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How to Get an Accurate Blood Oxygen Reading

Nina Ghamrawi, MS, RD, CDE
December 10, 2021
March 6, 2023

If you are new to monitoring blood oxygen, or have a new diagnosis, it is important to monitor your oxygen levels regularly. Blood oxygen is typically quite stable, but may vary depending on many factors, and slightly from one meter to the next. If you’ve noticed a difference in your readings and want to learn more, take these pointers!

Get a Baseline: What’s Normal for You?

If you are newly monitoring, or have a new diagnosis, you must first establish a baseline.

  • To get a baseline, measure your blood oxygen three times per day for 3 days.
  • The average of these 9 readings approximates your ‘baseline’.

Monitor Regularly

  • When first prescribed oxygen
  • Regularly with oxygen therapy
  • During or just after exercising
  • During sleep: when you have trouble sleeping or sleep apnea
  • When traveling to a higher altitude location

Ensuring Accuracy:

Most oximeters give a reading +/-2% of what your saturation would be if obtained by an arterial blood gas. The best reading is achieved when the hand is clean, warm, and relaxed.

Factors that could cause a higher-than-normal reading:

  • Smoking

Factors that could cause a lower-than-normal reading:

  • Nail polish & artificial nails
  • Very dark skin
  • Cold hands
  • Poor circulation
  • Sickle cell disease

Other factors that may cause an inaccurate or unmeasurable reading:

  • Bright light directly to the probe, like sunlight
  • Very low oxygen saturation levels (below 80%)
  • Movement during the measurement

Review Reliability: Blood oximeters do not require calibration because the design and measurement technique either incorporates a continuous calibration of the device, or does not require it by measurement technique. The SpO2 (blood oxygen) reading should always be considered an estimate of oxygen saturation. For example, if an FDA-cleared pulse oximeter reads 90%, then the true oxygen saturation in the blood is generally between 86-94%.