Ionized calcium measurement in vitro: is it necessary?

Experimental data obtained from a study on the inhibition of placental Ca-ATPase by ethacrynic acid were graphically analyzed using either total calcium or ionized calcium as the independent variable. Correct interpretation of the results required actual measurement of ionized calcium in the incubation medium. The assumption that ionized calcium is equal to the total calcium resulted in artifacts and led to erroneous conclusions. These observations emphasize the necessity for accurate measurement of ionized calcium in test systems where calcium effects are being examined.