Sue, it is my understanding that peripheral blasts are hopefully at a lower percentage than blasts found in the marrow. "The diagnosis of leukemia and MDS is based on BM blasts because, in most cases, the percentage of blasts is higher in BM than in PB".
http://www.nature.com/leu/journal/v1.../2403876a.html
I get my labs done at two different locations. My doctors want me to have the peripheral blast measured at just one of the locations for consistency. I often have the PB tests done halfway between my BMBs.