Pulse oximetry can be inaccurate in several situations that come up regularly in pediatric emergencies: poor perfusion, abnormal hemoglobin, excessive motion, darker skin pigmentation, and external interference from dyes or nail polish. For PALS (Pediatric Advanced Life Support), recognizing these limitations matters because a normal-looking SpO2 number can mask true hypoxemia, and a falsely low reading can trigger unnecessary interventions.
Poor Perfusion and Shock
Pulse oximeters work by detecting pulsatile blood flow through tissue. When perfusion drops, the signal weakens and readings become unreliable or disappear entirely. This is a critical limitation in PALS scenarios because the sickest children, those in shock, severe hypothermia, or cardiac arrest, are exactly the ones where you most need oxygen data and are least likely to get an accurate reading.
Research on low cardiac output states found that oximeters could still produce readings at a cardiac index as low as 2.4 L/min/m² and peripheral temperatures down to about 26.5°C (roughly 80°F). Below those thresholds, the devices often fail to display a reading at all. Vasoconstriction from cold exposure, vasopressor medications, or the body’s own shock response narrows blood vessels in the fingers and toes, further degrading the signal. In PALS, if a child is in decompensated shock or hypothermic, treat the clinical picture rather than relying on the oximeter number.
Abnormal Hemoglobin Types
Standard pulse oximeters use two wavelengths of light and assume the blood contains only two forms of hemoglobin: one carrying oxygen and one without. When other forms are present, the device gets confused.
Carbon monoxide poisoning is the classic example. Carboxyhemoglobin absorbs light almost identically to oxygen-carrying hemoglobin, so the oximeter reads it as oxygenated blood. A child with significant carbon monoxide exposure can show an SpO2 of 98% or higher while their tissues are dangerously starved of oxygen. If smoke inhalation or CO exposure is suspected, the SpO2 number is essentially meaningless on a conventional oximeter.
Methemoglobinemia works differently. Elevated methemoglobin causes the oximeter to underestimate true oxygen saturation at normal and high saturation levels. With methemoglobin levels between 4% and 8%, SpO2 readings were biased roughly 6 percentage points too low in one study. As methemoglobin rises further, conventional oximeters tend to drift toward a reading around 85% regardless of true oxygenation. In pediatrics, methemoglobinemia can result from certain topical anesthetics (like benzocaine), dapsone, or nitrate exposure. A child whose SpO2 seems stuck in the mid-80s despite high-flow oxygen should raise suspicion.
Motion Artifact in Children
Infants and toddlers don’t hold still, and movement is one of the most common reasons for inaccurate readings in pediatric practice. When a child kicks, squirms, or cries, the sensor picks up non-pulsatile motion and misinterprets it as arterial pulsation, producing erratic or falsely low numbers. Agitated or seizing children are especially prone to this.
Proper sensor placement helps. In neonates, the palm of the hand is the preferred site, followed by the sole of the foot if a palm reading is difficult. Securing the probe so it doesn’t shift during movement improves signal quality. Some newer oximeters use signal-processing algorithms designed to filter out motion artifact, but no device eliminates the problem entirely. In a PALS scenario, if the waveform on the monitor looks irregular or the number is bouncing wildly, the reading is likely artifact rather than a real change in oxygenation.
Skin Pigmentation Bias
Pulse oximeters tend to overestimate oxygen saturation in patients with darker skin. This bias has been documented in both adults and children and is significant enough to affect clinical decisions. A large pediatric study published in the New England Journal of Medicine found that in children with the darkest skin tones, one commonly used oximeter overestimated true oxygen saturation by an average of 2.4 percentage points, while another brand overestimated by 3.7 points. In children with the lightest skin, the same devices overestimated by only 0.9 and 1.4 points, respectively.
A 3 to 4 point overestimation might sound small, but it can be the difference between an SpO2 that reads 94% and a true saturation of 90%, which crosses a clinically important threshold. In PALS, this means a child with darker skin who appears to have a borderline-acceptable SpO2 may actually be more hypoxemic than the number suggests. Clinical signs of hypoxia, such as increased work of breathing, altered mental status, and central cyanosis (which is also harder to detect in darker skin), become even more important to assess alongside the oximeter.
Cyanotic Congenital Heart Disease
Children with cyanotic congenital heart defects live with chronically low oxygen saturations, often in the 70s or 80s. Pulse oximetry becomes less accurate at these lower saturation levels, and research has shown the devices tend to overestimate true oxygen saturation in this range. A child with a mixing lesion whose oximeter reads 82% may actually have an arterial saturation several points lower.
This matters in PALS because the goal for these children is not a “normal” SpO2. Their baseline is low, and what you need to detect is a change from their usual. If you don’t know the child’s baseline and the oximeter is simultaneously overestimating, you may underappreciate how compromised they truly are. Arterial blood gas measurement is the gold standard when precise oxygenation data is needed in these patients.
Nail Polish, Dyes, and External Interference
Anything that changes how light passes through tissue can throw off the reading. Nail polish is a well-documented culprit: black, blue, and green polishes significantly lower SpO2 readings, while darker reds and browns also interfere. The effect varies by color because each pigment absorbs light at different wavelengths, overlapping with the wavelengths the oximeter uses to calculate saturation.
In pediatric patients, stickers, temporary tattoos on fingers, or dried paint can cause similar problems. Certain intravenous dyes, particularly methylene blue (which is also used to treat methemoglobinemia, creating an ironic interference), cause dramatic drops in SpO2 readings that don’t reflect actual oxygenation. If a child has nail polish or dye on their fingers, placing the sensor on an unaffected digit, the palm, or the earlobe avoids the issue entirely.
Bright Light and Sensor Problems
Strong ambient light, including overhead surgical lights, phototherapy lamps used in neonatal care, and direct sunlight, can flood the oximeter’s photodetector and corrupt the reading. Covering the sensor with an opaque shield or a small piece of cloth solves this. A sensor that is too loose picks up ambient light and venous pulsation, while one that is too tight can dampen arterial flow and reduce signal quality. Using the correct sensor size for the child’s age and weight is a simple but frequently overlooked factor.
What the Waveform Tells You
The single most useful habit for catching inaccurate readings is looking at the plethysmograph waveform, the pulsing wave displayed alongside the SpO2 number. A clean, consistent waveform with smooth peaks that match the heart rate suggests the reading is reliable. An erratic, flat, or dampened waveform signals that something is interfering, whether it’s motion, poor perfusion, or a misplaced sensor. In PALS, if the waveform looks bad, the number is not trustworthy. Treat the patient, not the monitor.