Significance of a change between two consecutive measured values.

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.