Lets see if we can tease out this business of 'alignment' and 'calibration'!
(sorry - this is going to be a bit long winded - you can skip down to my recommendations below if you like!)
Because the IFCC HbA1c reference measurement system exists (re-defined HbA1c analyte + reference method), all manufacturers should be using it to calibrate their HbA1c assays because of the traceability requirements of the IVD directive (all calibrants and controls must have values assigned by a reference method of higher order if this exists). It is surprising that Primus have only just done this, as most other manufacturers did so over a year ago.
An assay cannot be made IVDD-traceable or metrologically traceable to 'DCCT', because the NGSP 'reference' (sic) method (BioRex70 column) used is not a method of higher order - it is in fact the least specific method for HbA1c that exists.
Assays are aligned to NGSP/DCCT by a process of adjustment using 'master equations', to make them fit with the HbA1c [DCCT] results and 'risk of complications vs HbA1c' relationships that emerged as a by-product from the DCCT trial. This is not calibration or standardisation in the correct sense, because true reference materials and methods are not involved. It is a pragmatic process of 'pseudo-standardisation by correlation' and the 'Certificate of Traceability' produced by NGSP isn't traceability in the accepted metrological or IVDD sense.
What should happen, IMHO, is this:
[1] manufacturers should strive to make their assays as specific as possible for HbA1c[IFCC] (glycosylated n-terminal hexapeptide of the beta chain of Hb)
[2] they should use calibrants which are metrologically traceable to HbA1c[IFCC] standards using the established network of reference laboratories, to calibrate their assays in terms of HbA1c[IFCC] numbers, so that these are the primary results output from their instruments,
[3] they should state clearly what equation they have used to convert IFCC to DCCT numbers
[4] they should give user laboratories the option of using this stated 'master equation' to convert their results in to HbA1c [NGSP] numbers, so that they can comply with national advice that they should produce 'DCCT-aligned' results, but also be ready to convert to HbA1c[IFCC] results when this is decided.
According to my understanding of the IFFC HbA1c Working Group plan, 2006 was supposed to have been the 'year of implementation' of HbA1c[IFCC] numbers, but little has happened. There has in fact been a deafening silence in the UK! Over 97% of UK laboratories are still producing HbA1c[NGSP] numbers. The new GP contract will perpetuate this no doubt! Nothing seems to have moved on since the 'Fudge' debate at Focus 2004 - or has it??
Proposals for the use of new unitage (mmol HbA1c / mol Hb = IFCC numbers x 10) have been made, but I have heard nothing further of this. Others have (wrongly IMHO) proposed that we should convert HbA1c results into blood glucose values using the DCCT plasma glucose vs HbA1c relationship - which has so much scatter as to render this quite inappropriate.
No doubt there was discussion of this at the AACC last month, but nothing has yet reached me about any decisions that were made.
What to do? Again, IMHO I suggest you
(a) ask your supplier
[1] to confirm the basis of their HbA1c calibration as being the IFCC reference measurement system
[2] to state whether their instrument can produce HbA1c[IFCC] numbers, or if it is tied to producing only 'DCCT' numbers
[3] to state the master equation they use to convert to HbA1c[NGSP] numbers
[4] to give you the option to decide which numbers to produce
(b) ask yourselves if you want to produce results for HbA1c which are analytically valid and traceable to the reference method or if you want to produce results which are aligned to an old, pretty nearly moribund, non-specific method
(c) engage with your colleagues and with national bodies as to when roll out of the new numbers is going to take place
(d) advise and educate your users accordingly
Hope this helps
JGM
Dr Jonathan Middle
Deputy Director, UK NEQAS Birmingham
0121 414 7300, fax 0121 414 1179
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-----Original Message-----
From: Clinical biochemistry discussion list on behalf of Hogan, Tim
Sent: Mon 14/08/2006 11:42
To: [log in to unmask]
Subject: DCCT or IFCC
Primus have recently changed their calibrant for Glycated Haemoglobins/HbA1c
to be in line with IFCC instead of DCCT, If we align ourselves with IFCC
this will result in a significant drop in the values (and will please GPs as
more of their patients will now be within published target limits!)
What are other UK users aligning their results with, and if they are using
IFCC, should the published targets now be modified.
Tim Hogan
Basildon
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