Oxygen Saturation and LOINC

Authors: Gilbert Hill, MD and Clement J. McDonald, MD
Date Written: 2007-10-19
Revised: 2009-07-09

1 Oxygen saturation

The term “oxygen saturation" is often used to refer to two distinctly different quantities, hemoglobin oxygen saturation (recommended symbol = _s_O₂) and fractional oxyhemoglobin (recommended symbol = _F_O₂Hb).

2 NCCLS 46-A defines the two as follows:

2.1 Hemoglobin oxygen saturation

Hemoglobin oxygen saturation = the amount of oxyhemoglobin in blood expressed as a percent of the total amount of hemoglobin able to bind oxygen (i.e., oxyhemoglobin (O₂Hb) + deoxyhemoglobin (HHb). Note that carboxyhemoglobin (COHb), methemoglobin (MetHb) and Sulfhemoglobin (SulfHb), the so-called dyshemoglobins (dysHb) are unable to bind oxygen, so are not included in the denominator of this fraction:

_s_O₂ = 100 x O₂Hb/(O₂Hb + HHb)

More important some instruments, e.g., the pulse oximeter, cannot pick up the dyshemoglobins. So they always report sO₂.

This quantity may be referred to as "oxygen saturation"; terms such as "functional" oxygen saturation or oxygen saturation of "available" or "active" hemoglobin, but NCCLS should not be used to name this quantity.

2.2 Fractional oxyhemoglobin

Fractional oxyhemoglobin = the amount of oxyhemoglobin expressed as a percent of the total hemoglobin (where total Hb = O₂Hb + HHb + [COHb + MetHb + SulfHb] taken all together O₂Hb HHb and the three DysHb's represent the total Hb.

_F_O₂Hb = O₂Hb/tHb

A key point here is that it takes a more sophisticated machine to measure Fractional Oxyhemoglobin than sO₂.

3 When there are no dyshemoglobins present (the usual situation), sO=FOHb

4 Oxygen saturation obtained by measuring pH and pO

An “oxygen saturation" can also be obtained by measuring pH and pO₂ and substituting the values into an empirical formula for the oxyhemoglobin dissociation curve (Hill equation). However, this calculated approach is prone to many kinds of errors and “oximetry” based on differential spectrophotometry is now the method of choice.

5 Three principal classes of oximetry

There are at least three principal classes of oximetry, commonly known as:

  • Pulse (or transcutaneous) oximetry (sensor attached to body surface)
  • Hemoximetry (sample of blood injected into instrument)
  • Co-oximetry

Historically a Co-oximeter measures Carbon monoxide bound hemoglobins and the other two dyshemoglobins and could only be done in the laboratory with an injected blood sample. Today (i.e., 2009) most laboratory blood analyzers measure all three dyshemoglobins and are really Co-oximeters, but may not be named as such.

A pulse co-oximeter was placed on the market in 2005. This device reports the oxyhemoglobin %, Carbon monoxide %, and the Pleth variability index; it also measures each of the dyshemoglobins. So it provides an accurate fractional oxygen saturation as well as information about CO poisoning.

6 Instruments used for pulse and hemoximetry

The instruments used for pulse and hemoximetry base results on calculations from readings at two different wavelengths, which means they do not reflect the presence of any dyshemoglobins, and therefore the result they produce will be _s_O₂ (LOINC component Oxygen saturation = NCCLS hemoglobin oxygen saturation).

7 Instruments used for co-oximetry

The instruments used for co-oximetry base results on calculations from readings at four to eight different wavelengths, which means they are able to reflect the presence of any dyshemoglobins, and therefore the result they produce will be _F_O₂Hb (LOINC component Oxyhemoglobin/Hemoglobin. total = NCCLS fractional oxyhemoglobin).

LOINC will name its components according to the NCCLS recommendation, and apply the corresponding NCCLS synonyms, i.e., sO₂ and FO₂Hb to the components, oxygen saturation and Hemoglobin Oxygen fraction, respectively and include the defining equation (see above) for the terms that carry these respective components. We will retain oxygen saturation as a synonym for both sO₂ and FO₂Hb so that mappers who may not know the official names will still be able to find the terms.

LOINC will use the property of MFR (mass fraction) to identify the property of these terms. There would only be an imperceptible difference between the numerical representation of this quantity as a MFR versus an SFR (substance or molar fraction). By LOINC convention we use MFR rather than SFR in such cases.

Depending upon the naming precision of laboratory, it may be difficult to determine whether a result called an Oxygen saturation is really an sO₂ or FO₂Hb. The other tests in the panel, and the inclusion of method names in the test order or battery and the source of the term (clinical laboratory, cardiac cath laboratory, respiratory therapy, nursing) all provide guidance for the mapping. But it may be necessary to contact the source to be sure in some cases.

If a battery reports both a fractional oxyhemoglobin and oxygen saturation, then the situation should be clear. If the method is co-oximetry or the battery includes a separate measure of Carboxyhemoglobin (or any of the other dyshemoglobins), then it will be reporting a fractional oxyhemoglobin. If the method is pulse oximetry, then you have an sO₂. Most central and ICU based blood gas measures will produce a fractional oxyhemoglobin.

Finally, in the U.S. these results are almost always reported with units of percent (%), but these results are reported as pure fractions (e.g., 20% becomes 0.2) in some environments.

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