2 ms·In the US this title seems to be held by Quest diagnostics.by Consultant32452 7y agoIn the US this title seems to be held by Quest diagnostics.dqv 7y agoWell Quest but also LabCorp. If we go on employee count, then LabCorp would have the bigger hold. I can't tell if Sonora Quest and Bioreference are subsidiaries of either of these two larger companies.anamexis 7y agoThere's also LabCorp.