In a clinical laboratory, a number on a screen can shape a life. A test result may help diagnose a rare metabolic disorder, guide treatment, or reassure a family that a feared condition is absent. Before that number reaches a doctor, however, the laboratory has to answer a quieter question: can we trust it? That question is the purpose of quality control, or QC. Laboratories repeatedly test samples with known properties and examine whether their instruments are producing results that remain accurate and precise. When something begins to drift, QC can reveal the problem before unreliable patient results are released. The principle sounds straightforward. In biochemical genetics laboratories, however, the reality is anything but simple.