Normal anion gap is mostly due to  :
Normal anion gap is due to Unmeasured proteins. Normally in the plasma, the concentration of anion and cations are equal to maintain electrical neutrality. Thus, no real anion gap exists in the plasma and this is purely a diagnostic concept. 
Guyton and Hall Textbook of medical physiology Pg-426
Unmeasured Na+ ion
Unmeasured Cl- ion
Unmeasured proteins
None of the above