Hi
Oh dear - another metrology rant coming! Please feel free to skip!
A chance concatenation of events (ACB west midlands meeting yesterday - Anne Dawnay gave a talk on Ischaemia-modified albumin (IMA) - and the arrival of the November Annals on my desk today containing articles by Orla Maguire et al and Bob Beetham et al on IMA) prompts me to wonder what it is we think we are doing with this 'analyte'.
It apparently is being 'measured' by an 'albumin cobalt binding' test which is 'standardised' using EDTA solutions. There is a burgeoning literature on ‘IMA’, we now have reference ranges and 'corrected IMA' to take into account albumin concentration, recommendations for clinical use and the FDA has approved it!
With my pedant metrologist hat on I was going increasingly ballistic whilst reading Orla's article (especially the bit about the manufacturer’s ‘reference’ method). I got to the last paragraph of the discussion which has the caveat: "Some issues remain about the ACB assay itself: the exact nature of the entity or entities the assay is measuring (sic) is not known and the fact that the calibrant supplied with the kit is a chelating agent (EDTA) rather than a known concentration of IMA." I breathed a sigh of relief - sense at last which states my main concerns! However, the paragraph goes on to say "Even with these uncertainties, the measurement (sic) of IMA by the ACB assay may have a role in the evaluation of patients with ACS ..."
Here is the rant ... ready?
In order to measure something you have to have a standard which is traceable back to the definition of the measurand - preferably in SI units. This assay does not 'measure' IMA (whatever that is) - it measures the remaining dithiothreitol cobalt binding capacity after interaction with a serum sample. The standards mimic the 'albumin' cobalt binding by chelating increasing amounts of the added cobalt.
Is it only me that worries about this? Is there not some basic scientific work to be done before we can make assertions about 'IMA', what it is, how it gets modified, is it a single entity which can be identified, purified, standardised, what else in serum binds cobalt etc, before we assert to clinicians that an entity IMA exists, we are measuring ‘it’ properly, and that its measurement is clinically useful?
If you agree with me, please send something in support. Or if you have a convincing argument that disposes of mine, please lets hear it!
Maybe I should start a new list - [log in to unmask] Then I needn't disturb anyone else!
Not entirely ironically,
Jonathan
PS ... By the way, are the IMA 'units' in Bob's article on the effect of storage at -20, the 'same' as the IMA 'kilounits' in Orlas's? Samples seem to have the same numerical values and the method appears to be the same.
Dr Jonathan Middle
Deputy Director, UK NEQAS Birmingham
0121 414 7300, fax 0121 414 1179
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