CalciumSerum calcium · Ca
Blood calcium, kept in a narrow range by hormones regardless of what bone holds.
Almost all the body's calcium is locked in bone. The small fraction circulating in blood is what this measures, and it is held within tight limits by parathyroid hormone and vitamin D, which move calcium in and out of bone and adjust how much the gut absorbs and the kidneys keep.
It matters because blood calcium says little about bone density. Someone can have thinning bones and a perfectly normal blood calcium, because the body will take calcium from bone to defend the blood level.
In practice roughly half of blood calcium is bound to albumin, so a low albumin drags the total down without the active calcium having changed. That is why reports also print an adjusted figure.