Measurement is the backbone of medical practice, and laboratory tests are becoming increasingly important in informing clinical decisions and managing patients. Much of the uncertainty in laboratory measurements cannot be completely eliminated, even if analytical irreproducibility and bias are minimized - because of biological variation. Therefore, as J.P. McCormack and D.T. Holmes, “uncertainty in laboratory measurements is not a solvable problem, but only a learned one.” Nevertheless, clinical laboratories have over time developed, improved, and implemented an arsenal of approaches, including verification experiments and daily quality control procedures, to ensure accurate and consistent results. In the last decade, efforts have been made to ensure that analytical performance (APC) is truly consistent with analytical requirements (APS) that are appropriate for the specific intended use of a particular measurand in clinical practice. Monitoring measurement uncertainty (ME), particularly intra-laboratory uncertainty (uRw), using internal quality control (IQC) plays a fundamental role in maintaining APC, and many efforts have been and are being made to improve IQC, including the integration of traditional real-time patient sample-based quality control programs. However, little attention has been paid to intra-lot and inter-lot variation, which can significantly affect the analytical and clinical reliability of laboratory results. In this issue of the journal, two articles provide new and important information for clinical laboratory professionals on the issue of differences in measurement results between different reagent lots.