Age- and influenza activity-stratified case definitions of influenza-like illness: experience from hospital-based influenza surveillance in South Korea.
OBJECTIVE
This study aims to identify clinical case definitions of influenza with higher accuracy in patients stratified by age group and influenza activity using hospital-based surveillance system.


METHODS
In seven tertiary hospitals across South Korea during 2011-2012 influenza season, respiratory specimens were obtained from patients presenting an influenza-like illness (ILI), defined as having fever plus at least one of following symptoms: cough, sore throat or rhinorrhea. Influenza was confirmed by reverse transcriptase-polymerase chain reaction. We performed multivariate logistic regression analyses to identify clinical variables with better relation with laboratory-confirmed influenza, and compared the accuracy of combinations.


RESULTS
Over the study period, we enrolled 1417 patients, of which 647 had laboratory-confirmed influenza. Patients with cough, rhinorrhea, sore throat or headache were more likely to have influenza (p<0.05). The most accurate criterion across the study population was the combination of cough, rhinorrhea, sore throat and headache (sensitivity 71.3%, specificity 60.1% and AUROC 0.66). The combination of rhinorrhea, sore throat and sputum during the peak influenza activity period in the young age group showed higher accuracy than that using the whole population (sensitivity 89.3%, specificity 72.1%, and AUROC 0.81).


CONCLUSIONS
The accuracy of clinical case definitions of influenza differed across age groups and influenza activity periods. Categorizing the entire population into subgroups would improve the detection of influenza patients in the hospital-based surveillance system.
Division of Infectious Diseases, Department of Internal Medicine, Korea University College of Medicine, Seoul, Korea ; Asian Pacific Influenza Institute (APII), Seoul, Korea.
Tae Un
TU
Yang
Division of Infectious Diseases, Department of Internal Medicine, Korea University College of Medicine, Seoul, Korea ; Asian Pacific Influenza Institute (APII), Seoul, Korea.
Hee Jin
HJ
Cheong
Division of Infectious Diseases, Department of Internal Medicine, Korea University College of Medicine, Seoul, Korea ; Asian Pacific Influenza Institute (APII), Seoul, Korea.
Joon Young
JY
Song
Division of Infectious Diseases, Department of Internal Medicine, Inha University School of Medicine, Incheon, Korea.
Jin Soo
JS
Lee
Division of Infectious Diseases, Department of Internal Medicine, Catholic University Medical College, St. Vincent's Hospital, Suwon, Korea.
Seong-Heon
SH
Wie
Division of Infectious Diseases, Department of Internal Medicine, Yonsei University, Wonju College of Medicine, Wonju, Korea.
Young Keun
YK
Kim
Division of Infectious Diseases, Department of Internal Medicine, Korea University College of Medicine, Seoul, Korea ; Asian Pacific Influenza Institute (APII), Seoul, Korea.
Won Suk
WS
Choi
Division of Infectious Diseases, Department of Internal Medicine, Hallym University College of Medicine, Chuncheon, Korea.
Jacob
J
Lee
Division of Infectious Diseases, Department of Internal Medicine, Chungbuk National University College of Medicine, Cheongju, Korea.
Hye Won
HW
Jeong
Division of Infectious Diseases, Department of Internal Medicine, Korea University College of Medicine, Seoul, Korea ; Asian Pacific Influenza Institute (APII), Seoul, Korea.
Woo Joo
WJ
Kim
D000293:Adolescent; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D005260:Female; D006760:Hospitalization; D006801:Humans; D007251:Influenza, Human / Q000175:diagnosis / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D009975:Orthomyxoviridae / Q000145:classification; D062486:Public Health Surveillance; D056910:Republic of Korea / Q000453:epidemiology; D012621:Seasons; D055815:Young Adult