Pulse Oximeter Accuracy: Cold Fingers, Nail Polish and Skin Tone

How the device can be fooled

The oximeter compares how red and infrared light pass through a pulsing fingertip. The calculation assumes that the main thing changing the light with each pulse is arterial blood, and that the blood contains the usual forms of haemoglobin. Anything that alters the light path, weakens the pulse signal or adds a source of interference can distort the result. The device may then show a number that looks credible but is wrong, and few models warn you clearly. For how the device works and how to use it, see pulse oximeters: how they work and how to use one.

Cold fingers and poor circulation

Cold weather, an air-conditioned room, anxiety, smoking, Raynaud's phenomenon, diabetes-related circulation problems and heart failure can all reduce blood flow to the fingers. With a weak pulse signal the device may fail to read, give very low numbers, or flicker. Older adults, who commonly have poor circulation, are more prone to this.

What to do:

• Rest indoors for a few minutes in a comfortably warm room.
• Warm the hands by rubbing them, holding them around a warm cup, or soaking them in warm (not hot) water, then dry.
• Try the middle or ring finger, which often have better flow, or the other hand.
• If the reading is still low but your parent looks well and breathes comfortably, wait and recheck. If they look unwell, do not wait.

Nail polish, artificial nails, henna and dirt

Dark nail polish, particularly blue, green, black and brown, and thick gel or acrylic nails can absorb or scatter the light and make the reading too low or erratic. Henna, turmeric stains, dirt and ink may have similar effects. Many Indian women apply nail colour and henna for festivals, so this is a frequent cause of odd readings.

What to do:

• Remove polish with remover from at least one finger and test on that finger.
• Wash and dry the hand, and avoid creams on the fingertip.
• If the nail cannot be cleaned, try another finger.
• Some devices can be placed sideways on the finger; the maker's guide explains.

Movement and tremor

Shaking, restlessness, coughing or talking disturb the signal. People with Parkinson's disease, essential tremor or agitation due to confusion may produce unreliable readings. See home monitoring with poor eyesight or tremor.

What to do:

• Rest the hand on a table or the chest, with the arm supported.
• Keep still for 30 seconds to a minute.
• Take the reading when your parent is calm and between bouts of coughing.
• If the number keeps changing, wait for it to settle and look for a steady waveform, if the device shows one.

Bright light and ambient interference

Strong sunlight or bright bulbs shining on the sensor can interfere. Shade the finger with your other hand or move to a softer-lit room.

Fit of the clip

A clip that is too loose lets light leak in, and one that is too tight can restrict blood flow. Very small or very large fingers, swollen fingers or fingers with arthritis may not fit well. Check that the fingertip touches the end of the clip and that the nail faces the sensor, according to the instructions.

Low blood pressure and shock

When blood pressure is very low, the pulse in the finger is weak and the oximeter may read poorly. A person with severe infection, heavy bleeding or heart failure may have cold hands and an unreliable number. In such a case, symptoms and signs (drowsiness, clammy skin, rapid pulse) matter more than the figure. Call 112.

Irregular heartbeat

In atrial fibrillation or frequent extra beats, the oximeter's pulse count and, at times, the saturation reading may fluctuate. The device may struggle to find a stable signal. Tell the doctor about any irregular pulse noted.

Carbon monoxide and other blood conditions

Carbon monoxide, which can build up from a faulty gas heater, a charcoal stove or generator fumes in a closed room, binds haemoglobin and can make an oximeter read falsely normal while the person is in serious danger. If your parent is drowsy, has a headache or is confused in a room with burning fuel, move them to fresh air and call 112. Some rare blood conditions and certain medicines can also distort readings. Severe anaemia may mean a normal percentage but too little oxygen delivered overall, so symptoms matter.

Skin pigmentation

Studies in recent years, including work reviewed by health regulators, have found that some pulse oximeters can overestimate oxygen saturation in people with darker skin, meaning that the number may look better than the true level, with a higher risk of missed low oxygen. The size of the effect varies by device. This is relevant in India, where skin tones vary widely. Practical points:

• Do not rely on a normal reading alone if your parent has symptoms such as breathlessness, confusion or blue-grey lips.
• Treat a downward trend from their usual baseline as important, even if the number is still in the "normal" range.
• Use the same device and finger each time to compare readings fairly.
• If your parent has a serious illness, ask the doctor about the best way to monitor.

Cheap and uncertified devices

A fingertip oximeter bought for convenience, without proper testing, may be less accurate than a hospital model. Quality varies. Choose a device from a trustworthy source, avoid unknown online listings with extravagant claims, and compare your device with the clinic's device or a hospital reading at least once. See health device budgets and quality checks in India. Replace the device if it gives inconsistent results or has been dropped. See calibration, batteries and replacement of home health devices.

A checklist for a reliable reading

1. Sit for five minutes in a warm room. 2. Warm and dry the hands. 3. Remove nail polish, gel or henna from the test finger. 4. Hold the hand at heart level, resting on a table or chest. 5. Choose the middle or index finger, and fit the clip snugly with the fingertip fully inserted. 6. Shade the clip from bright light. 7. Stay still, quiet and breathe normally. 8. Wait until the number is steady for several seconds, with a good waveform. 9. Record the stable value, the pulse and the time. 10. Repeat on another finger if the number surprises you.

Combine the number with the person

Always check for the following, whatever the number:

• Breathing: fast, laboured, unable to complete sentences.
• Colour: blue or grey lips or face.
• Mental state: confusion, drowsiness, restlessness.
• Pulse: very fast, very slow or irregular.
• Pain, fever, cough.

If these are present, seek urgent help even if the oximeter says 97. If the number is low but your parent is comfortable and well, recheck properly, then call the doctor if it stays low. See when oxygen saturation is low: what to do.

When to ask a professional

Ask a doctor, nurse or pharmacist to compare your oximeter with a clinic device and to show the best finger and technique for your parent. Ask whether your parent needs a personal target, particularly with lung disease or darker skin. Seek help if readings are repeatedly low or unstable, or if the number does not match how your parent looks and feels. Call 112 for severe breathlessness, blue lips, confusion, chest pain or collapse.