Ideally, a tumor marker would be detectable in the blood or other biological fluid, either as a primary substance or as a well-defined, stable metabolite. Most clinical laboratory methods are designed to use samples that are collected using minimally invasive or non-invasive techniques, namely whole blood, serum, plasma, and urine. Analyte-specific assays of other biological fluids, such as stool, saliva, breast milk, and semen have also been developed to help identify the expression of products produced by cancerous cells.
The benefits of sampling such fluids over performing biopsies of suspicious tissues generally include:
- Reduced risk of procedural complications, including infections and injuries
- Reduced cost of clinical laboratory analytical procedures
- Robust ability to identify and quantitate a disseminated marker in the blood and body fluids over the need to pinpoint the tissue sampling of a suspicious tissue site