A method of detecting the presence, or monitoring the severity of a condition characterised by the presence of fragments of a marker protein in the brain of a patient. The method comprises: (i) providing a sample comprising macrophages obtained from the patient; and (ii) detecting the presence of the marker protein or fragments thereof in the macrophages. The presence of abnormal levels of the marker protein and/or fragments thereof in the macrophages is indicative of the presence of the condition in the patient. The condition and the marker proteins can be: Alzheimer's Disease and the Abeta peptide, Parkinson's Disease and ubiquitin, Multiple Sclerosis and myelin basic protein, FrontoTemporal Dementia and tau, Amyotrophic Lateral Sclerosis and tau, Parkinson's disease, Lewy Body dementia or Alzheimer's Disease and alpha-synuclein.