Nomograms for calculating the concentration of ionized calcium of human blood plasma from total calcium, total protein and/or albumin, and pH.
Two nomograms are described. One is a Cartesian nomogram similar to the original McLean-Hastings nomogram but supplemented by an albumin scale and a pH scale to allow calculations in case of an abnormal albumin/globulin ratio or an abnormal pH. The other is an alignment nomogram. The underlying arithmetic equations, which are slightly different for the two nomograms are described. Both refer to 37 degrees C. Calculated and measured ionized calcium are compared for 24 specimens and it is concluded that the calculated value may be subject to a considerable bias amounting to 0.2 mmol/l in the worst cases. Nomograms size A4 and BASIC program are available on request.