Left: The baseline levels of a biomarker vary over time in multiple individuals (each line represents a unique individual). The range of healthy baseline levels is narrow, as indicated by the shaded area (peach). One individual’s biomarker concentration increases over time (red) and eventually goes outside the normal range, making it easy to detect an abnormal value. Right: The range of healthy baseline levels varies widely between patients. One individual’s biomarker concentration increases significantly over time (red) but stays within the normal distribution range. For this individual, it is important to compare the biomarker concentration value to previous measurements to ascertain that the level exceeds this individual’s baseline. Illustrated by Rachel Davidowitz.