Results from APMP comparisons on radioactivity measurements of 58Co, 88Y and 166mHo

International comparisons on radioactivity measurements of 58Co, 88Y and 166mHo were carried out within the framework of the Asia-Pacific Metrology Programme. Radioactive sources were also sent to the International Reference System (SIR) at the Bureau International des Poids et Mesures (BIPM) in order to link the results of these regional comparisons to the BIPM key comparison-reference values. The arithmetic mean for the activity results of 166mHo was in good agreement with the SIR reference value, but some disagreements were found for 58Co and 88Y results. The mean values for 58Co and 88Y were both higher than the existing SIR values by about 1.5% and 0.9%, respectively. Although there were some disagreements, it was successfully demonstrated that these regional comparisons could be linked to other international key comparisons through the SIR. The present results were also evaluated to estimate a reasonable uncertainty level for each laboratory, which is very important in the establishment of an adequate traceability scheme for radioactivity standards.