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For the last few months, I’ve started my mornings with the same routine: reviewing New York Times readers’ medical bills over coffee.

The documents are part of a project I began in August, asking readers to send in charges they’ve faced for coronavirus testing and treatment. The bills can reveal important information that hospitals and doctors often keep secret, like the true cost of a hospital stay or how much fees vary from one patient to the next. A few trickle in each day, and I look over every one.

If you read enough — I’m at 400 and counting right now — they can also show patterns in how providers bill patients. That’s how I came to my latest story, which looks at a doctor based in Greenwich, Conn., named Steven Murphy. Patients contend that he used public testing sites to run unnecessary and expensive tests. Dr. Murphy defended his billing practices and said he was providing a vital service to the community.

The story caught my eye only because of the high number of patients sending bills from his testing sites in New York City’s northern suburbs. Without that wave of reader submissions, I never would have known something was awry.

The first bill with this provider was sent to me on Aug. 3, the same day I started the collection project. A woman from outside New York City said she was “shocked” to see a drive-through coronavirus testing site bill her insurance $1,944. “How can this provider bill $480 for a 3 minute phone call giving test results?” she asked in her submission.

The next day, another submission from a patient of Dr. Murphy’s came in. “I can pay my bill but I’m astounded at the cost the provider is charging for the test,” the patient wrote. Four days later, there was another patient reporting the “exorbitant rates” she faced, also from Dr. Murphy.

When the first bill came in, I thought it was interesting but didn’t quite see a full story. The patient’s high charges could be an anomaly. By the end of the summer, I had six separate bills and an inkling something was amiss. I had slowly amassed a data set that showed that a doctor who was staffing public testing sites was repeatedly billing insurers more than $1,000 for coronavirus tests.

This is information that powerful health care lobbies typically try to keep secret. The American Hospital Association recently sued the Trump administration over new rules that would make health care prices public (they lost that challenge but have said they plan to appeal to a higher court). This makes it frustratingly difficult for reporters and patients to answer seemingly basic questions, like how much a coronavirus test costs in the United States.

The patients who went to one of these drive-through testing sites had no chance of knowing what charges they would face beforehand. Their bills, however, can help shine some light on the issue. They lay secret prices out in plain sight.

They also contain five-digit billing codes, which I’ve had to become more adept at reading the longer I’ve covered the health system. Those codes show exactly what service the doctor provided. In this case, those codes tipped me off that Dr. Murphy wasn’t just billing for coronavirus tests, as his patients thought. He was billing for 20 other respiratory pathogens, too.

The bills are important, but they are never the entire story. After I had amassed enough bills to start seeing a pattern, I began interviewing patients about their experiences. I spoke with Dr. Murphy about his billing practices. He said the use of the larger test was appropriate because it could catch a wider range of diseases, particularly for those who were symptomatic.

I talked to medical billing experts to get their insights, and to the elected officials who had set up the testing sites. I filed public records requests, and when they came back, combed through thousands of pages of emails between Dr. Murphy and town government workers.

In most cases, the patient bills I receive don’t turn into stories. Some don’t reveal new information. Of those that do, we often don’t have enough submissions to show a pattern or the ability to look into each one.

In this case, though, we got lucky: A critical mass of readers decided to take a few minutes to send us a medical bill they found odd. Their decisions allowed me to do my job better, and tell a story that otherwise may have gone untold.