Plasma Epstein-Barr virus (EBV) DNA will be measured in native plasma samples of nasopharyngeal carcinoma (NPC) patients, respectively, by three medical centers and a qualified laboratory in Southern China, the highest endemic area of NPC. Passing-Bablok regression and difference plots will be used to compare results from each center to the all-method median and mean values. Agreement among methods will be evaluated against bias derived from a biological variation.

The investigators intend to collect EBV DNA positive plasma samples of NPC patients, distribute the samples to four test centers and compare the measurement results to obtain the current status of the agreement of plasma EBV DNA measuring at different centers with various measurement systems in the endemic regions of NPC. A series of statistical analyses including Cohen's Kappa, intraclass correlation coefficient, and Passing-Bablok regression will be adopted for the consistency test. A harmonization study by using the same calibrators will be conducted.

Inclusion Criteria:

• Age between 18-70 years
• Histologically confirmed NPC or Non-NPC with positive plasma cell-free EBV DNA

Exclusion Criteria:

• unsatisfactory performance status: Karnofsky scale (KPS) <= 70
• Any severe intercurrent disease
• Any contraindication of blood sampling

NPC patients or Plasma EBV DNA positive people