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General reference information for laboratory and clinic staff. It is not medical advice and does not describe how to interpret a result for any individual patient.
Short answer
Co-oximetry is hemoglobin measurement with light at several wavelengths. Open Critical Care defines the term as devices that use at least four wavelengths, which lets them tell oxyhemoglobin and deoxyhemoglobin apart from other hemoglobin forms such as carboxyhemoglobin (CO-Hb) and methemoglobin (Met-Hb). A conventional pulse oximeter uses two wavelengths and cannot do that.
Definition and scope
The Open Critical Care Oxygen FAQ (“What is co-oximetry”) says the term CO-oximetry refers to devices that use at least four wavelengths of light. Those devices measure oxyhemoglobin and deoxyhemoglobin and also other hemoglobin forms, with CO-Hb and Met-Hb given as examples. The same page notes that “CO-oximeter” is often used interchangeably with an arterial blood gas analyzer, which is a point of confusion covered below.
This page covers the measurement concept and the questions a lab or clinic can ask about an instrument. It does not cover what to do about any particular reading. For the wider device family, see what a blood gas analyzer is.
How it works in practice: more wavelengths, more hemoglobin forms
The principle in the sources is simple. Each additional wavelength gives the instrument more information, so it can separate more kinds of hemoglobin. According to Open Critical Care, most conventional pulse oximeters use two wavelengths of light and can only detect oxyhemoglobin or deoxyhemoglobin. Their output is a “functional” saturation, while multiwavelength devices report “fractional” saturation.
A 2008 review in Current Opinion in Anaesthesiology (Barker and Badal, PubMed 18997533) describes the same progression for pulse oximetry. Older oximeters used two wavelengths and reported SpO2 only. Newer multiwavelength pulse oximeters can noninvasively estimate total hemoglobin and the two most common dyshemoglobins, carboxyhemoglobin and methemoglobin. The abstract’s author is associated with a pulse oximeter manufacturer, so treat it as a description of what the technology claims to do rather than an independent performance test.
Comparison: pulse oximetry, pulse CO-oximetry and benchtop co-oximetry
Sources for the table: the Open Critical Care FAQ and the Barker and Badal review.
| Device type | Wavelengths of light | What it detects or reports |
|---|---|---|
| Conventional pulse oximeter | Two (most devices) | Oxyhemoglobin or deoxyhemoglobin only; a “functional” saturation |
| Pulse CO-oximeter | About 10 for some devices | Additional hemoglobin forms; the review describes estimates of carboxyhemoglobin, methemoglobin and total hemoglobin |
| Benchtop CO-oximeter | At least four by definition; benchtop units “may use 128 to 256” wavelengths | Oxy, deoxy and other hemoglobin forms such as CO-Hb and Met-Hb; a “fractional” saturation |
Where it matters for labs and clinics: the CLSI C46 guideline
The Clinical and Laboratory Standards Institute lists C46, Blood Gas and pH Analysis and Related Measurements, Second Edition (C46-A2), published 24 February 2009. CLSI’s page describes it as a consolidation of six CLSI documents and projects that addresses blood gas, pH and related measurements. It defines terminology and discusses performance characteristics, preanalytical variables, analytical considerations and quality control. It covers in vitro measurement; in vivo monitoring devices are not specifically addressed. The page marks the document as archived, meaning it is no longer being reviewed through the CLSI consensus process, and also says it is technically valid.
A question list built from the guideline’s own scope
The checklist below only turns CLSI’s stated scope items into questions. It is not a purchasing rule and no source ranks one instrument above another.
- Terminology: does the instrument documentation say whether it reports functional or fractional saturation, and does your staff use those terms the same way?
- Wavelengths: how many wavelengths does the device use, and is it a pulse device or a benchtop one? The table above shows these are different classes.
- Performance characteristics: which performance characteristics does the manufacturer document for the measurements you intend to rely on?
- Preanalytical variables: who controls sample handling before the sample reaches the instrument?
- Quality control: what quality control does your program run, and where is it recorded? See what calibration is for the general concept.
- Scope: is the device an in vitro analyzer (the scope of C46) or an in vivo monitor (not specifically addressed by C46)?
For bedside and near-patient programs, point-of-care testing and what a pulse oximeter is cover the neighbouring ground.
Common misunderstandings
“A pulse oximeter is a co-oximeter.”
Not by the definition above. Most conventional pulse oximeters use two wavelengths and detect only oxyhemoglobin or deoxyhemoglobin. Some multiwavelength pulse devices are called pulse CO-oximeters and use more wavelengths (about 10 for some), which is a different class from the benchtop unit.
“Co-oximeter and blood gas analyzer mean the same thing.”
Open Critical Care notes the terms are often used interchangeably. The defining feature of co-oximetry, though, is the multiwavelength hemoglobin measurement, not the rest of what a blood gas analyzer may report. Check the instrument documentation for which measurements a given model includes.
“More wavelengths always means a better result.”
The cited sources make no such claim and give no accuracy figures for comparing devices, so this page does not either.
FAQ
What is co-oximetry?
It is the measurement of hemoglobin forms using light at four or more wavelengths, so that forms such as carboxyhemoglobin and methemoglobin can be distinguished from oxyhemoglobin and deoxyhemoglobin (Open Critical Care).
How many wavelengths does a co-oximeter use?
At least four by definition. Some pulse CO-oximeters use about 10, and benchtop CO-oximeters may use 128 to 256 (Open Critical Care).
How is co-oximetry different from pulse oximetry?
Conventional pulse oximeters mostly use two wavelengths and give a functional saturation. Multiwavelength co-oximetry gives a fractional saturation and can detect additional hemoglobin forms.
Is there a CLSI guideline for blood gas measurements?
Yes. CLSI C46-A2 (2009) addresses blood gas, pH and related measurements in vitro. CLSI lists it as archived but technically valid (CLSI).
Sources
- Open Critical Care, Oxygen FAQ: What is co-oximetry. Checked 2026-11-01.
- Clinical and Laboratory Standards Institute: C46, Blood Gas and pH Analysis and Related Measurements. Checked 2026-11-01.
- Barker SJ, Badal JJ. The measurement of dyshemoglobins and total hemoglobin by pulse oximetry. Curr Opin Anaesthesiol 2008;21(6):805-810. PubMed 18997533. Checked 2026-11-01.
