A 2 year old child is being evaluated for persistent metabolic acidosis. Blood tests show Na+ 140mEq/1. K+ 3mEq/1, Ca2+ 8mg/1, Mg+2 mEq/l. phosphate 3 mEq/1, pH 7.22, bicarbonate 16mEq/1 and chloride 112mEq/1. The plasma anion gap is: