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ACB-CLIN-CHEM-GEN  April 2012

ACB-CLIN-CHEM-GEN April 2012

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Subject:

Re: RCV for creatinine

From:

"O'Connor John (Royal Devon and Exeter Foundation Trust)" <[log in to unmask]>

Reply-To:

O'Connor John (Royal Devon and Exeter Foundation Trust)

Date:

Mon, 2 Apr 2012 11:28:40 +0100

Content-Type:

text/plain

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Thanks Callum

Particularly in respect of pointing out that we are only looking at an
increase and so I should have used a Z score of 1.65 for 95%
probability. But the issue is still very relevant as is your clear and
concise opinion on this, which is as always very helpful. 

BW John


Hi John

Thank you for your post regarding the definition of what actually is a
significant change in serum creatinine.

I think that many of the difficulties seen  come from the fact that many
studies on markers in AKI use a rise of 50% in serum creatinine as the
"gold standard" and, if smaller changes are used (as they should be),
then significant increases in creatinine would be seen much earlier than
most think and most literature suggests.

The RCV formula that you have is generally useful but not quite right in
this context. You have used 2.77 as the multiplier, which is the square
root of 2 multiplied by 1.96. The 1.96 is a two-sided (two-tailed)
Z-score (number of Standard Deviates). Since we are looking at "rise" or
"increase" in this clinical setting and not "change", then we need to
use one-sided (one-tailed) Z-scores - these are 1.65 for  95%
probability and 2.33 for 99% probability. This is explained in my
Editorial in the first issue of the Annals of Clinical Biochemistry of
2012. Use of these Z-scores actually makes what is a significant rise in
creatinine smaller than you have it and certainly smaller than 50%.
Moreover, as you point out, a rise of 26 umol/L is a different
percentage rise depending on the baseline creatinine and comes form
clinical studies that have used 0.3 mg/dL as the indicator of
significant rise.  Not very objective?

Moreover, in that article, you will see that I advocate using a
rearrangement of the RCV equation to give the probability (from the
one-sided Z-score) that any rise is significant. In my view, this would
be the really clever way ahead. And, of course, we seem to think of 95%
probability as "significant" always, although this interpretation is a
research tool and not really what happens in practice. Have the renal
people involved in this ever been asked or ever thought about what
probability they want the rise to tell that there might be AKI. This
might be interesting.

NHS Tayside has flagged significant and highly significant changes on
reports since well before the turn of the millennium. See page 85 in
Fraser CG. Biological Variation: From Principles to Practice, AACC
Press, 6th printing, 2010. You will also see a graph of probability of
change on the y-axis versus change on the x-axis - it would be really
easy for labs to use their own analytical impression and the database
estimate of within-subject biological variation to create a graph of
probability versus rise in creatinine. Users like nomograms and the
like.

Best regards. Callum


PS. You can post the body of this if you wish, John. Cheers. callum

Professor Callum G Fraser

Phone +44 [0] 1382 553799
FAX    +44 [0] 1382 425679

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