At 2000-11-21 22:59 +0000, Aubrey Blumsohn wrote:
>This debate seems to pop up at regular intervals on this mailbase.
>Interpretation of the serum calcium/PTH relationship is complex in a FEW
>patients. However to say that "It is meaningless to discuss ANY error in
>calcium metabolism unless ionized calcium is measured!!" is wrong.
Several correlations between ionized and total (or albumin corrected)
calcium has been published. Indeed good correlation coefficients have been
reported and therefore the wrong conclusion has been draw that total
calcium should be a good indicator of ionized calcium. This is a typical
example of mis-use of correlation coefficient.
It is obvious that a patient with highly pathological value of total
calcium also has a great probability of having a pathological ionized
calcium. But in such a case the patient should be treated (if
hyperparathyreotic: parathyroidectomy) and the result of this treatment
should be monitored. Then the patient is supposed to be normocalcaemic and
then this should be measuered with the most specific method, i.e. ionized
calcium.
But hyperparathyroidism (there are other possible diagnoses too but this is
most debated) is a slowly progrediating disease and the patient has
suffered increasing symptoms during several years and during this time the
patient has often visited several doctors without being accordingly diagnosed.
Each visit costs 75-150 Euro, and probably several (normal or borderline)
assays for total calcium has been performed during these years. Ionized
calcium costs a few Euro more but the correct diagnosis can be establised
several years earlier, saving not only thousands of Euros but also a lot of
suffering for the patient.
It is true that normo(total)calcaemic hyperparathyroidism is found in few
patients, but (nearly) all those cases will be correctly diagnosed if using
ionized calcium. A very small fraction of the patients remain "borderline"
also using ionized calcium, but then a trend analysis should be done.
A positive trend (sampling each 4-6 months) in ionized calcium is
pathognomic for hyperparathyridism.
Ionized calcium is the most accurately regulated analyte in plasma. It has
a very narrow reference range and the within-individual variation is
extremely low for healthy persons (or animals too!).
It is true that this debate pops up regularly. This is obviously because
ionized calcium is not the primary method when investigation errors in
calcium metabolism. This regularly yields inconsistant results needed to be
discussed.
If ionized calcium had been performed, such inconsistant results are
apparently abolished and no debate was necessary. This is not only true for
hyperparathyroidism but for all kinds of errors in calcium metabolism.
Therefore ionized calcium should ALWAYS be done in all suspected error in
calcium metabolism!
Mr Sten Öhman, PhD
Sten Öhman, PhD
Elfin Lab & Milieuconsult
P O Box 133
S-590 70 Ljungsbro
Sweden
Tel Nat: 013-368940 Int: +46 13 368940
Fax Nat: 013-368941 Int: +46 13 368941
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