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Measurement of TCO2 is useful in determining the acid– base status of a patient. The TCO2 in serum or plasma exists in three major chemical forms: 95% as bicarbonate anion, 3% as dissolved CO2, and 2% as carbamino derivatives of plasma protein.1 Other quantitatively minor forms are carbonic acid and carbonate ions. The use of the term “bicarbonate” to describe the measured parameter is not correct since the measured TCO2 includes dissolved CO2, carbamino compounds, and carbonic acid and not strictly bicarbonate. Some laboratories use the term “TCO2” to describe the measured bicarbonate. Two commonly employed methods to quantitate TCO2 content in serum or plasma involve acidification of the sample to convert all carbon dioxide forms to CO2 gas and measure the amount of gas formed using PCO2 electrode. The second method utilizes the enzymatic reaction of bicarbonate with phosphoenolpyruvate carboxylase coupled to a spectrophotometric indicator reaction. Most assays that specifically measure bicarbonate have reagents at an alkaline pH. This converts other forms of CO2 to bicarbonate, so these assays are also measuring total CO2. Bicarbonate can also be calculated from blood–gas analysis using the Henderson–Hasselbalch equation. The equation used to calculate bicarbonate is [HCO3 −] = αPCO2 (antilog [pH − pKa]). This equation is based on the assumption that the dissociation constant (pKa) and solubility coefficient (α) are 6.1 and 0.03 respectively. However, pKa is affected by changes in pH, ionic strength, and temperature while the values of solubility coefficient (α) vary with the composition of the solution, such as the presence of increased salts proteins or lipids.2 Therefore, the calculated bicarbonate values may have significant error under certain circumstances irrespective of the sample type, making the reliability questionable. Previously done studies to assess the agreement between calculated bicarbonate and measured TCO2 have shown good agreement in some studies3-5 while revealing 1Assistant Professor, 2Senior Resident 1,2Department of Biochemistry, ESIC Medical College & Postgraduate Institute of Medical Sciences and Research Chennai, Tamil Nadu, India

Comparison of measured Serum Total Carbon Dioxide with calculated Bicarbonate calculated from Arterial Blood Gas Analysis