Researchers are analyzing pools of patient information collected from routine checkups to help doctors better diagnose their patients.

This type of data is easier to mine thanks to the rise in electronic health records that contain information collected in regular doctor visits.

In one instance, a group of researchers looked at data from patients with sore throats and came up with a way to help determine whether people should see a doctor for a strep throat test or stay home and take aspirin. In another, a pediatrician was able to follow a hunch to study a connection between an eye disease and allergies in young patients with arthritis.

The quantity of data that is collected during routine medical visits is huge. It can include everything from X-rays and blood tests to doctors' written observations.

"We should be learning from the record of routine medical practice," says Nigam H. Shah, assistant professor of medicine at Stanford University School of Medicine. He is one of the authors on a 2013 study published in the journal Pediatric Rheumatology that mined electronic health records of young patients with arthritis to see if there was a connection between allergies and an eye inflammation condition called uveitis.

Some early data-mining results are starting to influence care at clinics through published papers, presentations at conferences and a push by federal agencies interested in finding ways for academic centers and hospitals across the country to learn from big data.

Big data generally refers to information that is too large—terabytes to petabytes or even exabytes of memory—to process with older standards of processing power. Researchers say it is important to do additional studies beyond data mining to learn more.

The data is anonymous. Identifying information about patients is hidden when used for research. But many in the field are still figuring out how to protect privacy and collect standardized, useful information from multiple sites. Large national efforts, some supported by funding from the National Institutes of Health, are trying to come up with better ways to extract useful information from these searches.

Spotting a Crucial Link | Jennifer Frankovich, a pediatric rheumatologist, examines Elijah Taurke in Palo Alto, Calif. She helped find an association between allergies and uveitis in juvenile arthritis patients. Lucile Packard Children's Hospital Stanford

Kenneth D. Mandl of Boston Children's Hospital is one of the authors of a 2013 paper in Annals of Internal Medicine looking at records from patients who visited a retail health clinic chain with a sore throat. He said many ideas are generated by patients or caregivers during routine care. Data-mining offers a way to test ideas and hypotheses, said Dr. Mandl: "It gives you data over time on millions and millions of patients."

His research team aimed to create a score to help figure out the types of patients who didn't need to come in right away for strep throat testing. The researchers used data from 71,776 patients over the age of 15 with sore throats who visited a chain of health clinics over a 15-month period. Uniform data about the patients was gathered and kept in an electronic health record. The study estimated that 230,000 unnecessary doctor's visits could be avoided annually for those with low enough strep throat risk to stay home.

A 2014 paper published in JAMA Neurology grew out of Dr. Mandl's efforts to help a family member with inflammatory bowel disease and epilepsy. Dr. Mandl wondered if the two might be linked. Neither the neurologist nor the gastroenterologist he consulted saw a connection. So Dr. Mandl's team turned to big data from doctor visits and prescription information submitted to a large health insurance plan.

The paper found that patients with autoimmune diseases, such as Crohn's disease and rheumatoid arthritis, have a fivefold increased risk of epilepsy, and that seizures tend to begin within the first one to two years after an autoimmune disease diagnosis. Dr. Mandl said more studies need to be done, but that doctors who see people with autoimmune diseases should be monitoring for development of seizures. "Things become evident on the population level that are not evident to the individual practitioner," he said.

Jennifer Frankovich, a pediatric rheumatologist at Lucile Packard Children's Hospital Stanford in Palo Alto, Calif., follows pediatric patients who have juvenile idiopathic arthritis. Doctors know that these patients are at higher risk of getting an eye inflammation disease called uveitis. If not detected and treated in time, uveitis can cause serious eye damage, including blindness.

The difficulty for doctors is predicting which patients will get the condition. Sometimes the children experience blurry vision but don't tell anyone. Other times, there are no obvious signs of a problem until serious damage is done.

As a result, eye exams are done on the patients at random times. When several families mentioned during routine checkups that the uveitis seemed to act up when their children had allergies, Dr. Frankovich wondered if there might be a connection. Stanford researchers used data from electronic health records of unnamed patients from Lucile Packard Children's Hospital from 2000 to 2011, including clinical notes. They concluded that there was an association between allergies and flare-ups of uveitis in patients with juvenile arthritis.

While the doctor says more research must happen to prove the connection, the study suggests that when a patient has allergies, an eye exam should also be scheduled.

Elijah Taurke, a 12-year-old from Pacific Grove, Calif., plays the trombone and is on a travel soccer team. He was diagnosed at age 6 with juvenile idiopathic arthritis and within a year got uveitis too, says his father, Alex Taurke.

Elijah has a cataract in one eye caused by the uveitis. Elijah suffers from sinusitis, and learning of the association between allergies and uveitis has led his parents to be "more vigilant to go see the ophthalmologist when he has nasal issues," his father says.

Dr. Frankovich says that when she is with patients, she collects extensive information, giving detailed physical exams and asking questions about everything from new pets to changes at school or at home.

She wants to know as much as possible, she says, both to help patients now and because, down the road, "we will mine the data."

Write to Amy Dockser Marcus at amy.marcus@wsj.com