check albumin- drop in albumin by 1 drops Ca by 0.8, so calculate the real Ca level first if albumin is low.
If corrected calcium is still low then look for the cause:
- Low PTH
- Renal failure- low Vit D
- Hypomagnesemia -Mg needed for PTH function
- Low Vit D absorption
Note: if PO4 is low, PTH must be high (PTH causes excretion of PO4).