ImportancePotentially inappropriate medications (PIMs) and potential prescribing omissions (PPOs) are used in research to reflect the quality of drug treatment in older people and have been suggested for inclusion in core outcome sets for evaluation of interventions for improved prescribing. Their validation so far, however, is primarily restricted to expert opinion–based processes.ObjectiveTo evaluate the performance of 3 explicit PIM/PPO criteria sets as diagnostic tools to identify inadequate drug treatment in older patients.Design, Setting, and ParticipantsThis diagnostic study analyzed patients aged 65 years or older consecutively included from 2 primary health care centers from October to November 2017. Data were analyzed from February to August 2022.ExposuresThe PIMs/PPOs were concordantly identified by 2 specialist physicians (2018-2019) retrospectively after a planned physician visit, using 3 European PIM/PPO criteria sets and without knowledge of this diagnostic study.Main Outcomes and MeasuresArea under the receiver operating characteristic (ROC) curve, reflecting the ability of PIM/PPO criteria sets to identify the reference standard of inadequate drug treatment, determined by 2 specialist physicians in consensus. Inadequate drug treatment implied that additional action related to the medication could be medically justified before the next regular visit.ResultsA total of 302 patients were analyzed (median age, 74 [IQR, 69-81] years; 178 women [59%]; median number of drugs in the medication list, 6 [IQR, 3-9]); 98 patients (32%) had inadequate drug treatment. A total of 0 to 8 PIMs/PPOs per patient were identified using the Screening Tool of Older Persons’ Prescriptions (STOPP)/Screening Tool to Alert to Right Treatment (START) criteria, 0 to 6 with the European EU(7)-PIM list, and 0 to 12 with the Swedish set of indicators of prescribing quality. The areas under the ROC curve for the 3 sets to identify the reference standard for inadequate drug treatment were 0.60 (95% CI, 0.53-0.66) for the STOPP/START criteria, 0.69 (95% CI, 0.63-0.75) for the EU(7)-PIM list, and 0.73 (95% CI, 0.67-0.80) for the Swedish set. For comparison, the area under the ROC curve was 0.71 (95% CI, 0.65-0.78) using the number of drugs in the medication list.Conclusions and RelevanceIn this diagnostic study, the evaluated PIM/PPO sets had poor to fair performance as diagnostic tools to identify inadequate drug treatment, comparable with a simple count of the number of drugs in the medication list. These findings suggest that use of PIMs/PPOs as indicators of drug treatment quality in core outcome sets for the evaluation of interventions for improved prescribing may need reconsideration.