As many as 1 in 8 patients treated for pneumonia in hospitals are misdiagnosed, leading to poor medical outcomes and contributing to the growing problem of antibiotic resistance, new research suggests.

A study that analyzed the data of more than 17,000 adults treated for pneumonia at 48 hospitals in Michigan found that about 12% were improperly diagnosed. Misdiagnoses were most common among older patients, people with dementia and those with an “altered mental status,” such as cognitive disorders. 

The majority of patients who received an inappropriate diagnosis were treated with a full course of antibiotics, researchers found. The study was published Monday in JAMA Internal Medicine.

These findings are significant because treating patients inappropriately with antibiotics was linked to a higher risk of “antibiotic-associated adverse events,” the researchers wrote. These events include delayed recognition and treatment of acute, chronic and novel diagnoses and “may lead to unnecessary antibiotic use, adverse effects and antibiotic resistance.”

Though antibiotics are used to prevent and cure bacterial infections, bacteria can develop in ways that render the antibiotics powerless – leading to antibiotic resistance. When antibiotic-resistant bacteria infect people, they are much harder to treat than non-resistant bacterial infections.

Community-acquired pneumonia, or CAP, is the most common cause of medical hospitalization in the United States and a frequent source of antibiotic overuse.

For this study, researchers wanted to learn more about how often CAP is misdiagnosed and the ramifications of inappropriate diagnoses.

Some of the reasons why doctors may misdiagnose pneumonia is because it is so prevalent, especially among older adults, and because its symptoms often mimic those of other cardiopulmonary diseases. Also, older patients especially suffer poor outcomes from CAP, so physicians may “prefer overtreatment,” according to the researchers.

Recommended medical guidelines encourage the reconsideration, de-escalation or halting of antibiotics within 48 to 72 hours after a pneumonia infection has been ruled out. “Full-course antibiotic treatment of those inappropriately diagnosed with CAP may be harmful,” the researchers wrote.

Noting that the study had important clinical and policy implications, “balancing harms of underdiagnosis and overdiagnosis of CAP remains essential,” they wrote.

Researchers with the Veterans Affairs Ann Arbor Healthcare System and the University of Michigan Medical School analyzed data collected in Michigan hospitals between 2017 and 2020.