The detection of recombinant human erythropoietin (rhEPO) is difficult and becomes more challenging when only microdoses are administered intravenously. Twenty-three subjects were divided into two groups: EPO group (n = 7) and CONTROL group (n = 16). Seven urine and blood samples per subject were collected at least 5 days apart to determine within- and between-subject standard deviations in the percentage of migrating isoforms by the MAIIA test. Six injections of 50 IU/kg bw (boosting dosage) of epoetin beta (Neorecormon, Roche Diagnostics, Hvidovre, Denmark) were performed intravenously during a 3-week period, followed by two microinjections of only 10 IU/kg bw. Blood and urine samples were collected 2, 6, 12, and 72 h after the microinjection, as well as 72 h after the last boosting dose. Sensitivities and specificities of the MAIIA test were examined by absolute and passport thresholds. Sensitivity was 100% for at least 12 h after the microinjection, with ∼30% of plasma samples still exceeding the 99.9% passport threshold 72 h after a microinjection. The specificity was higher for the passport approach compared to the absolute approach, but there were no differences in sensitivities between approaches or between specimens (urine and plasma). We conclude that the MAIIA test shows potential for detecting very small doses of rhEPO.