Systems and methods for processing sample processing devices. The system can include a sample processing device comprising a detection chamber, a motor configured to rotate the sample processing device about an axis of rotation, and an optical module operatively positioned relative to the sample processing device and configured to determine whether a selected volume of material is present in the detection chamber of the sample processing device. The method can include rotating the sample processing device about an axis of rotation, and determining whether a selected volume of material is present in the detection chamber, while rotating the sample processing device. In some embodiments, determining whether a selected volume of material is present can be performed by optically interrogating the detection chamber for an optical property of the material.