Other tests to rule out reversible disease might include serum electrolytes, B12 levels, CBC, Venereal Disease Research Laboratory (VDRL), and CT or MRI. Diagnosis of Alzheimer's disease remains a diagnosis of exclusion. However, the insidious and subtle onset, with few focal signs (except for higher mental functioning) and a slowly progressive course are characteristic. Careful attention to the pattern of cognitive defects also improves diagnostic accuracy. The intensity of investigation will depend on numerous factors, including age, presence of atypical findings, and the timing of presentation. There is as yet no definite consensus on the most appropriate plan of investigation.