Influenza Testing: Study Finds Older Patients Are Under-Tested and Under-Diagnosed

The worst flu season in years adds immediacy to a new study tracking flu test utilization patterns based on age. The key finding: Older adults hospitalized with fever or respiratory symptoms during flu seasons are less likely than younger patients to have a provider-ordered flu test.

Age as a Factor in Flu Risk
Existing evidence suggests that older adults are disproportionately hospitalized for the flu and that early diagnosis and treatment improves outcomes. The U.S. Centers for Disease Control and Preventions says that a total of 8,990 laboratory-confirmed influenza-associated hospitalizations were reported between Oct. 1, 2017, and Jan. 13, 2018. During that period:

• The overall hospitalization rate was 31.5 per 100,000 people;

• The highest rate of hospitalization was among adults 65 and over (136.5 per 100,000); and

• The next highest rate was among followed by adults age 50 to 64 (33.2 per 100,000) followed by children age 0 to 4 (22.8 per 100,000).

The Study
Published online in the Journal of the American Geriatrics Society on Jan. 17, 2018, the study was performed by researchers from Vanderbilt University led by Lauren Hartman, M.D., who assessed influenza testing among 1,422 adults hospitalized with acute respiratory illness or nonlocalizing fever at four hospitals (one academic and three community facilities) in Tennessee between November 2006 and April 2012. They prospectively performed reverse- transcriptase polymerase chain reaction (RT-PCR) influenza testing for all patients, even if the patients' providers had not ordered it. The researchers then compared demographic and clinical characteristics of patients whose providers had ordered testing with those of patients for whom laboratory-based diagnostic tests had not been ordered.

The Findings
The researchers found that over the study period providers requested tests for just over one-fourth of patients (28%). They also found an age disparity between tested and untested patients. Thus, the average age for tested patients was 58 versus 66 for untested patients. Part of this difference is attributable to the fact that tested patients were more likely to have flu-like symptoms (e.g. fever, cough, and/or sore throat), which decreased with age.

RT-PCR testing identified flu in 10% of patient; but among those patients with confirmed flu, 43% did not have test orders placed by their providers. Patients receiving care in the academic hospital were more likely to have provider-ordered influenza tests (41% versus 20% in community hospitals). The 450 provider-ordered tests were primarily for antigen detection (97.0%), 7.3% were for viral culture and 8.5% were for RT-PCR.

The Challenge Ahead
"The challenge of influenza diagnosis in hospitalized older adults is to not only identify cases clinically, but select an appropriate sensitive diagnostic test such as RT-PCR," write Hartman and her colleagues. "Further strategies are needed to increase clinician understanding of these challenges in clinically identifying influenza in older adults, as well as the limitations of diagnostic tests, to better diagnose and treat cases of influenza in this vulnerable population."

Takeaway: When it comes to flu, older, hospitalized patients are undertested and under-diagnosed. And even when testing is ordered, it is most often not for sensitive diagnostic tests, like RT-PCR.