As access to an ionized calcium is not always available a corrected calcium may be used instead. To calculate a corrected calcium in mmol/L one takes the total calcium  in mmol/L and adds it to ((40 minus the serum albumin in g/L) multiplied by 0.02). There is, however, controversy around the usefulness of corrected calcium as it may be no better than total calcium. It may be more useful to correct total calcium for both albumin and the anion gap.