BACKGROUND: In Spain, primary healthcare (PHC) referrals for diagnostic procedures are subject to long waiting-times, and physicians and patients often use the emergency department (ED) as a shortcut. We aimed to determine whether patients evaluated at a hospital outpatient quick diagnosis unit (QDU) who were referred to ED from 12 PHC centers could have been directly referred to QDU, thus avoiding ED visits. As a secondary objective, we determined the proportion of QDU patients who might have been evaluated in a less rapid, non-QDU setting. METHODS: We carried out a cross-sectional retrospective cohort study of patients with potentially serious conditions attended by the QDU from December 2007 to December 2012. We established 2 groups of patients: 1) patients referred from PHC to QDU (PHC-QDU group) and 2) patients referred from PHC to ED, then to QDU (PHC-ED-QDU group). Two observers assessed the appropriateness/inappropriateness of each referral using a scoring system. The interobserver agreement was assessed by calculating the kappa index. Multivariate logistic regression analysis was performed to identify the factors associated with the dependent variable 'ED referral'. RESULTS: We evaluated 1186 PHC-QDU and 1004 PHC-ED-QDU patients and estimated that 93.1% of PHC-ED-QDU patients might have been directly referred to QDU. In contrast, 96% of PHC-QDU patients were found to be appropriately referred to QDU first. The agreement for PHC-QDU referrals (PHC-QDU group) was rated as excellent (ϰ=0.81), while it was rated as good for PHC-ED referrals (PHC-ED-QDU group) (ϰ=0.75). The mean waiting-time for the first QDU visit was longer in PHC-QDU (4.8 days) than in PHC-ED-QDU (2.6 days) patients (P=.001). On multivariate analysis, anemia (OR 2.87, 95% CI 1.49-4.55, P<.001), rectorrhagia (OR 2.18, 95% CI 1.10-3.77, P=.01) and febrile syndrome (OR 2.53, 95% CI 1.33-4.12, P=.002) were independent factors associated with ED referral. Nearly one-fifth of all QDU patients were found who might have been evaluated in a less rapid, non-QDU setting. CONCLUSIONS: Most PHC-ED-QDU patients might have been directly referred to QDU from PHC, avoiding the inconvenience of the ED visit. A stricter definition of QDU evaluation criteria may be needed to improve and hasten PHC referrals.