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**Reference Change Value**

For serial results to be significantly different the difference in numerical results must be greater than the combined variation inherent in the two results. Traditionally called the *Critical Difference *it is now better known as* Reference Change Value (RCV) *to keep nomenclature consistent with international recommendations. The value takes account of the biological and analytical variation within the result and calculated using the following formula-

RCV = 2^{1/2} * Z * (CV_{A}^{2} + CV_{I}^{2})^{ 1/2}

Where: -

CV_{A} = the analytical coefficient of variation,

CV_{I} = within subject biological variation

If Z = 1.96 then a change in any direction (2 tailed)= to the RCV is *“Significant”* at 95% probability

If Z = 2.58 then a change in any direction (2 tailed)= to the RCV is *“Highly Significant” *at 99% probability.

If Z = 1.65 then a unidirectional change (1 tailed) = to the RCV is *“Significant”* at 95% probability

If Z = 2.35 then a unidirectional change (1 tailed) = to the RCV is *“Highly Significant” *at 99% probability

The RCV may be reported as a percentage or as an absolute value with same unitage as the analyte.

See RCV Calculator on the Tools page of this site

*Ref: Fraser CG. Biological Variation: From Principles to Practice. 2001, 67-90. Published by the American Association of Clinical Chemistry. ISBN 1-890883-49-2
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