Biological Variation: Information Site for Laboratory Medicine.
biologicalvariation.com  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 = 21/2 * Z * (CVA2 + CVI2) 1/2

Where: -

CVA = the analytical coefficient of variation,

CVI = 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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