Why do you have two data source tables with redundant data?
You can start with this relationship:
Datasource1::ID# = Datasource2::ID#
THen, from a layout based on Datasource1, perform a find for all records in Datasource 1 that meet your criteria by entering find mode and putting an = in Datasource1::BloodTest and an asterisk in Datasource2::BloodTest. If you perfom this find by hand, you'll need to add the BloodTest field from DataSource2 to this layout so that you can enter criteria into it.
Is ID# the patient's ID or the database's record ID? If it is the patientID and it would be the same for a given person entered from either data source, Phil's answer would work. If not then you would have to match by name, which would be nearly impossible since one data source could enter Robert Smith and the second enter Bob Smith or there could be multiple people named Bob Smith.
Thanks. ID is the patient's ID.
The datasource files are reports from 2 different sources--one is hospital records and the other is from the lab.