Plasma Cl:Na ratio: a simple alternative to Stewart's Strong Ion equation for detection of elevated lactate or unmeasured anions in metabolic acidosis

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Introduction

There has been a resurgence of interest in the interpretation of acid base abnormalities using Stewart's Strong Ion Theory [1], central to which is the solving of a complex quadratic equation that accurately quantifies the contribution of both organic and inorganic anions to pH. In accordance with this theory, we hypothesised that an increase in organic anions (lactate and unmeasured anions) during metabolic acidosis would cause a compensatory fall in inorganic chloride thus preserving electroneutrality. The aim of this study was to determine if this compensatory hypochloraemia, expressed as a fall in the Cl:Na ratio, could be a useful method to detect these elevated organic anions in patients with metabolic acidosis.

Patients and methods

Paired blood samples were taken on admission and at 24 h from 341 patients (median weight 8 kg, IQ range 3–15) admitted to a regional PICU, from which serum electrolytes, lactate and blood gases were measured and the strong ion gap calculated to quantify unmeasured anions (UMA). A metabolic acidosis was defined as being present if the base deficit (BD) was ≥ 5 mmol/l and was chosen rather than pH since it better defines the metabolic component of an acidosis [2].Using the above criteria for acidosis, 136 samples were selected and further divided into two subgroups using a Cl:Na ratio ≤ 0.74 as the cutoff value and compared for acid base parameters, lactate and UMA concentrations. The cut off value of 0.74 represents the 1st quartile for Cl:Na ratio derived from analysis of the 341 paired samples (median 0.76, IQ range 0.74–0.78). This approach was necessary since there are no normal values for Cl:Na ratio.

Results

In the episodes characterised by a BD ≥ 5 mmol/l (n=136), the blood lactate and UMA concentrations were significantly greater in the subgroup with a Cl:Na ratio ≤ 0.74 despite having similar conventional acid base parameters (pH, pCO2 and BD).

Conclusion

A low plasma Cl:Na ratio (≤ 0.74) is a simple alternative test for detecting the presence of abnormal elevations in lactate (>2.4 mmol/l) or UMA (>4.3 mmol/l) concentrations in patients with metabolic acidaemia. This obviates the need to solve the complex quadratic equations required for calculation of UMA using the Strong Ion Theory of Stewart.

Table 1 Median values; (1st and 3rd quartiles)

Table 2 ROC curve values for Cl:Na ratio ≤ 0.74. Values in mmol/L