I don't think it's going to be easy to compare Quest and Labcorp on accuracy across different assays. I think you can count on the published ranges on a per assay basis and compare these. I've never done it, but I have strong suspicion you won't see a big difference on most common measures.
You should be able to find details on a test menu page, like this one (it's showing a strange image in the post, but the link is to a details page about fasting blood glucose):
Although methods for glucose analysis exhibit low imprecision at the diagnostic decision limits of 7.0 mmol/L [(126 mg/dL), fasting] and 11.1 mmol/L [(200 mg/dL), postglucose load], the relatively large intraindividual biological variability (CVs of ~5% to 7%) may produce classification errors. On the basis of biological variation, glucose analysis should have analytical imprecision <3.4%, bias <2.6%, and total error <8.0%.1,2