Correspondence Misconceptions about hyperchloremic acidosis Nagaoka et al [1] suggest that the difference between sodium and chloride and the chloride/sodium ratio are good tools to disclose inorganic apparent strong ion difference (SIDai) acidosis. Although this highlights the importance of chloride ions in the care of critically ill patients, it is not the chloride that changes the acid-base status directly, but instead it is the stoichiometric relationship with bicarbonate that is responsible for the acid-base changes. The authors state that: “hyperchloremia is the most frequent etiology of metabolic acidosis in critically ill patients.” This is not true, because it is clear from the substance HCl that chloride is a proton acceptor, and therefore, it is not an acid but a base. The term “hyperchloremic acidosis” is therefore incorrect and should not be used. Instead, either “hyperchloremia and acidosis” or “normal anion gap acidosis” may be the preferred nomenclature, showing that a high level of chloride is related to a decrease in [HCO 3 - ] to remain electroneutrality. A rather large variation exists in normal values of Na and Cl and an overlap between normal and abnormal Na/Cl difference and ratio, and therefore, it is not justified to use the relation of Cl to Na as an indirect surrogate acid-base parameter. Without chronic hyperventilation, a low bicar- bonate level will result in metabolic acidosis, but with the use of the Na/Cl difference or ratio, there will be always doubt if this is a variation of normal or a pathological test result. It is not remarkable that both Na-Cl and Cl/Na showed excellent correlation with SIDai, as Na + and Cl - are the 2 ions with the highest plasma concentrations and the largest concentration variances in the SIDai formula. Inorganic apparent strong ion difference is also not a proper standard to compare metabolic acidosis with, because SIDai is not a criterion standard of metabolic acidosis. As the authors state, multiple laboratorial measurements can result in errors. This is another reason to use bicarbonate as a direct acid-base parameter. As an example, when large amounts of normal saline are used, an increase in the [Cl - ] and a decrease in [HCO 3 - ] will provide ample evidence for a normal anion gap acidosis, due to fluid resuscitation. The authors also compared 128 patients with 14 healthy volunteers with “acidosis.” However, the patients had a pH of 7.33 ± 0.09, and the healthy volunteers had a pH of 7.34 to 7.39 (median, 7.36), and therefore, these groups cannot be compared [1], because only patients with a pH below that of the healthy volunteers (b7.34) should have been used. Kenrick Berend MD, PhD E-mail addresses: kenber@attglobal.net, kenber@scarlet.an doi:10.1016/j.jcrc.2010.06.006 Reference [1] Nagaoka D, Nassar Junior AP, Maciel AT, Taniguchi LU, Noritomi DT, Azevedo LC, et al. The use of sodium-chloride difference and chloride-sodium ratio as strong ion difference surrogates in the evaluation of metabolic acidosis in critically ill patients. J Crit Care 2010 [Epub ahead of print]. Comment about Misconceptions about hyperchloremic acidosis We thank the comment “Misconceptions about hyper- chloremic acidosis,” a letter on the study “The use of sodium- chloride difference and chloride/sodium ratio as strong ion difference surrogates in the evaluation of metabolic acidosis in critically ill patients” [1]. Currently, there are several methodologies for the interpretation of acid-base distur- bances which are actually complementary [2]. Moreover, there are theoretically different “independent” variables to drive the concentration of [H] + ; in this way, we have used the methodology described by Stewart [3] because this tech- nique and its variants allow a quantitative analysis of the acid-base metabolism. The classical bicarbonate-PaCO 2 (Boston) methodology also allows the quantification of the analysis but with the use of complex mathematical calcula- tions [4]. The methodologies of acid-base interpretation are easily interchangeable at the level of their most basic elements. These interchanges can disclose the limitations of each technique and how combined approach can be used to achieve a more complete understanding about acid-base physiology [2]. As stated in our article [1], the standard base excess was used as a metabolic measure of the acid-base 0883-9441/$ – see front matter © 2010 Elsevier Inc. All rights reserved. Journal of Critical Care (2010) 25, 532–535