The conversion rate of ALA to EPA/DHA has been well studied in humans. What about EPA conversion to DHA?

A direct assessment requires the synthesis of a deuterated ("tagged") EPA which is fed to humans and then the appearance of deuterated EPA in blood/other tissues is measured. We await such important research to be done. Interestingly, the studies that fed concentrated EPA (essentially devoid of DHA)to humans for relatively short time periods found no net rise in blood levels of DHA.