ctHb is the concentration of total hemoglobin in blood. Total hemoglobin, in principle, includes all types of hemoglobin, i.e., deoxy-, oxy-, carboxy-, met-, and sulfhemoglobin. The very rare and non-oxygen-carrying sulfhemoglobin is not included in the reported ctHb.
ctHb = cO2Hb + cHHb + cCOHb + cMetHb
The systematic symbol for arterial blood is ctHb(a). The analyzer symbol may be tHb or ctHb.
What does ctHb tell you
ctHb is a measure of the total potential for oxygen transport in the blood, except for the physically dissolved O2, usually 1-2 % of ctO2. However, the actual oxygen transport capacity is defined by the effective hemoglobin concentration (i.e., ctHb minus the concentration of dyshemoglobins).
The arterial blood’s oxygen transport status is in turn determined by the amount of hemoglobin (ctHb), the fraction of oxygenated hemoglobin (FO2Hb), and the oxygen tension (pO2).
ctHb(a) reference range (adult):
male: 8.4-10.9 mmol/L (13.5-17.5 g/dL)
female: 7.4-9.9 mmol/L (12.0-16.0 g/dL)
High values of ctHb normally result in a high blood viscosity, which may precipitate cardiac failure through an increase in left ventricular afterload; furthermore, the risk of thrombotic incidents is increased due to increased microcirculatory sludging.
Common causes of high values of ctHb (polycytemia):
- chronic lung disease
- chronic heart disease
- living at high altitude
- trained athletes
Low concentrations of total hemoglobin or effective hemoglobin imply a risk of tissue hypoxia because of the lowered arterial oxygen content (ctO2).
Physiological compensatory mechanisms for a low total concentration of hemoglobin are an increase in cardiac output and an increased red-blood-cell production. An increase in cardiac output may be inexpedient in the case of ischemic heart disease or impossible in the case of impaired myocardial contractility or flow obstruction.
Common causes of low values of ctHb (anemia):
- impaired red-cell production
- dilution (overhydration)
- multiple blood samples (particularly in neonates)
A normal total concentration of hemoglobin does not guarantee a normal oxygen transport capacity. If dyshemoglobins are present in high concentrations, the effective transport capacity will be significantly reduced. The figure below demonstrates the effect of ctHb on oxygen content, and the effect of a decreased ctHb is shown.
ceHb = ctHb (1- FCOHb - FMetHb)
Be aware of the risk of preanalytical errors (inhomogeneous sample) on ctHb values.
For more information, go to Preanalytical considerations.