Significance of a change between two consecutive measured values

Abstract

Background. The interpretation of a change between two consecutive measured values of a given quantity in the same patient is still controversial and it is not commonly found in clinical laboratory reports. We present here a manageable and affordable approach for all clinical laboratories to help physicians to interpret these differences. Methods. We selected all pairs of two consecutive measured values of serum albumin concentration and serum thyrotropin concentration, both within the biological reference interval, delivered by our clinical laboratory in a 2-year period (2008–2009). We calculated the relative difference between pairs from which we estimated percentiles 2.5 and 97.5 in order to set an interval defining the significance of a change. We verified these intervals with data from the year 2010. Results. During the 2-year period, we found 122 626 consecutive pairs of serum albumin concentration and 9 374 pairs of serum thyrotropin concentration, with both measured values within our biological reference interval. The intervals defining the significance of a change between two consecutive measured values were: (−14.0 to 17.6)% for the relative differences of albumin concentration, and (−61.4 to 177.8)% for the differences of thyrotropin concentration, which determined 12.7% and 18.4% of significant relative differences, respectively. Similar results were obtained from data for the year 2010. Conclusions. The interval that defines the significance of a change could easily be estimated with the patients’ measured values obtained daily in the laboratory. Our proposal is more appropriate to each specific population than the conventional intra-individual biological variation approach.

Key Words::

• Albumin
• critical difference
• data interpretation
• significant change
• thyrotropin

Declaration of interest: The authors report no conflict of interest. The authors alone are responsible for the content and writing of the article.