Blood and Saliva Composition in Cancer: Similarities or Differences?

Saliva is of great interest for diagnosing and monitoring various diseases, including cancer. Saliva contains a wide range of proteins, some of which are found in blood plasma, while others are unique. Despite the obvious advantages of using saliva for diagnostics and patient monitoring, difficulties in interpreting salivary values remain. Furthermore, the extent to which salivary results correlate with blood test results remains unclear. In this review, we have collected and analyzed all currently available parallel studies of saliva and blood on the nature of changes in biochemical parameters, cytokines, growth factors, hormones, and tumor markers in cancer patients. The most contradictory and divergent changes in saliva and blood were observed when measuring biochemical parameters. Cytokines, growth factors, hormones, and tumor markers in both saliva and blood have a higher reproducibility between independent studies. It is important to consider that the causes and mechanisms behind a particular indicator in saliva may differ from those underlying the same indicators in the blood. Furthermore, these indicators may not always be directly related to cancer. We suggest that comparing identical parameters in blood and saliva is useful in the context of "pathological process routing." The paper also provides interpretations and hypotheses regarding the causes and nature of changes in saliva and blood composition in cancer.