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Haemoglobin concentration influences the chloride–bicarbonate but not the strong ion difference–bicarbonate relationship


Chloride and bicarbonate concentrations share an inverse reciprocal relationship during either acidosis or alkalosis. This relationship is, in part, due to the red cell chloride shift. However, according to the Stewart quantitative approach to acid–base balance, it seems conceivable to expect a greater relationship between the strong ion difference (SID) and bicarbonate, rather then between chloride and bicarbonate. We propose that, with decreasing haemoglobin (Hb) levels, the SID preserves its independent role with respect to bicarbonate, while chloride gradually loses its relationship.


We retrospectively collected blood gas analysis and electrolytes, from 206 patients, measured on a single blood sample taken on admission. We calculated the apparent SID through the following formula: [Na+] + [K+] + [Ca2+] + [Mg2+] - [Cl-] - [Lact-] (mEq/l). We divided patients into three groups based on Hb levels: group A (n = 54) with Hb levels between 12 and 15 g/dl, group B (n = 104) with Hb levels between 9 and 12 g/dl, and group C (n = 48) with Hb levels below 9 g/dl. We calculated Pearson's coefficients between the SID and bicarbonate and between chloride and bicarbonate in these three groups of patients.


Correlation strength between the SID and HCO3- was high and significant even at a Hb concentration below 9 g/dl (see Table 1). Pearson's coefficients for chloride and bicarbonate showed a moderate but significant inverse correlation in group A and group B; eventually this correlation was completely lost in group C.

Table 1 (abstract P40


These results give further validation to Stewart's theories: the SID appears to maintain the role of an independent variable with respect to bicarbonate even at low haemoglobin levels, while chloride loses this relationship at haemoglobin levels below 9 g/dl.

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Mercieri, M., Marcelli, A., Claroni, C. et al. Haemoglobin concentration influences the chloride–bicarbonate but not the strong ion difference–bicarbonate relationship. Crit Care 11, P409 (2007).

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  • Bicarbonate
  • HCO3
  • Haemoglobin Level
  • Significant Inverse Correlation
  • Bicarbonate Concentration