Arterial blood gas analysis of a patient reveals-pH 7.2, HCO3 36 mmol/L pCO2 60 mm of Hg. The abnormality is -
In this patient
pH → decreased (acidosis) N → 7.4
HCO3- → Increased (N → 24 mm Hg ± 2)
PCO2 → Increased (N → 40 Hg ± 2 )
First step

Determine whether it is acidosis or alkalosis
Decreased pH (7.2) suggests acidosis.

Second step

observe HCO3-/PCO2 to determine the cause of acidosis or alkalosis
PCO2 is increased which indicates respiratory acidosis.
HCO3- is increased which indicates metabolic alkalosis
The abnormality which is in sync with the change in pH is increased pCO2 i.e., increased pCO2 is responsible for decreased pH or acidosis So the primary abnormality in this case is respiratory acidosis.

Third step

This step requires determining whether the patient's compensation is appropriate 
Change in PCO2 → 20 mm Hg. (N — 40, observed 60)
Expected change in HCO3- → 2 mmol/L for 20 mm Hg change in PCO2
(For every 10 mm increase in PCO2, HCO3- increases to 1 mmol/L)
So the expected value of HCO3- → 26 mmol/L
But the observed value of HCO3- → 36 mmol/L
This suggests that some other abnormality other than respiratory acidosis exists which has increased the HCO3- value to more than expected.
A disorder which increases the HCO3- concentration is metabolic alkalosis
Thus this is a case of mixed disorder, where respiratory acidosis and metabolic alkalosis exist together.
Respiratory acidosis with metabolic alkalosis
Respiratory acidosis
Respiratory alkalosis with
Respiratory acidosis with metabolic acidosis