A pulse oximeter is placed in the hand of a COVID-19 patient. A team of researchers from the University of Nottingham (UoN) have developed a laboratory-based test to assess the performance of POs under the effects of varying melanin (pigment responsible for colour) concentration and oxygen saturation (SpO2) values.
| Photo Credit: File photo

Given that the performance of pulse oximeters (POs) is known to vary with the colour of skin, a team of researchers from the University of Nottingham (UoN) have developed a laboratory-based test to assess the performance of POs under the effects of varying melanin (pigment responsible for colour) concentration and oxygen saturation (SpO2) values.

A study pertaining to this, titled ‘Pulse oximeter bench tests under different simulated skin tones’, has been published in Medical and Biological Engineering and Computing, a peer reviewed medical journal.

Pulse oximeters were the most commonly used devices during the covid pandemic as they allowed for non-invasive, painless, continuous and inexpensive real-time monitoring of oxygen levels in the blood using light (commonly, red and infrared light).

Need for caution

Suvvi K. Narayana Swamy, lead author of the study, told The Hindu that POs did not show any deviations in SpO2 readings due to skin colour in the laboratory tests. “However, the tests also did not conclusively demonstrate that melanin does not affect SpO2 measurements. So, we need to be cautious not to over interpret these results. Further research is underway,” she said.

“In the United Kingdom, the COVID Oximetry @home programme was launched as part of the National Health Service (NHS) response to the pandemic. Through this programme, commercially available peripheral POs were delivered to allow patients with COVID-19, or at high risk, to remotely monitor their SpO2 levels at home. These devices were widely used globally by healthcare professionals to make timely clinical decisions when admitting patients (particularly with the absence of any noticeable symptoms) to hospitals for critical care and treatment,” Ms. Swamy said.

The study gains significance in a context where several recent retrospective clinical studies have highlighted that SpO2 may be overestimated in patients with non-white skin types. This phenomenon is generally termed occult hypoxemia, where SpO2 measured by a PO is greater than 90%, despite true oxygen saturation value being less than 88%,” she explained. 

“In this study, the performance of POs used in the NHS COVID Oximetry @home programme and in hospitals in UK was investigated. The test enabled the POs to be subjected to low signal and varying melanin concentration, which are the two main principal effects of melanin in the skin through simulation. The effects of low signal levels were simulated by neutral density filters (NDFs) that reduced red and infrared radiation light in the same manner while the effects of melanin concentration were achieved by utilising melanin filters (MFs) that decreased both red and IR light by unequal amounts.In these tests, the SpO2 overestimation, which has often been observed clinically, was not reproduced,” she explained.

Significant variations

Stating that a laboratory simulation was used to mimic different SpO2 values (~ 70 to 100%), Ms Swamy said different neutral density and synthetic melanin filters were used to reproduce low signal and varying melanin reduction levels. “POs from six different brands were reviewed and their response documented. Significant variations were observed in the recorded SpO2 values among different POs when exposed to identical simulated signals.  Differences were greatest for lower SpO2 (<80%) where empirical data is limited,” Ms. Swamy said.

“All PO responses under low signal and melanin concentration did not change across various simulated SpO2 values. The tests do not provide conclusive evidence that melanin does not affect in vivo SpO2 measurements. Research in the areas of instrument calibration, theory and design needs to be further developed,” she added.