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https://www.wada-ama.org/en/questions-answers/epo-detection
There's also tests that measure new blood cells and reticulites and other wonderful shit so they can tell if you've been using EPO even if there's no EPO in your system.
http://www.ncbi.nlm.nih.gov/pubmed/15228161
"Five different hematopoietic parameters were initially chosen as indirect markers of rEPO abuse: concentration of serum EPO, concentration of serum-soluble transferrin receptors (sTFr), hematocrit, percentage of reticulocytes, and percentage of macrocytes. New models considering only hemoglobin, serum EPO concentration, and percentage of reticulocytes are simpler and seem to be more sensitive when low doses of rEPO are used. A more direct method of urine analysis (isoelectrofocusing, double blotting, and chemiluminescent detection) based on the charge differences between rEPO and endogenous EPO, related to their carbohydrate composition, provides proof of rEPO use."
This is only because at that stage they had no other way of establishing EPO abuse.