Using point-of-care tests for influenza to aid clinical decisions in primary care setting

Abstract

BACKGROUND: Influenza in primary care setting commonly presents with acute respiratory tract symptoms and fever, and it is often difficult to distinguish influenza virus infections from those caused by other respiratory pathogens during influenza epidemics. Primary care doctors usually rely on clinical symptoms, especially presence of fever, to judge the likelihood of influenza, but its accuracy is affected by the prevalence of influenza and other respiratory pathogens and the test sensitivity and specificity of symptom(s) . We conducted a study in outpatient clinics during influenza epidemics to evaluate the diagnostic performance of influenza-like illness (ILI) criteria and a rapid point-of-care test for influenza, and explore their optimal use for aiding clinical decisions. METHOD: Patients presenting with acute respiratory illness (ARI: 2 of 7 symptoms: fever ≥37 .8°C, cough, sore throat, runny nose, headache, myalgia and phlegm) to private out-patient clinics within 72 hours of illness onset were enrolled and tested for influenza A and B viruses by rapid antigen test (QuickVue®) and subsequently by RT-PCR . ILI is defined as fever ≥37 .8°C plus cough or sore throat. The sensitivities and specificities of ILI and rapid test among enrolled patients were calculated using RT-PCR results as the gold standard. Combined application of ILI and rapid test for diagnosing influenza virus infection and their overall performances were explored . RESULTS: 626 ARI patients were recruited through 20 private out-patient clinics over three influenza epidemics from July 2014 to August 2015. The patients’ ages ranged from 10 months to 94 years (median 32 years). 255 (40 .7%) subjects were positive for influenza A or B by RT-PCR. Presentation of symptoms included fever (63 .1%), cough (81 .5%), runny nose (79 .6%), sore throat (72 .8%), headache (67 .9%), myalgia (65 .5%) and phlegm (62 .1%); with 362 (57 .8%) of them fulfilling the ILI criteria . 99 (15 .8%) of all ARI patients tested positive for influenza A or B by the rapid antigen test . The sensitivity (SN), specificity (SP), positive predictive value (PPV) and negative predictive value (NPV) of the ILI criteria were 79 .2%, 56 .9%, 55 .8% and 79 .9%, while those for the rapid antigen test were 37 .3%, 98 .9%, 96 .0% and 69 .6%, respectively . The net SN, SP, PPV and NPV of their serial application were 29 .5%, 99 .5%, 97 .8% and 67 .3%, while those of their parallel application were 87 .0%, 56 .3%, 57 .7% and 86 .3% . CONCLUSION: ILI was a sensitive criteria to detect influenza infection and a negative result could allow primary care doctors to have 80% chance to rule out influenza and consider other pathogens during influenza epidemics . The rapid antigen test can provide highly specific results for influenza among the ARI patients and guide specific treatment for them, though the negative test results would have missed 30% of the ARI episodes caused by influenza A or B . Using rapid antigen test in parallel can improve the net PPV and NPV compared with using ILI criteria alone .