spec_keep_cols = c('specimens.specimenGuid', 'specimens.sampleKitGuid')
metadata_keep_cols = c('metadata.subject.cohort', 'metadata.sample.sampleKitGuid', 'metadata.subject.subjectGuid', 'metadata.subject.biologicalSex', 'metadata.subject.birthYear', 'metadata.sample.visitName')
read_keep_cols = c('file.pool', 'file.batchID','sample.sampleKitGuid') 

sample_sheet <- merge(out$specimens[spec_keep_cols], out$metadata[metadata_keep_cols], by.x='specimens.sampleKitGuid', by.y='metadata.sample.sampleKitGuid')
sample_sheet <- merge(sample_sheet, fileDescToDataframe(list(list(read_out[[1]]$descriptors)))[read_keep_cols], by.x='specimens.sampleKitGuid', by.y='sample.sampleKitGuid')