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. 2025 Feb;19(2):e70069.
doi: 10.1111/irv.70069.

Accuracy of ICD Influenza Discharge Diagnosis Codes in Hospitalized Adults From the Valencia Region, Spain, in the Pre-COVID-19 Period 2012/2013 to 2017/2018

Ainara Mira-Iglesias  1   2 , Mónica López-Lacort  1   2 , Hélène Bricout  3 , Matthew Loiacono  4 , Mario Carballido-Fernández  5   6 , Joan Mollar-Maseres  7 , Miguel Tortajada-Girbés  8 , Germán Schwarz-Chávarri  9 , F Xavier López-Labrador  2   10 , Joan Puig-Barberà  1 , Javier Díez-Domingo  1   2   11 , Alejandro Orrico-Sánchez  1   2   11
Affiliations

Accuracy of ICD Influenza Discharge Diagnosis Codes in Hospitalized Adults From the Valencia Region, Spain, in the Pre-COVID-19 Period 2012/2013 to 2017/2018

Ainara Mira-Iglesias et al. Influenza Other Respir Viruses. 2025 Feb.

Abstract

Background: International Classification of Diseases (ICD) codes obtained from real-world data can be used to identify influenza cases for epidemiological research but, without validation, may introduce biases. The objective of this study was to validate ICD influenza discharge diagnoses using real-time reverse transcription-polymerase chain reaction (RT-PCR) laboratory-confirmed influenza (LCI) results.

Methods: The study was conducted during six influenza seasons (2012/2013-2017/2018) in the Valencia Hospital Surveillance Network for the Study of Influenza (VAHNSI). Patients aged 18+ years were identified via active-surveillance and had to meet an influenza-like illness (ILI) case definition to be included. All patients were tested for influenza by real-time RT-PCR. Main and secondary influenza discharge diagnosis codes were extracted from hospital discharge letters. Positive predictive values (PPVs) and the complementary of the sensitivities (1-Sensitivity) of ICD codes with corresponding 95% credible intervals (CrIs) were estimated via binomial Bayesian regression models.

Results: A total of 13,545 patients were included, with 2257 (17%) positive for influenza. Of 2257 LCI cases, 1385 (61%) were not ICD-coded as influenza. Overall, 74.73% (95% CrI: 63.24-84.44) of LCI were not-ICD coded as influenza (1-Sensitivity) after adjustment. Sensitivity improved across seasons and with increasing age. Average PPV was 74.02% (95% CrI: 68.58-79.17), ranging from 43.71% to 81.57% between seasons.

Conclusion: Using only main and secondary discharge diagnosis codes for influenza detection markedly underestimates the full burden of influenza in hospitalized patients. Future studies, including post-COVID context, using prospective surveillance for ILI are required to assess the validity of hospital discharge data as a tool for determining influenza-related burden of disease.

Keywords: ICD codes; active surveillance; diagnosis codes; discharge diagnoses; hospitalizations; influenza; influenza‐like illness; laboratory‐confirmed influenza.

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Conflict of interest statement

AMI has received fees for conferences/experts' meetings from Sanofi and for educational events from MSD. MLL has received fees for conferences meetings from Sanofi and for educational events from MSD. MLL has attended to several congresses whose registration, travel and accommodation costs have been covered by MSD, AZ and Sanofi. HB and ML are employees of Sanofi and may hold shares and/or stock options in the company. JDD has attended to several congresses whose registration, travel and accommodation costs have been covered by MSD, GSK and Sanofi. JDD and his institution received research grants from Sanofi and GSK related to RSV preventive strategies. JDD acted as advisor for these immunization strategies to Sanofi. AOS has attended to several congresses whose registration, travel and accommodation costs have been covered by MSD, GSK, Novavax and Sanofi. AOS and his institution received research grants from Sanofi and MSD related to RSV preventive strategies. AOS acted as advisor for these immunization strategies to Sanofi and Moderna. FXLL, MCF, MTG, JMM, GSC and JPB declare no conflicts of interest.

Figures

FIGURE 1
FIGURE 1
Participant disposition flow diagram. ICD, International Classification of Diseases; ILI, influenza‐like illness; PPV, positive predictive value; RT‐PCR, real‐time reverse transcription‐polymerase chain reaction.
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