A 10 days old neonate is posted for pyloric stenosis in surgery. The investigation report shows a serum calcium level of 6 mg/dL. What information would you like to know before you supplement calcium to this neonate –
Blood glucose
Serum protein
Serum bilirubin
Oxygen saturation
Normal calcium level is 8.9-10.1 mg/dl (total calcium).
Slightly less than half of the total serum calcium exists in free or ionized form. Remainder is bound to protein (mostly albumin). ionized calcium is relevant for cell fuction.

"There are few clinical situations in which the total calcium is not an adequate surrogate for the ionized Ca++ concentration. The most common and severe problem is the presence of hypoalbuminemia".

Each 1 gin/di of albumin in the serum binds about 0.8 mg/dl of calcium.
A low total calcium concentration may be normal in a patient with significant hypoalbuminemia.
Now, it is clear from above that a low level of serum protein lowers the total plasma calcium but not the ionized calcium, So, before treating hypocalcemia, measure the serum protein level.