Pulse Oximeter

Last Updated: May 21, 2021

Pulse Oximeter – Illustrative

The pulse oximeter, a small device which was unknown to most, found its way into the home medicine cabinet amidst the global COVID-19 pandemic. It measures the oxygen saturation in the blood. Early detection of the low blood oxygen saturation could help to initiate oxygen therapy early to prevent further deterioration and the possible need for intubation and ventilation.

Traditionally, pulse oximetry is used in a hospital setting to measure the oxygen saturation of the blood during and following a surgery, in intensive care units and emergency departments, usually on a continuous basis. The wearables can be wirelessly connected to a central station, which is continuously monitored by the nursing staff. An alert may be sent to the nurse’s pager in case the patient’s condition deteriorates.

Oximeters for home use have been incorporated in spectacles and personal mobile phones, but the finger pulse oximeter remains the most popular. Though having an app in a mobile phone that scans the finger tip and estimates the oxygen saturation appears to be a convenient and cost-effective option, false apps have emerged that have been incriminated of stealing biometric data; thus security remains a concern while using these apps.

The pulse oximeter measures the oxygen saturation of the blood.

The red blood cells contain hemoglobin, a pigment which carries oxygen from the lungs to the different parts of the body. The percentage of oxygen in the hemoglobin reflects the lung function. A drop in the oxygen saturation can damage the brain, heart, kidneys and other organs. The pulse oximeter can be used as an early warning device to detect dropping blood oxygen levels, particularly to detect “silent hypoxemia” or “happy hypoxia”, a situation caused by COVID-19, where the oxygen levels in the blood fall without causing breathlessness, until the later stages.

The pulse oximeter used for home monitoring measures the oxygen saturation in the blood intermittently. The oximeter transmits light through the finger, which passes through the skin and the blood vessels of the finger. In some oximeters, a sensor on the opposite side of the finger detects the light and calculates the oxygen saturation based on algorithms. Other types of oximeters detect reflected light. Thus, the light is emitted from one side of the device is reflected to a sensor on the same side of the device. However, concerns have been raised about the accuracy of devices measuring reflected light.

Selection of a pulse oximeter for home monitoring should be based on price, and more importantly, accuracy.

The accuracy of the pulse oximeter remains a major cause of concern, especially when the oxygen saturation falls below 90%. Most devices are tested in normal individuals; therefore their reliability in patients with low oxygen levels is not established, especially when the level falls below 70%. It is therefore important to note the trend of the readings over a period, by taking multiple readings during the day, rather then concluding from a single reading. For example, if the reading consistently falls, the healthcare team should be alerted, without waiting for the oxygen saturation to fall to critical levels. It is also important to remember that after an initial slow fall, they could be a sudden steep fall, which necessitates an early visit to the hospital.

Devices that are FDA or ISO approved have been tested for accuracy.

The below link provides a search engine for a list of devices approved by the FDA.


The FDA does not provide a certificate for any device, and therefore any FDA-approval certificate for a device is likely to be fake. Over-the-counter devices may not be FDA certified for medical use.

ISO certified devices can be found at the following website:


Other options to verify ISO certification are to check on the certified body website, or request the body for more information.

The procedure of recording the oxygen saturation should be properly followed to ensure that the readings obtained are free from errors.

  • Understand the instruction sheet in detail before using the instrument.
  • The measurement should be taken when the patient is at rest, breathing quietly without speaking for several minutes before the measurement. Smoking and alcohol intake should be avoided before taking the readings.
  • The reading should be taken indoors. High intensity ambient light may interfere with the reading.
  • The device should be placed securely on the middle or ring finger. The finger should be completely inserted into the cavity. The patient should not move while the measurement is being taken. There should be no nail polish or artificial nails on the finger. If the finger is cold, it should be warmed before taking the reading.
  • The reading should be taken for 30 to 60 seconds and the number which appears most commonly when the pulse signal is strong should be considered.
  • Multiple readings should be taken during the day, according to the advice of the doctor. The readings should be recorded and shared with the healthcare provider.

A pulse oximeter usually measures two or three parameters.

  • The oxygen saturation of blood, in percentage. The normal oxygen saturation in the arterial blood at sea level is between 95 to 100%. It is lower at higher altitudes, and in people with prior lung problems. A consistent fall in the oxygen saturation could raise red flags about deteriorating lung function.
  • The pulse rate, or the number of heart beats per minute
  • The perfusion index, which measures the strength of the pulse. Select a device that provides information on the strength of the pulse signal, through a reading called the perfusion index. The normal perfusion index is 3% or greater.

A pulse oximeter may sometimes give incorrect readings.

Some people may experience low blood supply through the fingers, as a result of which the oximeter may give erroneous readings. An alternative in these patients could be to measure the oxygen saturation on the forehead or the earlobe, though such devices may not be easily available.

The pulse oximeter could give erroneous readings in patients with blood disorders like anemia, abnormal hemoglobin or high carbon dioxide levels in the blood. Pulse oximeters that measure the parameters such methemoglobin, carboxyhemoglobin levels, total hemoglobin along with several other parameters, and may be useful in such patients.

Symptoms such as bluish colour of the face, lips or nails, shortness of breath, difficulty with breathing, chest tightness and a fast pulse rate could indicate the immediate need for oxygen. Under such circumstances, the physician should be contacted at the earliest irrespective of the oxygen saturation reading on the pulse oximeter.


Michard F et al. COVID-19: Pulse oximeters in the spotlight. J Clin Monit Comput. 2020: 1–4. doi: 10.1007/s10877-020-00550-7

Braun F et al. Evaluation of a Novel Ear Pulse Oximeter: Towards Automated Oxygen Titration in Eyeglass Frames. Sensors (Basel). 2020; 20(11): 3301. doi: 10.3390/s20113301

Torp KD et al. Pulse Oximetry. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK470348/

Hafen BB and Sharma S. Oxygen Saturation. https://www.ncbi.nlm.nih.gov/books/NBK525974/


Luks AM and Swenson ER. Pulse Oximetry for Monitoring Patients with COVID-19 at Home: Potential Pitfalls and Practical Guidance. Ann Am Thorac Soc 2020 Sep;17(9):1040-1046. doi: 10.1513/AnnalsATS.202005-418FR


This article is meant for informational purposes only and not intended to replace the relationship between the doctor and the patient. It should not be used for self-treatment by patients or by doctors to treat patients. Consult your doctor before using any of the above information.

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