The interpretation of acid-base status (simple and mixed disturbances) is made possible by evaluation of three independent variables: 1. pCO2. 2. The strong ion difference (SID), which is the difference between the sums of all the strong (fully dissociated, chemically non-reacting) cations and all the strong anions. 3. Non-volatile weak acids (Atot) are formed by inorganic phosphate and albumin. None of the other acid-base variables, re gularly presented with acid-base findings (pH, [HCO3 – ] or BE) can change primarily. They are dependent variables and change only, when one of the independent variable changes. The paper presents calculations of SID, non-measured anions (UA – ) and the anion gap. The last parameter has to be corrected for hypoalbuminaemia. Also [Cl – ] has to be corrected in states with hypo- or hypernatraemia. Examples of various pathological situations and their influence on calculated parameters are presented by means of model states. The values of measured and calculated parameters in the group of 9 healthy volunteers are presented also as well as examples of mixed acid-base disturbances from clinical practice.

        Key words: acid-base status, strong ion difference, non-measured anions, anion gap, mixed acid-base distur-