In Tietz Textbook of Clinical Chemistry 3rd ed. p288 the formula for the 95% confindence interval of a true change is:

(x1-x2) +/- 2 * root[sqr(sd1)+sqr(sd2)]
(x1-x2) +/- 2 * sd * root[2]

i.e. +/- 2.7 times standard deviation!
Or did I get it wrong?

Dieter

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Dr. med. Dieter Auch
Inst. f. Klin. Chemie und Pathobiochemie
Gaffkystr. 11
D-35392 Gießen
Tel: +49 641 99 41552
Fax: +49 641 99 41569
 

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Gesendet: Donnerstag, 30. November 2000 04:29
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Betreff: Significant change

I'd like to share the contents of a memo from our principal scientist to the
director of pathology about what can be regarded as a significant change in
glycated Hb results.  I do so of course with his permission - I think it's a
clear statement of analytical goals.

Gary,

The basic statistical rule for interpreting serial trends is:

There is a 95% probability that there is a real change if the difference between
the two results is greater than 2.7 times the long term assay standard
deviation. This SD will vary with concentration - so you need to know the