It is difficult to provide an unequivocal response to your question regarding the accepted population measured blood serum levels for DHA and EPA as well as arachidonic acid (AA). The levels of all three of these fatty acids is strongly dependent upon dietary intakes such that those populations (eg., Japanese) with higher intakes of the omega-3 fatty acids as DHA/EPA from fish will show considerably higher levels in the blood serum lipids and moderately lower levels of AA omega-6. With respect to the risk of fatal ischemic heart disease, there are various studies in the populations showing that higher levels of DHA and EPA we well as DHA/EPA (combined) in the blood serum lipid biomarker (phospholipid) are associated with a lower risk of cardiovascular disease and the risk of heart attacks. For example, a published study by Lemaitre et al. (Am. J. Clin. Nutr., 77:279-280 (2003)) indicated that older adults with levels of DHA/EPA (combined) in their blood plasma phospholipid reaching at least 4.6% of total fatty acids had a 70% lower risk of fatal ischemic heart disease as compared to those with much lower levels.