Integration of acid-base and electrolyte disorders.

To the Editor: In his review on acid–base disorders, Seifter (Nov. 6 issue)1 attempts to integrate the “traditional, bicarbonate-centered model” described by Davenport2 and Boron3 with Stewart’s strong-ion-difference model4: strong ion difference (in mmol per liter) = [Na+] + [K+] + [Ca2+] + [Mg2+] – [Cl−]. In unbuffered salt solutions, strong ion difference = [OH−] – [H+] and is thus a pH surrogate. However, adding buffers as strong ion salts (e.g., sodium bicarbonate equilibrated with carbon dioxide, or sodium lactate equilibrated with lactic acid) increases the strong ion difference while having variable or even no effects on pH. Thus the strong ion difference does not uniquely define pH. Changes in the strong ion difference are a consequence of adding acids or bases as strong ion salts; they do not cause pH to change. Stewart4 makes the fundamental error of mistaking correlation for causation. Proteins are generally sensitive to pH per se, not the strong ion difference. No biologic mechanisms exist for directly sensing or regulating the strong ion difference in cells, blood, or other compartments (e.g., the cerebrospinal fluid) — domains where pH is both sensed and regulated. Seifter advocates the strong ion difference as a diagnostic tool, rather like the anion gap. Although this analysis may indirectly provide information on acid–base control, the strong ion difference offers no new mechanistic insight because it does not have a causal role in pH changes.