Capillary action is one mechanism microfluidics uses to draw liquid autonomously in a substrate without the need of external energy. This behavior can be exploited to collect accurate volumes of liquids such as blood in narrow columns known as capillary tubes and help the development of inexpensive, user-friendly personalized biomedical tools. Precision bore glass capillaries demonstrate the "state of the art" for volume accuracy and precision, but height and radius must be carefully chosen in order to exploit the capillary action behavior efficiently. This Article investigates the influence of surface glass aging, due to prolonged exposure to humid air, and hematocrit level on the blood capillary rise. It provides also the tools to correctly define the optimum capillary dimensions to collect an accurate volume of blood in a glass capillary tube.