Our group of Clinical Biochemists has been reviewing the MRCPath short
questions published monthly in ACB News (Deacon’s Challenge). In the answer
to Deacon’s Challenge # 8 (20th Oct 2001) an equation was alluded to for
calculation of a critical change (%CC) in 2 values for serum cholesterol,
given the analytical and biological co-efficients of variation (Cva and
Cvi). His statement was:
“For 2 results to be significantly different they have to be at least 2.8
SD’s apart. The derivation for this can be found on page 105 of Clinical
Investigation and Statistics in Laboratory Medicine, Richard Jones and
Brian Payne, Venture Publications 1997.”
Equally, when Callum Fraser wrote an invited essay on this and other topics
on Westgard’s website (www.westgard.com/guest12.htm) he gave a similar
equation for Reference Change Value (RCV, which I take to be
interchangeable with the term Critical Change):
%RCV = Z * SQRT(2)* SQRT(Cva-squared + Cvi-squared)
Clearly the equation is built from 3 terms: the first is the standard
variate, Z, which would be 1.96 for 95% confidence; the second is root two;
the third is a combined factor that takes account of both analytical and
biological variability of the marker in question (1.96* SQRT(2) = 2.8).
In Jones and Payne’s statistical text, the suggestion is that when the
analytical and biological Cv’s are equal it is possible to replace the two
Cv's with one value and include a multiplier term, and since Cv is now a
double term we should use ‘root two’ as the multiplier. I can see that we
might use:
%CC = 1.96 * SQRT(2) * (Cv)
However I can not see why the ‘root 2’ term should be applied when both Cva
and Cvi are used individually in the equation, as in the example from
Callum Fraser's paper on Westgard's website.
So, my question is this. What is the source of the ‘root two’ term and why
does Callum Fraser use it when both Cvs are also used? Can anyone please
shed some light?
------ACB discussion List Information--------
This is an open discussion list for the academic and clinical
community working in clinical biochemistry.
Please note, archived messages are public and can be viewed
via the internet. Views expressed are those of the individual and
they are responsible for all message content.
ACB Web Site
http://www.acb.org.uk
List Archives
http://www.jiscmail.ac.uk/lists/ACB-CLIN-CHEM-GEN.html
List Instructions (How to leave etc.)
http://www.jiscmail.ac.uk/
|