Normal value ranges may vary slightly among different laboratories
Low: <8.9 mg/dL (Ionized: <4.8 mg/dL or <1.20 millimol/L)
Critical low: <6.6 mg/dL (Ionized: <3.5 mg/dL)

Correct for hypoalbumimia
Corrected Ca = (0.8 *(Normal Alb - Patient's Alb)) + Serum Ca
Corrected Ca = (0.8 *(Normal Alb - Patient's Alb)) + Serum Ca
15% bound to anions (phosphate, lactate, citrate)
40% bound to albumin
45% free (regulated by PTH, Vit-D)
Misc
Shock
Sepsis
Pancreatitis
Hypomag
Rhabdo (phosphate overload)
Massive transfusion
Systemic Hydrofluoric Acid toxicity
DiGeorge syndrome
Decreased absorption
Vitamin D deficiency
Increased excretion
Alcoholism
Renal Failure
Diuretics
Endocrine
Hypoparathyroidism
Drugs
Cimetidine
Phenytoin
Lasix, loop diuretics
Norepinephrine
Glucagon
Glucocorticoids
Magnesium sulfate
Nitroprusside
Shock
Sepsis
Pancreatitis
Hypomag
Rhabdo (phosphate overload)
Massive transfusion
Systemic Hydrofluoric Acid toxicity
DiGeorge syndrome
Vitamin D deficiency
Alcoholism
Renal Failure
Diuretics
Hypoparathyroidism
Cimetidine
Phenytoin
Lasix, loop diuretics
Norepinephrine
Glucagon
Glucocorticoids
Magnesium sulfate
Nitroprusside
Hypocalcemia
Chorea
Hypernatremia
Sydenham's chorea
Hyponatremia
Neuroleptic malignant syndrome
Ketonemia
Serotonin syndrome