Abstract Background Cerebrospinal Fluid (CSF) biomarkers are useful in the definitive diagnosis of Alzheimer’s disease (AD) when basic investigations and neuroimaging are inconclusive. In our tertiary memory clinic, the standard CSF biomarker panel consists of Amyloid Beta (AB) 42, Total tau, and Phosphorylated tau. AB42/40 is an additional CSF biomarker which has been shown to improve diagnostic accuracy for AD but is not part of our standard CSF biomarker panel (1). We examined the use of AB42/40 in patients attending our service. Methods In our service, diagnosis of dementia is based on a multidisciplinary consensus meeting (including Geriatricians, Neurologists, Psychiatrists) who review clinical presentation and neuroimaging. In a minority of cases, CSF analysis is performed. We retrospectively evaluated how the addition of AB40/42 to our standard CSF panel impacted diagnostic accuracy for AD, taking a ratio of <0.067 to indicate AD (2). Results Of 1853 patients attending the Memory service between September 2022 and December 2023, CSF was available for 107 patients. Of these 107, average age was 68. 56 were female (52%) and 51 were male (48%). At the multidisciplinary consensus meeting, 81 of these 107 patients were diagnosed with AD (75%), with the remaining 26 (24%) patients being diagnosed with a non-AD process (Vascular disease, Lewy Body etc) or an inconclusive diagnosis. After the addition of AB42/40 to CSF, 8 of the 26 patients (31%) with non-AD or an inconclusive diagnosis were identified as potential AD. Of the 81 with a diagnosis of AD, none were reclassified after addition of AB42/40. Receiver Operating Characteristic (ROC) curve analysis was performed to determine relevant cut off values. Conclusion This study suggests the addition of AB42/40 ratio to our standard CSF biomarker panel improves identification of potential AD cases in our sample. This biomarker is being added to the standard CSF panel for our Memory service.