The cups of urine travel by express mail to the Comprehensive Pain Specialists lab in an industrial park in Brentwood, Tenn., not far from Nashville. Most days bring more than 700 of the little sealed cups from clinics across 10 states, wrapped in red-tagged waste bags. The network treats about 48,000 people each month, and many will be tested for drugs.

Gloved lab techs keep busy inside the cavernous facility, piping smaller urine samples into tubes. First there are tests to detect opiates that patients have been prescribed by CPS doctors. A second set identifies a wide range of drugs, both legal and illegal, in the urine. The doctors’ orders are displayed on computer screens and tracked by electronic medical records. Test results go back to the clinics in four to five days. The urine ends up stored for a month inside a massive walk-in refrigerator.

The high-tech testing lab’s raw material has become liquid gold for the doctors who own Comprehensive Pain Specialists. This testing process, driven by the nation’s epidemic of painkiller addiction, generates profits across the doctor-owned network of 54 clinics, the largest pain-treatment practice in the Southeast. Medicare paid the company at least $11 million for urine and related tests in 2014, when five of its professionals stood among the nation’s top billers. One nurse practitioner at the company’s clinic in Cleveland, Tenn., single-handedly generated $1.1 million in Medicare billings for urine tests that year, according to Medicare records.