Misconceptions in reporting oxygen saturation

We describe some misconceptions that have become common practice in reporting blood gas and cooximetry results. In 1980, oxygen saturation was incorrectly redefined in a report of a new instrument for analysis of hemoglobin (Hb) derivatives. Oxygen saturation (sO2) was redefined as the ratio of oxyhemoglobin (O2Hb) to total Hb instead of the ratio of O2Hb to active Hb (O2Hb + desoxyhemoglobin). In addition, the new terms "functional saturation" and "fractional saturation" were introduced. Since the new parameter was implemented in a widely used cooximeter, its use is now widespread and has caused misunderstandings.

In this report, we review the development of the definitions and measurements of sO2 and related quantities and contend that the misconceptions should be resolved by standardizing instrument read-outs and clinical reports, so that sO2, defined as the ratio of O2Hb to active Hb, should replace FO2Hb and be reported along with the total Hb concentration and the common dyshemoglobin fractions (%CO-Hb and % methemoglobin [metHb]).

The redefinition of sO2 as the %O2Hb or FO2Hb did not address the confusion that might result from interchanging these two often-similar but different terms. The term fractional saturation is an inappropriate terminology and lacks clear physiological meaning. We see frequent cases of confusion: (a) the difference between the sO2 in pulse oximetry and the FO2Hb in cooximetry is called the "pulse oximeter gap;" (b) sO2 results are described as "method dependent;" and (c) reference ranges for these terms are substituted.

Although either parameter could be used by clinicians who fully understand the relatively simple difference between these parameters, we find clear evidence that there is widespread confusion of these terms, even among experts in the field. Standardization of the reporting format would help, and instrument manufacturers could contribute by standardizing the reporting format for cooximetry results.