Urine testing for patients with chronic pain has grown explosively over the past decade amid a rising death toll from opioid abuse. Pain doctors say drug testing helps them make sure patients are taking the drugs as prescribed and not mixing them with illegal substances.

Yet the testing boom costs billions of dollars annually and has raised concerns that some labs and doctors run urine tests needlessly – or charge exorbitant rates – to boost profits.

Some insurers have refused to pay, which can leave patients threatened with ruinously high bills they had no idea they had incurred.

“Surprise bills loaded with unexpected expenses and little explanation inflict sticker shock on vulnerable patients,” said James Quiggle, communications director of the Coalition Against Insurance Fraud, whose members include insurers, consumer groups and government agencies. Quiggle said many “puffed-up bills straddle a fine line between abuse and outright fraud.”

Prices for urine tests can vary widely depending upon complexity and the technology used. Some doctors’ offices use a simple cup test, which can detect several classes of drugs on the spot. These tests rarely cost more than $200, and typically much less.

Bills climb higher when labs check for levels of multiple drugs and bill for each one, a practice insurers argue is seldom medically justified. An egregious example is featured in an NPR story about Sunset Labs LLC in Houston. After a patient had back surgery in late 2015, her surgeon prescribed an opioid painkiller and a follow-up drug test that seemed routine – until the lab slapped her with a bill for $17,850.

It charged $4,675 to check her urine for a slew of different types of opioids: $2,975 for benzodiazepines, a class of drugs for treating anxiety, and $1,700 more for amphetamines. Tests to detect cocaine, marijuana and phencyclidine, an illegal hallucinogenic drug also known as PCP or angel dust, added $1,275 more.

Indeed, Kaiser Health News now refers to the urine drug testing business as “Liquid Gold.”

Kaiser Health News, with assistance from researchers at the Mayo Clinic, analyzed available billing data from Medicare and private insurance billing nationwide, and found that spending on urine screens and related genetic tests quadrupled from 2011 to 2014 to an estimated $8.5 billion a year.

There are virtually no national standards regarding who gets tested, for which drugs and how often. Medicare has spent tens of millions of dollars on tests to detect drugs that presented minimal abuse danger for most patients, according to arguments made by government lawyers in court cases that challenge the standing orders to test patients for drugs.

Payments have surged for urine tests for street drugs such as cocaine, PCP and ecstasy, which seldom have been detected in tests done on pain patients. In fact, court records show some of those tests showed up positive just 1 percent of the time.

Urine testing has become particularly lucrative for doctors who operate their own labs. In 2014 and 2015, Medicare paid $1 million or more for drug-related tests billed by health professionals at more than 50 pain management practices across the U.S. At a dozen practices, Medicare billings were twice that high.

Thirty-one pain practitioners received 80 percent or more of their Medicare income just from urine testing, which a federal official called a “red flag” that may signal overuse and could lead to a federal investigation.