Nonlinearity of assay results within the presumed linear primary measurement range: examples from analysis of CAP linearity survey data

Tatiana Villatoro, Thomas Jefferson University Hospital
Laura J. McCloskey, MD, Thomas Jefferson University Hospitals, Philadelphia, PA
Douglas F. Stickle, PhD, Thomas Jefferson University Hospitals, Philadelphia, PA


We recently described nonlinearity within the primary measurement range (PMR, undiluted specimen measurement range) of a lipase assay as the cause of inaccuracy in lipase results among samples undergoing automated repeat-on-dilution [1]. Nonlinearity of this assay was also evident in interlaboratory data reported in the CAP linearity survey by the changing slope (Δresult/ Δdilution) across sample dilution series results.

As a follow-up to these observations, our objective in this study was to examine CAP linearity survey results for other assays in use at our laboratory (Roche Cobas 500 assays) to determine whether nonlinearity in the primary measurement range could be a factor affecting overall accuracy of assay results.