Blog entry by Meguid El Nahas

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by Meguid El Nahas - Friday, 16 November 2012, 6:57 AM
Anyone in the world

This is a post by Dr Pierre Delanaye posted on OLA French that I translated and share in view of its importance:

Biological variability and critical difference are concepts of clinical biology nephrologists often poorly familair with, but are essential for clinicians and clinical decision making.


Variability of a biomarker (eg serum Creatinine, or PTH) is the intra-individual coefficient of variation (CVi) without taking into account the analytical variability (due to imprécisison of the measurement = CVa). One could say that the CVi is the variability of a marker or a biological that is "normal" or physiological. It is determined by measuring from the same patients, for example, the marker at 1-week intervals for 6 weeks. The subject is compared each time itself.
The critical difference integrates the CVi and CVa and represents the change needed to be reached for the change in value in the same patient longitudinal monitoring to be considered significant. For example, with serum creatinine, it is around 10-15% (depending on the method used and its CVa). This means that for a given patient, if two successive values ​​of creatinine did not reach 15%, these results are in fact superimposable and "not different".
The critical difference for serum PTH as we have determined (publication pending) is 40-50%! which is huge. This means a difference of more than 50% between PTH values ​​for a patient to say that it has truly changed...!!!!
These concepts are important to grasp to assist with decision making when changes in a biomarker can trigger a clinical management decision.
+ See also the very interesting work of our English colleague EJ LAMB
[ Modified: Thursday, 1 January 1970, 1:00 AM ]