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Flu-related hospitalization rates underestimated
National surveillance data continued to underestimate rates of influenza-related hospitalizations in the United States despite the use of highly sensitive diagnostic tests, according to recent findings.
“Researchers have examined rates of influenza-associated hospitalization during different influenza seasons,” researcher Alexander J. Millman, MD, physician and epidemic intelligence service officer at the CDC’s Influenza Division, and colleagues wrote. “However, comparing rates between seasons can be inaccurate without accounting for changes in the sensitivity of diagnostic testing used.”
Alexander J. Millman
The researchers analyzed data from the CDC’s Influenza Hospital Surveillance Network (FluSurv-NET) to identify the testing platforms used for various age groups during the 2003-2013 influenza seasons. Literature was searched to determine the sensitivity ranges for a variety of tests used during the study period, including reverse transcription PCR (RT-PCR), viral culture, direct fluorescent antibody staining and rapid influenza diagnostic tests (RIDTs). The investigators calculated observed rates of influenza-associated hospitalization by age group and adjusted rates based on the sensitivity of these tests.
Millman and colleagues found a shift in the use of diagnostic influenza tests among identified cases, especially after the 2009 pandemic. Before then, RIDTs were used most commonly, and they identified about 70% of cases reported through FluSurv-NET. After 2009, however, RT-PCR — the most sensitive test among all age groups — supplanted RIDTs as the most commonly used tool. Its use in identifying cases rose from less than 10% before 2009 to approximately 70% afterward.
Influenza diagnostic tests had the poorest sensitivity in adults aged 65 years and older, the researchers added.
The researchers also found seasonal disparities in observed and adjusted rates of influenza-associated hospitalization per 100,000 population for all age groups. Observed hospitalization rates ranged from 7.3 during 2011-2012 to 50.5 during 2009-2010 in children aged younger than 18 years; from 3 during 2006-2007 to 30.3 during 2009-2010 for adults aged 18 to 64 years; and from 13.6 during 2008-2009 to 181.8 during 2012-2013 for adults aged 65 years and older.
After adjusting for test sensitivity, those rates increased approximately 15% for children aged 18 years and younger, 20% for adults aged 18 to 64 years, and 55% for adults aged 65 years and older.
“The primary reason for this underestimation is that diagnostic test sensitivity is imperfect, so true cases of influenza are missed,” Millman and colleagues wrote. “Furthermore, test sensitivity varies with patient age, and all types of influenza diagnostic tests, but especially RIDTs, have comparatively poor sensitivity in older persons. Adjusting hospitalization rates on the basis of diagnostic test sensitivity enables more accurate and timely comparisons of associated disease activity in hospitalized patients over time.” – by Jen Byrne
Disclosure: The researchers report no relevant financial disclosures.