How do we decide primary or secondary acid base disturbance?
For instance, so we have respiratory alkalosis as a compensation for metabolic acidosis? The best and most reliable way is clinical assessment. The analysis of blood gasses can get impressively complicated when mixed acid base disorders are present and we might have to consider the effect of things like the effect of altitude or spurious lab results. It is possible to determine the primary process from the blood gas alone but the situational awareness will suffer if this is done. Confusion and error is more likely.
Does this patient have chronic COPD and always have elevated HCO3-? Now that we have situational awareness we can understand that metabolic alkalosis in this situation is due to chronic respiratory acidosis.
A patient with diabetic ketoacidoais has Kussmal respiration. Now that we have situational awareness we can understand that respiratory alkalosis is secondary to metabolic acidosis
In the classroom in the study of acid base chemistry we know nothing but the values of the blood gasses. There is also the assumption that no errors of laboratory analysis are present. This is a useful exercise but not necessary or appropriate to the clinical environment. The worst thing that can happen in the classroom is that someone writes and F on a piece of paper. In the clinical environment we can have morbidity or mortality.