The term CO-oximetry refers to devices that use at least four wavelengths of light to measure not only oxy and deoxy-hemoglobin, but also other forms of hemoglobin (e.g. CO-Hb, Met-Hb). While most conventional pulse oximeters use two wavelengths of light and can only detect oxyhemoglobin or deoxyhemoglobin, some pulse oximeters contain many more wavelengths and the ability to function as ‘pulse CO-oximeters.’ (Note: these devices are clearly marked with this function and often are considerably more expensive than other devices).
Despite this, the term ‘CO-oximeter’ is often used synonymously with ‘arterial blood gas analyzer’ because historically it was only multi-function arterial blood gas analyzers that possessed CO-oximetry functionality.  Some pulse CO-oximeters have ~10 wavelengths of light, whereas benchtop CO-oximeters may use 128 to 256 wavelengths of light.
Of note, the terms “CO-oximeter” and “Hexometer” are brand names coined by the companies Instrumentation Laboratories and Radiometer, respectively. The generic term for a device with these capabilities is a “multiwavelength oximeter,” but the term “CO-oximeter” is widely used to refer to these devices, regardless of brand.
Keywords: CO-oximetry, arterial, carboxyhemoglobin, methemoglobin