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      A Simple Estimate of the Effect of the Serum Albumin Level on the Anion Gap


      American Journal of Nephrology

      S. Karger AG

      Intensive care, Anion gap, Serum albumin, Serum t[CO2]

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          Background: The serum anion gap (serum [Na<sup>+</sup>]-Cl<sup>-</sup>]-[CO<sub>2</sub>]) is still the first-line approach to metabolic acidosis. However, while it is generally acknowledged that hypoalbuminemia mandates a downward adjustment of the expected anion gap, a specific correction factor for the anion gap in the face of low serum albumin has never been demonstrated. Methods: We reviewed initial laboratory data from 432 consecutive patients admitted or transferred to the medical intensive care unit at Nassau County Medical Center over a 6-month period and correlated the serum albumin with the anion gap and the serum [tCO<sub>2</sub>] using multivariate analysis. We looked at the anion gap as a function of <Delta> (albumin), the difference between normal and actual serum albumin, defined as 4.0 - measured serum albumin g/dl. We also assessed [tCO<sub>2</sub>] as an independent variable. Results: For patients with normal or high serum tCO<sub>2</sub>, the ratio of change in anion gap (<Delta> anion gap) to <Delta> (albumin) was 1.46 and 1.45, respectively. For patients with serum tCO<sub>2</sub> <22 mEq/l this ratio was 1.89. In the latter group, anion gap was best predicted taking both <Delta> (albumin) and serum tCO<sub>2</sub> into account: anion gap = 36.2 - serum tCO<sub>2</sub> - 2.3*<Delta> (albumin) (r = 0.71, p < 0.0001). Conclusion: For intensive care patients with normal or high serum tCO<sub>2</sub> (>21 mEq/l) a simple bedside adjustment of the anion gap by subtracting 1.5 times the difference between measured serum albumin and the 'normal' level of 4.0 g/dl gives a close estimate of the actual anion gap. For intensive care patients with serum tCO<sub>2</sub> <22 mEq/l, correction of the anion gap is well predicted by adding about twice the <Delta> (albumin) to the calculated gap.

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          The fall of the serum anion gap.

          Using modern electrode technology (Beckman ASTRA analyzer), we evaluated the reference range for the anion gap (calculated as sodium minus chloride minus bicarbonate concentrations) in serum to determine whether the 8 to 16 mmol/L reference range in common use is still valid. After measurement of electrolytes in (1) serum from 29 healthy volunteers, (2) aqueous standards verified against National Bureau of Standards reference material, and (3) serum from 120 blood donors, we drew the following conclusions. (1) The reference range for the anion gap has shifted downward (to 3 to 11 mmol/L in one of our laboratories), primarily because of an upward shift in chloride values. (2) Using the ASTRA analyzer, a majority of normal individuals can be expected to have serum anion gaps of 6 mmol/L or less unless chloride calibration is deliberately altered. (3) If the anion gap is to remain an effective tool in diagnosing acid-base disorders, clinicians need to be aware that the traditional reference range may not be appropriate with new instrumentation.

            Author and article information

            Am J Nephrol
            American Journal of Nephrology
            S. Karger AG
            October 2000
            15 November 2000
            : 20
            : 5
            : 369-372
            Department of Medicine, Nassau County Medical Center, East Meadow, N.Y. and State University of New York Health Sciences Center, Stony Brook, N.Y., USA
            13618 Am J Nephrol 2000;20:369–372
            © 2000 S. Karger AG, Basel

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            Page count
            Figures: 1, Tables: 2, References: 7, Pages: 4
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            Clinical Study

            Cardiovascular Medicine, Nephrology

            Serum t[CO2], Serum albumin, Anion gap, Intensive care


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