Definition
FCOHb is the ratio between the concentrations of COHb and tHb:
The systematic symbol for arterial blood is FCOHb(a). The analyzer symbol may be COHb or FCOHb.
What does FCOHb tell you
Carbon monoxide binds reversibly with the heme-group ferrous ions, but the affinity of hemoglobin for carbon monoxide is 200 to 250 times as great as the affinity for oxygen.
Carboxyhemoglobin is incapable of transporting oxygen and furthermore increases the affinity for oxygen of the remaining binding sites. This results in a decreased oxygen transport capacity along with an impaired peripheral oxygen release due to the left shift of the ODC.
Reference ranges
FCOHb(a) reference range (adult): 0.5 - 1.5 % (0.005 - 0.015)
disclaimer
Clinical interpretation
Carboxyhemoglobin levels are normally below 2 %, but may be up to 9-10 % in heavy smokers. Newborns may present levels of up to 12 % of carboxyhemoglobin because of an increased hemoglobin turnover combined with an immature respiratory system. With acute exposure headache, nausea, dizziness, and chest pain usually occur at levels of 10-30 %. Severe headache, general weakness, vomiting, dyspnea, and tachycardia are common at 30-50 %. Above 50 %, seizures and coma are seen, and death usually ensues.
Considerations
Duration and timing of exposure is important in the clinical evaluation of these patients, as patients with long exposure time may be severely affected despite relatively low blood concentrations of carboxyhemoglobin. If significant carboxyhemoglobinemia is suspected, 100 % oxygen should be given and hyperbaric oxygen therapy considered in accordance with history and neuropsychiatric symptoms.
Be aware of the risk of preanalytical errors (inhomogeneous sample) on FCOHb values.
For more information, go to Preanalytical considerations.
