The goal of this observational study is to explore the diagnostic performance of blood biomarkers from the internal jugular vein for the diagnosis of Alzheimer's disease (AD) in patients with cognitive symptoms or concerns. The main question it aims to answer is:

Whether blood biomarkers from the internal jugular vein outperform those from the median cubital vein which is the routine site for venous blood collection?

Inclusion Criteria:

1. age: 40-90 years old (including 40 and 90 years);
2. patients with cognitive complaints or concerns;
3. Willing and able to comply with all aspect of the protocol and provide written informed consent. If a subject lacks capacity to consent in the investigator's opinion, the subject's assent should be obtained, if required in accordance with local laws, regulations and customs, plus the written informed consent of a legal representative should be obtained (capacity to consent and definition of legal representative should be determined in accordance with applicable local laws and regulations).

Exclusion Criteria:

1. Severe medical conditions which are not stably and could affect the subject's safety;
2. Presence of contraindications for PET examination, cerebrospinal fluid or blood collection;
3. Unwilling and refuse to participate in this study.

Patients with cognitive symptoms or concerns were consectively recruited from memory clinics.