ignoring nail polish, dyes, and ambient light. Blue, green, and black nail polish can absorb the probe’s wavelengths and produce artifactual readings. Methylene blue administered intravenously will transiently drive SpO₂ down. Bright overhead lights on a neonatal warmer can interfere with probe accuracy.
- Missing occult hypoxemia in darker-skinned patients. SpO₂ may report values in the acceptable range while the true SaO₂ is significantly lower. When clinical concern is high, co-oximetry removes the ambiguity.
- Assuming SpO₂ tracks oxygenation alone. SpO₂ reflects hemoglobin saturation, not oxygen delivery. A patient with severe anemia can have SpO₂ of 99% and critically low oxygen content (CaO₂). Always interpret saturation in the context of hemoglobin concentration and cardiac output.
Board Exam Pearls
- CO poisoning gives a falsely normal SpO₂. Co-oximetry is required to measure COHb; pulse oximetry cannot detect it.
- Methemoglobinemia plateaus near 85%. The reading will not reflect the true saturation above or below that threshold.
- Pulse oximetry requires pulsatile flow. Poor perfusion, hypothermia, and motion artifact all invalidate the reading.
- SpO₂ is accurate to about ±2–3%in the 70–100% range under ideal conditions; reliability falls below 70%.
- Co-oximetry measures all hemoglobin fractions. It is the only bedside-accessible test that quantifies COHb, MetHb, and functional SaO₂ simultaneously from a single arterial sample.
FAQ
Why does SpO₂ read normal in carbon monoxide poisoning?
Pulse oximetry uses only two wavelengths of light and assumes hemoglobin is either oxygenated or deoxygenated. Carboxyhemoglobin absorbs red light (660 nm) almost identically to oxyhemoglobin, so the pulse oximeter cannot distinguish the two. The result is a falsely normal or high SpO₂ reading even when the patient has severe tissue hypoxia. Co-oximetry on an arterial blood sample is required to measure the true COHb fraction.
What is the difference between SpO₂ and SaO₂?
SaO₂ is the true arterial oxygen saturation measured by co-oximetry on a blood sample — it reflects the fraction of functional hemoglobin actually carrying oxygen. SpO₂ is the noninvasive estimate from pulse oximetry. In a healthy patient without dyshemoglobinemias, they track closely (within ±2–3%). In the presence of carboxyhemoglobin or methemoglobin, SpO₂ can be substantially higher than the true SaO₂.
What conditions make a pulse oximetry reading unreliable?
Reliability depends on adequate pulsatile flow and the absence of interfering substances. Common causes of inaccuracy include poor peripheral perfusion (hypotension, vasoconstriction, hypothermia), motion artifact, nail polish (especially blue, green, or black), bright ambient light, severe anemia, intravenous dyes such as methylene blue, and dyshemoglobinemias (COHb and MetHb). Darker skin pigmentation can cause SpO₂ to overestimate saturation at low values, resulting in occult hypoxemia that the monitor does not flag.
When should I obtain co-oximetry instead of relying on SpO₂?
Obtain co-oximetry — an arterial blood gas with hemoximetry — whenever a dyshemoglobinemia is possible or the clinical picture does not match the SpO₂. Key indications include suspected carbon monoxide poisoning (fire, smoke inhalation, generator use indoors), suspected methemoglobinemia (dapsone, nitrates, topical anesthetics), unexplained altered mental status with a normal SpO₂, and any situation where peripheral perfusion is poor enough to make the SpO₂ waveform unreliable.
Put it to work
When SpO₂ does not tell the whole story, quantify how well oxygen is crossing into the blood. The A–a gradient calculator does it from a gas.
Open the A–a Gradient calculator →Related Resources
Sources
- American Association for Respiratory Care. AARC Clinical Practice Guideline: Pulse oximetry. Respir Care. 1991;36(12):1406-1409.
- Davis MD, Walsh BK, Sittig SE, Restrepo RD. AARC Clinical Practice Guideline: Blood gas analysis and hemoximetry: 2013. Respir Care. 2013;58(10):1694-1703.