This paper proposes an information-based analytic method for calculating the conditional standard error of measurement (CSEM) in multistage testing (MST) using maximum likelihood estimation. The accuracy of the proposed method was evaluated by comparing CSEMs computed using the analytic method with those obtained from simulation across the same four MST designs. The results show that analytic and simulation-based CSEMs converge as test length increases, indicating that the proposed method provides a reliable approximation for longer tests. However, shorter tests and more complex MST designs require additional items to achieve comparable accuracy. The study also compared the proposed method with Park et al.'s analytic approach. Practical implications of the proposed method are discussed.