QbTest is a commercially available, computerised test of attention, impulsivity, and activity designed to assist with the diagnosis of attention deficit hyperactivity disorder (ADHD). Health Innovation East Midlands (formerly East Midlands AHSN), led the implementation of the QbTest on behalf of the 15 Health Innovation Networks across Child and Adolescent Mental Health services (CAMHS) and Paediatric sites in England between April 2020 and March 2023. We evaluate the impact of this programme on diagnostic assessment at participating sites. A mixed-methods approach was used including: case-note data collected on 10-30 cases per site pre and post QbTest implementation; interviews with healthcare staff working with QbTest; and surveys to explore perspectives of healthcare staff and patients/carers. Case-note data was descriptively analysed to compare time to diagnosis (number of appointments and days) pre/post QbTest implementation. Survey data was analysed descriptively. Qualitative interview data was explored using thematic analysis. Case-note data was provided by 20 sites across England. Comparison of mean values pre- and post-QbTest implementation identified a decrease of 0.37 (11.5%) in number of appointments to reach a diagnostic decision, a 55-day (12.5%) increase in days from initial referral to diagnosis, and a 12-day (10.3%) increase in days to reach a diagnostic decision. Exploratory analyses indicated greater benefit for Paediatric services over CAMHS, in terms of a decrease in days from referral to diagnosis and number of appointments to diagnosis. Interviews with healthcare staff (n=21) revealed that the QbTest was perceived to support a faster, more efficient diagnostic process. Survey data (n=65 healthcare staff, n=22 patients/carers) identified that the QbTest helped patients understand their symptoms and the diagnostic decision. Although some logistical issues (e.g., room requirements) and patient issues (e.g., sensory sensitivity) were identified, healthcare staff considered that QbTest was easily incorporated into the ADHD assessment pathway. The national implementation of QbTest in ADHD clinics resulted in a small reduction in the number of clinical appointments needed to reach a diagnostic decision, with greatest benefit demonstrated in Paediatric sites. Data were impacted by COVID-19 therefore, further evaluation is warranted.