Several researchers have called for schools to interpret universal screening results using posterior probabilities. Following this recommendation could require schools to move away from direct-route, single-measure screening unless base rates of risk fall within a narrow range. In this descriptive study, we investigated two questions surrounding the feasibility of adopting a probability-based approach in schools. First, how many districts had base rates of risk that would allow educators to classify all students using a single screener? Second, to what extent would yearly fluctuation in base rates require planned flexibility in screening procedures? We used public state test data to estimate base rates of risk for each district in Florida, Oregon, and Wisconsin across four school years. Districts' base rates of risk were rarely within the required range for a single screener to classify all students based on posterior probabilities. Many districts had base rates conducive to direct route screening in at least 1 year but very few had base rates in the required zone across the 4 years. Using posterior probabilities to make screening decisions may require more changes to existing practices than simply interpreting screening results as probabilities. Additional research to promote the use of this approach in schools is needed. [This is the online first version of an article published in "Psychology in the Schools."]