Definition
cHCO3- is the concentration of bicarbonate (hydrogen carbonate) in the plasma of the sample. It is calculated using the measured pH and pCO2 values. The systematic symbol for arterial blood is cHCO3-(aP). The analyzer symbol may be cHCO3- or cHCO3-(P).
What does cHCO3- tell you
The actual bicarbonate is calculated by entering the measured values of pH and pCO2 in the Henderson-Hasselbalch equation. An increased level of cHCO3- may be due to a primary metabolic alkalosis or a compensatory response to primary respiratory acidosis. Decreased levels of cHCO3- are seen in metabolic acidosis and as a compensatory mechanism to primary respiratory alkalosis.
Reference ranges
cHCO3-(aP) reference range (adult) [24]:
male: 24-31 mmol/L
female: 22-31 mmol/L
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Clinical interpretation
The plasma bicarbonate level depends both on “consumption” through titration of acids unrelated to carbonic acid/pCO2 (the actual titration produces water and carbon dioxide, of which the latter is excreted by ventilation), on active regulation of renal bicarbonate excretion, and on actual carbonic acid concentration (i.e., pCO2 as per the Henderson-Hasselbalch equation).
A high bicarbonate concentration may thus be due to either a true relative deficit of non-CO2-related acid (e.g., in continuous gastric aspiration), to a compensatory increase of renal proton excretion (resulting in bicarbonate retention) - in both instances constituting an element of “metabolic alkalosis” - or to a high pCO2 level.
Conversely, a low bicarbonate concentration is caused, either by titration of excess non-CO2-related acid (e.g., in hyperlactatemia), by a compensatory reduction in renal proton excretion, or by low pCO2.
The evaluation of bicarbonate levels must therefore always be done in conjunction with evaluation of the pCO2 and the pH.
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