Calcium may be bound to both albumin and globulin. Determinations of serum calcium should be done in conjunction with plasma proteins, as changes in serum proteins can alter the total calcium.
Calcium in the blood is present as:
(1) protein-bound calcium (around 40%)
(2) ionized calcium (around 45-50%)
(3) calcium complexed with anions (around 15%) , which may be significant if the serum phosphate level is high.
|
Protein
|
Calcium Bound
|
|
1 g/dL albumin
|
0.8 mg/dL
|
|
1 g/dL globulin
|
0.16 mg/dL
|
calcium protein bound in mg/dL =
= ((albumin in g/dL) * 0.8) + ((globulin in g/dL) * 0.16))
approximate percentage of calcium protein-bound =
= ((8 * (albumin in g/dL)) + (2 * (globulin in g/dL)))+ 3)
Normal reference range:
(1) total calcium = 8.5 - 10.5 mg/dL
(2) albumin = 3.5 - 5.5 g/dL.
Patients with Altered Albumin Levels
In patients with abnormal serum albumin levels the total serum calcium may reflect the amount bound to the serum albumin while the umbound fraction is unchanged. When faced with a low serum albumin and/or globulin then consider measuring ionized (unbound) calcium.
If the serum albumin is low, then the comparable total serum calcium that would be seen if the serum albumin was normal is
approximate total serum calcium with normal albumin, in mg/dL =
= (current total serum calcium in mg/dL) + (0.8 * (4 - (current serum albumin in g/dL)))
which takes into account the amount bound to albumin (see above).
This can be simplified to (Payne, 1973) with some loss of accuracy:
approximate total serum calcium with normal albumin, in mg/dL =
= (current total serum calcium in mg/dL) + 4 - (albumin in g/dL)
where:
• The approximate total serum calcium level is that which the patient would have if s/he had a normal albumin.
• 4 is the midpoint of the reference range for albumin (3.5-5.5 g/dL)
A different equation was proposed by Orrell (1971)
"corrected" calcium in mg/dL =
= (total calcium in mg/dL) - (0.707 * ((serum albumin in g/dL) - 3.4))
which can be rewritten:
"corrected" calcium in mg/dL =
= (total calcium in mg/dL) + (0.707 * (3.4 - (serum albumin in g/dL) ))
