Accurate measurements of the presence or absence of an analyte (e.g., MRSA) in a sample are provided. For example, the sample can be subjected to an activation reagent (potentially after an initial reagent has already been added), which can cause a flash signal that increases and then decreases over time. Signal data points can be measured from the flash signal using a detector. A quadratic regression function that fits the signal data points can be determined. An accuracy of the quadratic fit can be determined, as well as a signal-to-background ratio. A difference between a signal-to-background term and an accuracy term can be used as a score that is compared to a threshold to determine whether the analyte is present in the sample.