Influenza
| Numbers (% or LR) | |
|---|---|
| Incidence | |
| Influenza Season (December to March, USA) | 4% [1] |
| Non-Influenza Season (April to November, USA) | 0.5% [1] |
| Risk Factor | |
| 1. Up to date vaccination | LR 0.8 [2] | Symptoms/Syndrome |
| 1. Fever | LR 1.5 [3] |
| 2. Nonproductive cough | LR 2.0 [3] |
| 3. Myalgias | LR 2.0 [3] |
| 4. Nasal Congestion | LR 2.0 [3] |
| 5. Staphylococcus aureus pneumonia | LR 1.3 [4,5] |
| 6. No clinical symptoms or syndrome | LR 0.2 |
| Test | Clinical sensitivity | Clinical specificity |
| 1. Antigen testing (rapid influenza diagnostic test) | 54% [1,2] | 97% |
| 2. Rapid molecular test | 66% [1,2] | 98% |
| 3. Molecular test (including RT-PCR) | 98% | 99% [6] |
| Other |
| Explanation for + test without disease: Asymptomatic or mildly symptomatic disease, or viral shedding after recovery from disease for PCR testing |
| Explanation for - test with disease: Poor testing technique, specimen handling, or false negative if using RIDT or rapid molecular assays |
| Example of high value use: Those at high risk of complication when prevalence is high in the community. |
| Example of low value use: Asymptomatic testing during periods of low prevalence. |
| Choosing wisely or other guidance: CW: Do not routinely order broad respiratory pathogen panels unless the result will affect patient management. |
Discussion
Incidence and risk factors:
The incidence, both for influenza season and off season were calculated using data published by the CDC from the 2018 – 2019 influenza season, to eliminate the effect of the COVID pandemic.1 Rates vary by year and local epidemiology.
The protective effect of vaccination as demonstrated by the positive likelihood ratio of 0.8 was calculated based on predicted illnesses avoided by vaccination as calculated by the CDC from 2020.[2] Degree of protection from vaccine varies by year.
Symptoms/Syndromes:
The likelihood ratio for symptoms of fever, nonproductive cough, mylagias, and nasal congestion were estimated utilizing the likelihood of symptoms being present in laboratory documented cases of influenza by Monto et al.[3]
Test sensitivity & specificity:
Sensitivity and specificity were documented in IDSA clinical practice guidelines from 2018. Of note, the sensitivity and specificity of antigen testing and rapid molecular testing reported in comparison to PCR as the gold standard, thus their values were adjusted slightly to reflect the reference.[6]
References
Overview of Influenza Testing Methods
National Center for Immunization and Respiratory Diseases C for DC and P. FluView Interactive.
National Center for Immunization and Respiratory Diseases C for DC and P. Key Facts About Seasonal Flu Vaccine.
Monto AS, Gravenstein S, Elliott M, Colopy M, Schweinle J. Clinical Signs and Symptoms Predicting Influenza Infection. Archives of Internal Medicine. 2000;160(21):3243. doi:10.1001/archinte.160.21.3243
Kollef MH, Shorr A, Tabak YP, Gupta V, Liu LZ, Johannes RS. Epidemiology and Outcomes of Health-care–Associated Pneumonia. Chest. 2005;128(6):3854-3862. doi:10.1378/chest.128.6.3854
Finelli L, Fiore A, Dhara R, et al. Influenza-Associated Pediatric Mortality in the United States: Increase of Staphylococcus aureus Coinfection. Pediatrics. 2008;122(4):805-811. doi:10.1542/peds.2008-1336
Uyeki TM, Bernstein HH, Bradley JS, et al. Clinical Practice Guidelines by the Infectious Diseases Society of America: 2018 Update on Diagnosis, Treatment, Chemoprophylaxis, and Institutional Outbreak Management of Seasonal Influenzaa. Clinical Infectious Diseases. 2019;68(6):e1-e47. doi:10.1093/cid/ciy866
Authors: Ravi Tripathi