A combined sterilization indicator incubator and reader system, including a sterilization indicator vial containing one of a variety of selected biological indicators and a liquid, at least two incubator blocks, each incubator block independently operable to incubate the sterilization indicator vial at a plurality of independently selectable temperatures, each of the incubator blocks including at least one heating element operable to heat the incubator block to any one of the plurality of independently selectable temperatures; a light source, a photodetector and a control system configured to operate the combined system to determine the efficacy of a variety of sterilization processes. The system calculates and compares a slope to a predetermined threshold slope for the biological indicator, and provides output based only on the comparison of the calculated slope to the predetermined threshold slope without first determining either a baseline or a minimum value of the output from the photodetector.