The best replacement for ionized calcium is calcium.

The best alternatives to ionized calcium are rated as following:
★★★★★  calcium
★★★★★  serum calcium
★★★★★  total calcium
★★  corrected calcium
★  regular calcium
★  normalized calcium
★  calcium level
  adjusted calcium

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Speaking about ionized calcium, one probably shouldn't forget about parathyroid.

Other search terms related to ionized calcium and their ratings are:

★★★★★ parathyroid
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ionized calcium
serum calcium