Base deficit is actually a measure of metabolic acidosis. It refers to the amount of base needed to titrate a serum pH back to normal (between 7.35 and 7.45). Let me explain it further, base deficit (or base excess) is the amount of base ( or acid ) needed to titrate a serum pH back to normal. Incidentally, ABG analyzer derives this base deficit using a nomogram based on the measurement of pH, HCO3 and buffer Hemoglobin in few minutes.

The buffers that maintains pH are carbonic acid (H2CO3), bicarbonate (HCO3?), and carbon dioxide. When bicarbonate ions combine with free hydrogen ions it becomes carbonic acid and when hydrogen ions are removed it lessens the pH.