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August '98
by Gary E. Frank, B.A., B.Ed., R.N.
Palliative Care Nurse Consultant, Edmonton Regional Palliative
Care Program
The total serum calcium level is
made up of three components:
- 45% is protein bound, predominantly
to albumin, and is biologically inert
- 50% is free or ionized calcium
and is metabolically active
- 5% is complexed with citrate and
is also biologically inert.
When the serum albumin level is low a higher percentage of total
serum calcium will be free and metabolically active. Therefore,
even though the total serum calcium may be low the patient may
not be metabolically hypocalcemic. Since the ionized serum calcium
level is a direct measure of the amount of metabolically active
serum calcium no other calculation need be done when it is available.
Unfortunately, this test is not always easily available.*
However, adding .02 mMol/litre to
the total serum calcium level for every 1 gram/litre that
the serum albumin is less than 40 grams/litre will give a
clinically reliable total serum calcium measurement for the
hypoalbuminemic patient. Mathematically this is represented
as follows:
Corrected Total
Serum Calcium = x mMol/L + .02 mMol/L (40 g/L - y g/L)
| Where |
x mMol/L |
= patient's
total serum calcium |
| |
y g/L |
= patient's serum albumin |
| |
g/L |
= normal serum albumin |
| |
.02 mMol/L |
= amount by which the total serum calcium value should
be increased for every g/L that the serum albumin is below
40 g/L (in order to compensate for the increased percentage
of the total serum calcium that will remain unbound when
the serum albumin is decreased) |
* In cases of multiple myeloma it
is recommended that an ionized serum calcium level be done
if at all possible, as a significant percentage of the total
serum calcium may be bound to myeloma proteins.
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