Abstract We developed an Observer-Reported Outcome (ObsRO) survey instrument to be applied in a multicenter, placebo-controlled, crossover randomized controlled trial of dichloroacetate in children with pyruvate dehydrogenase complex deficiency. The instrument quantifies a subject's at-home level of functionality, as reported by a parent/caregiver, who were instrumental in providing the clinical descriptors and domains that formed the instrument's content. Feasibility testing of the ObsRO tool showed it to be easy to use and comprehensive in capturing the major clinical functional limitations of affected children and requires less than 5 min for a parent/caregiver to complete daily. Highlights • There are no validated markers that predict functionality following therapeutic intervention in patients with primary mitochondrial diseases. • We developed and determined the feasibility of a Reported Outcome (ObsRO) survey tool to measure home functionality. • The ObsRO survey is being applied as the primary outcome measure in a controlled trial of dichloroacetate in children with pyruvate dehydrogenase complex deficiency. [ABSTRACT FROM AUTHOR]