CLIA-certified testing laboratories such as ZRT are required to provide reference ranges as an aid to interpretation of test results.
Ideally, reference ranges provide the expected range of values for a healthy population. When methodology and equipment is identical for testing a particular analyte among different laboratories, reference ranges for that analyte should be the same.
However, when methodologies or equipment for the same test differ somewhat from lab to lab, each lab must provide its own reference range established with its own methods. These ranges are usually very close, but may differ slightly depending on method. An example would be testing testosterone in blood or saliva by extraction and LC-mass spectrometry vs. direct testing by immunoassay.