A patient with salicylic acid poisoning has the following arterial blood gas analysis report: pH is 7.12 , PCO2 is 18 mm Hg HCO3– is 12 mmol/L. The resulting acid-base abnormality can be best labelled

First step Determine whether it is acidosis or alkalosis Low pH (7.2) means that it is acidosis Second step Observe HCO3-/ CO2 to determine the cause of acidosis or a decrease in pH Decreased PCO2 means respiratory alkalosis Decreased HCO3- means metabolic acidosis The abnormality which is in sync with the change in pH, i.e., the abnormality which is causing acidosis is decreased HCO3- So, the primary abnormality in the case is metabolic acidosis Third step The third step requires determining whether the patient's compensation is appropriate or not. Refer to the table Change in HCO3- →12 mmol / L Expected change in PCO2 → 1.25 x 12 mmol =15 for 12 mmol change in HCO3-  (PaCO2 is expected to decrease 1.25 mm Hg for each mmol/L decrease in HCO3-) Thus, the decrease in PCO2 is more than the expected. This suggests that there must be coexisting respiratory alkalosis, which causes more than the expected decrease in PCO2.