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At 2001-05-11 13:02 +0100, Aubrey Blumsohn rote: >We need to impress on clinicians that the correction is simply a rule of >thumb. Whether we need more than a rule of thumb in most patients is >questionable in my view. >The rationale for measuring ionised calcium (by no means a perfect >measurement) in everyone depends on the perceived utility of detecting >subtle degrees hyper/hypocalcaemia. Anders Kallner pointed out that no reference was cited in this debate and he also mentioned two of the most outstanding scientists in this field. Indeed the number of scientific articles on total calcium, corrected calcium, and ionized calcium is overwhelming. The following was published in 1979 (Larson L, Öhman S. Clin Biochem 12:138-41) and it has been confirmed by several studies since then. It is of utmost importance correctly to handle the samples before analysis. The most important rule is: NEVER, NEVER REMOVE THE CAP!! In blood collected in plain vacuum tubes serum ionized calcium is stable (+-0.01 mmol/L) for at least 4 days at +4C after centrifugation and if the stopper is unremoved. But after removal of the stopper carbon dioxide escapes and ionized calcium rapidly (within half a minute) changes. Modern instruments for ionized calcium have a coefficient of variation of less than 2% and most of them less than 1%. This is considerably better than most methods for total calcium and (for corrected calcium) albumin. Thus ionized calcium is rather "more perfect" than total calcium. The opinion that ionized calcium is "not a perfect measurement" is due to its sensitivity to pre-analytical errors, i.e. carbon dioxide loss leading to an increased pH which, in turn, increases the calcium binding capacity of blood proteins. Not only the analytical variation is less for ionized calcium than total and corrected. In 84 healthy subjects these analytes were measured at two occations with 5 years interval. The intra-individual CV was 1.5% for ionized calcium, 4.8% for total calcium, and 3.3% for corrcted calcium. Mr Sten Öhman, PhD