Objective The objectives of this study are to refine the measurement of appropriate emergency department (ED) use and to provide a natural observation of appropriate ED use rates based on professional versus patient perspectives. Setting Taiwan has a population of 23 million, with one single-payer universal health insurance scheme. Taiwan has no limitations on ED use, and a low barrier to ED use may be a surrogate for natural observation of users' perspectives in ED use. Participants In 7 years, there were 1 835 860 ED visits from one million random samples of the National Health Insurance Database. Measures Appropriate ED use was determined according to professional standards, measured by the modified Billings New York University Emergency Department (NYU-ED) algorithm, and further analysed after the addition of prudent patient standards, measured by explicit process-based and outcome-based criteria. Statistical analyses The area under the receiver operating characteristic curve (AUC) was used to reflect the performance of appropriate ED use measures, and sensitivity analyses were conducted using different thresholds to determine the appropriateness of ED use. The generalised estimating equation model was used to measure the associations between appropriate ED use based on process and outcome criteria and covariates including sex, age, occupation, health status, place of residence, medical resources area, date and income level. Results Appropriate ED use based on professional criteria was 33.5%, which increased to 63.1% when patient criteria were added. The AUC, which combines both professional and patient criteria, was high (0.85). Conclusions The appropriate ED use rate nearly doubled when patient criteria were added to professional criteria. Explicit process-based and outcome-based criteria may be used as a supplementary measure to the implicit modified Billings NYU-ED algorithm when determining appropriate ED use.