According to teaching-learning theories, interventions in educational practice require a valid diagnosis of students' learning preconditions. Cognitive dispositions at the beginning of studies are the strongest predictor of knowledge acquisition. In the context of increasing numbers of beginning students with heterogeneous preconditions, valid entry diagnostics are indispensable to inform education practitioners about students' study-related preconditions and to design effective measures for the admission phase. In practice, entry diagnosis is based on indicators such as the grade of university entrance qualification, and general achievement tests. Numerous studies show that such indicators have limited validity in predicting academic performance. Therefore, recent research suggests subject-specific entrance tests to improve entry diagnostics. The use of a test in educational practice for diagnostic purposes can only be justified if the test is able to explain academic performance better than other (established) indicators. In a Germany-wide, large-scale representative study in higher education economics (6,610 beginning students at 46 universities), we investigate whether a validated domain-specific test can provide more valid entry diagnostics than other indicators. We examine the relation between this test and other indicators and demonstrate its diagnostic value, and discuss how the test can be used in the study entry phase.