Several symptom screening instruments have been developed to identify trauma survivors at risk for chronic posttraumatic stress disorder, but few of these have been thoroughly evaluated to date. In this study, a range of symptom combination scoring rules derived from the literature were applied to the Posttraumatic Diagnostic Scale and evaluated in 4 different samples of trauma survivors (total N = 522) regarding their power to identify people with posttraumatic stress disorder. Results were replicated in a fifth sample (N = 253). Most scoring rules showed lower diagnostic efficiencies than in the original reports. The most stable results were obtained for cutoffs on the Posttraumatic Diagnostic Scale total scale and a new subset of 8 items. The results underscore the need to cross-validate findings before using screening instruments for clinical applications.