["Stewart's and Fencl's methods have recently been proposed to interpret acid-base disorders where traditional theory has proven inadequate. Our objectives were to evaluate: (1) the occurrence of acid-base disturbances in critically ill patients and their trend over the first 3 intensive care unit days, (2) whether Stewart's theory offers advantages over the traditional theory in the diagnosis of acid-base metabolic disturbances and (3) whether variables derived from Stewart's and Fencl's methods offer advantages over the traditional method to predict patient mortality.", "A prospective cohort study in a general intensive care unit. Blood samples were analysed for arterial blood gases, electrolytes and proteins. PaCO2, pH, bicarbonate, base excess, standard base-excess, sodium, potassium, chloride, phosphorous, calcium, magnesium and lactate were measured. Anion gap, Stewart's and Fencl's variables were calculated.", "When using Stewart's method, metabolic acidosis and metabolic alkalosis were found in 92.9% and 93.4% of samples, respectively. Corresponding figures obtained with the traditional method were 15% and 18.7%. In 245 (64.5%) samples, Stewart's method revealed that metabolic acidosis and alkalosis were simultaneously present, whereas the traditional method revealed a normal acid-base status. Strong ion gap increased significantly over the first 3 intensive care unit days. Strong ion gap and lactate were independent predictors of 28-day mortality."] Is the early phase of critical illness a progressive acidic state due to unmeasured anions?