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The test, which gauges a person's average blood sugar control over the past three months, is routinely used to diagnose and monitor type 2 diabetes. But guidelines say it should be done only once or twice a year if a patient has been showing good blood sugar control, according to the study.
All of the patients in the current study fell into that category. Yet nearly 55 percent underwent A1C tests three or four times per year. Another 6 percent had at least five tests per year, according to findings published online Dec. 8 in the journal BMJ.
"I think part of the problem is that we often think more testing is better," said lead researcher Dr. Rozalina McCoy, a professor of medicine at the Mayo Clinic, in Rochester, Minn.
But, as in other areas of medicine, that is not necessarily true in diabetes care, McCoy explained.
If someone is unlikely to benefit from frequent A1C tests, she said, the downsides -- like extra costs and inconvenience -- can't really be justified.
Plus, there are potential risks to patients' health, McCoy pointed out. In this study, frequent A1C tests raised the odds that patients would be started on additional medications to control their blood sugar.
The worry, McCoy said, is that those more-intense regimens would boost patients' risk of dangerously low blood sugar.
The study's findings are based on insurance claims made between 2001 and 2013 for more than 31,000 type 2 diabetes patients age 18 and older.
All of the adults in the study had A1C levels that were consistently below 7 percent within the previous two years -- which meant their blood sugar was under good control.
In general, McCoy said, there's little chance that such patients would benefit from having A1C tests more than once or twice a year. Yet, most patients were tested more often than that over the study period.
But at a certain point, tighter control has "diminishing returns," said Dr. Rodney Hayward, a professor of medicine at the University of Michigan, in Ann Arbor.
According to Hayward, who wrote an editorial published with the study, clinical trials have found that lowering A1C from 8.5 percent to 7 percent can "modestly" lower heart attack risk. But there's also evidence that tight blood sugar control can shorten people's lives, he noted.
So what should people with type 2 diabetes do? McCoy said that when your doctor orders blood work, feel free to ask: "What tests am I having? And how will it improve my care?"
Hayward stressed the importance of asking questions when your doctor wants to add a new diabetes medication. "Discuss what kind of benefit you can expect to get from taking more medication," he said.
He also cautioned that blood sugar is not the only thing that matters. Getting high blood pressure under control and taking a statin are actually the most effective ways to cut the risk of diabetes complications like heart disease and kidney failure.
The study couldn't uncover the reasons for overzealous A1C testing. It's possible, McCoy said, that many doctors, or patients, prefer close monitoring.
"One of the hardest things for doctors is to do less," she said.
"Fragmented care" could be another reason, McCoy added. Study patients with multiple doctors were more likely to receive three or more tests a year, the research revealed.
Still, the researchers did find a positive trend: After 2009, excessive A1C testing started to decline, the study said.
"That's encouraging," McCoy said. "We don't know what's driving the change. But it may be related to growing awareness that tight blood sugar control may not benefit patients."
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