At 2001-05-11 09:44 +1000, Greg Watts wrote:
>Sten,
>
>What you say is correct, however this really means that Total Calcium is
>also a waste of time. Surely it is better to report a corrected calcium,
>with its errors than just a total calcium with even more unknowns.
This is a conditional statement!
Generally speaking any systematic error in the clinical value of a
biochemical measurement (assuming no analytical errors) may be compensated
for by a formula including one or more other biochemical measurement(s).
Such a formula is expected to improve the clinical value (i.e. the clinical
sensitivity and/or specificity) but ONLY if the random error of the
measurements are low compared with the systematic errors.
Thus, for corrected calcium, if serum albumin is close to normal, the
formula compensates only for a slight systematic error, but any random
error in the albumin measurement is added to the total analytical error.
In conclusion: When serum albumin is close to normal total calcium is
"better" than corrected. At some point (depending on the precision of the
methods) this situation is changed in favor of corrected calcium.
As I mentioned before, albumin corrected calcium only compensates for one
part of the protein-binding of calcium. In children (which was the original
question) the globulin concentration is lower and more variable than for
adults. Hence the clinical value of corrected calcium in children has a
lower reliability than for adults.
Living cells do not react to neither total nor albumin corrected calcium
but only to the "free" fraction, i.e. ionized calcium. Fortunately there
exists instruments capable of a direct measurement of this fraction. This
method is slightly more expensive than total and/or corrected calcium, but
there is never any doubt on its interpretation.
Total or albumin corrected calcium may be advocated for screening purpose,
but in any case where an error in calcium metabolism is suspected, ionized
calcium is the method of choice. If you don't have such an instrument in
your laboratory, don't hesitate to send it to another laboratory having it!
Mr Sten Öhman, PhD
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