This inquiry is an investigation of item response theory (IRT) proficiency estimators' accuracy under multistage testing (MST). We chose a two-stage MST design that includes four modules (one at Stage 1, three at Stage 2) and three difficulty paths (low, middle, high). We assembled various two-stage MST panels (i.e., forms) by manipulating two assembly conditions in each module, such as difficulty level and module length. For each panel, we investigated the accuracy of examinees' proficiency levels derived from seven IRT proficiency estimators. The choice of Bayesian (prior) versus non-Bayesian (no prior) estimators was of more practical significance than the choice of number-correct versus item-pattern scoring estimators. The Bayesian estimators were slightly more efficient than the non-Bayesian estimators, resulting in smaller overall error. Possible score changes caused by the use of different proficiency estimators would be nonnegligible, particularly for low- and high-performing examinees.