From the Guidelines
Yes, anemia can affect SpO2 readings, as pulse oximetry measures the percentage of hemoglobin saturated with oxygen, not the absolute amount of oxygen in the blood. According to the British Thoracic Society guideline for oxygen use in adults in healthcare and emergency settings 1, oxygen is transported in the blood in two forms: a small amount dissolved in plasma and the majority bound to the hemoglobin molecule. In anemia, the reduced number of red blood cells means less total hemoglobin is available to carry oxygen, even though the percentage of available hemoglobin carrying oxygen may appear normal. This discrepancy is particularly important in severe anemia, where a patient may have normal SpO2 readings but still experience tissue hypoxia.
The guideline also discusses anaemic hypoxia, which results from a reduced level of hemoglobin available for oxygen transport, leading to tissue hypoxia despite normal PaO2 and SpO2 levels 1. Furthermore, the guideline recommends aiming for an oxygen saturation of 94–98% in anemic patients, or 88–92% if the patient is at risk of hypercapnic respiratory failure 1.
Key points to consider:
- Pulse oximetry may show falsely normal or elevated readings in anemic patients
- Anemic patients may experience tissue hypoxia despite normal SpO2 readings
- Clinicians should interpret SpO2 readings cautiously in anemic patients and consider measuring arterial blood gases for a more accurate assessment of oxygenation status
- Other factors like poor peripheral perfusion, certain dyes, nail polish, and skin pigmentation can further affect the accuracy of pulse oximetry readings in anemic patients.
From the Research
Effect of Anemia on SPo2 Readings
- Anemia can affect the accuracy of pulse oximeter readings, particularly at low oxygen saturation levels 2.
- Studies have shown that pulse oximeters can give a value for oxygen saturation even in cases of severe anemia, but the value may not always be accurate 3.
- The error in pulse oximeter readings due to anemia is more pronounced at lower oxygen saturation levels, with a greater bias observed at SaO2 levels below 75% 2.
- The type of pulse oximeter used can also affect the accuracy of readings in anemic patients, with some devices showing greater errors than others 2.
Mechanism of Error
- The error in pulse oximeter readings in anemic patients is thought to be due to the decreased amount of hemoglobin available to bind to oxygen, which can affect the absorption of light by the pulse oximeter 2.
- The decreased hemoglobin concentration can also affect the calibration of the pulse oximeter, leading to inaccurate readings 2.
Clinical Implications
- The inaccuracy of pulse oximeter readings in anemic patients can have significant clinical implications, particularly in patients with severe anemia or those who require close monitoring of oxygen saturation levels 3, 2.
- Clinicians should be aware of the potential for error in pulse oximeter readings in anemic patients and take steps to verify the accuracy of readings, such as using alternative methods to measure oxygen saturation 3, 2.