Thus, Na + + unmeasured cations (UC) = Cl − + HCO 3 − + unmeasured anions (UA) and the anion gap, Na + − (Cl − + HCO 3 −) = UA − UC The predominant "unmeasured" anions are phosphate (PO 4 3 −), sulfate (SO 4 −), various negatively charged proteins, and some organic acids, accounting for 20 to 24 mEq / L (20 to 24 mmol / L).