Very High Sensitivity Of A Rapid Influenza Diagnostic Test In Adults And ...

Discussion

Based on a meta-analysis, RIDTs (i.e., antigen detection tests based on the immunochromatography) were reported to show low sensitivity (42.6%; 95%CI: 34.8–50.9) for influenza A in adult patients [9]. In contrast, an RIDT used in Japan, ImunoAce Flu, showed much 97.1% sensitivity in the detection of influenza A viruses (95%CI: 93.8–98.9) in adult patients (Table 1). The performance of RIDTs used in Japan might be higher than those reported in the meta-analysis [9]. In the same meta-analysis, it was also reported that RIDTs that utilize analyzer devices showed higher sensitivity in the detection of influenza viral antigens than RIDTs without analyzer devices. For example, the BD Veritor System, classified as “a digital immunoassay” in the meta-analysis report [9], showed 83.0% sensitivity and 97.5% specificity, which was much higher in comparison to RIDTs without analyzer devices. Although the BD Veritor System is also used in Japan, this superiority is not recognized. The minimum viral titer required for a positive reaction for influenza A viruses using the BD Veritor System (103−104 TCID50 per 100 μl) was similar to that required by ImunoAce Flu [13].

The sensitivity of RIDT is dependent on the viral load in the upper respiratory tract [1]. However, the timing at which samples were collected was not described in the recent meta-analyses that reported the low sensitivity of RIDTs [9, 10]. The Clinical Practice Guidelines by the Infectious Diseases Society of America (IDSA) emphasize that clinicians should collect upper respiratory tract specimens as soon as possible, preferably within 4 days after the onset of symptoms [18]. Accordingly, we believe that the recent reports [9, 10] included the results of many patients tested with RIDT kits at 4–5 days after the onset of illness. In contrast, in our study, most patients (89.8%) visited the outpatient department of our hospital within 48 hours after the onset of illness. Infectious influenza virus levels in the upper respiratory tract of persons with uncomplicated influenza peak during the first 1–2 days after illness onset, and decline to undetectable levels within a week [18]. In Japan, the early diagnosis and treatment of influenza are now standard practice [2].

Another important factor related to the high sensitivity of RIDTs in Japan was the measure used to collect samples for RIDTs. In our study, nasopharyngeal swabs were used as samples for all tests. In contrast, in the meta-analysis report, nasopharyngeal swabs or aspirate were used as test samples for <44% of tests [9]. The IDSA guidelines emphasize that nasopharyngeal specimens should be collected rather than other upper respiratory tract specimens in order to increase the detection of influenza [18].

ImunoAce Flu showed 89.2% specificity (95%CI: 84.1–93.1), which is relatively low (Table 1). When the study population was limited to patients who were tested within 48 hours after the onset of illness, the specificity was 90.4% (95%CI: 85.1–94.3) (Table 2). In contrast, the specificity was >99.9% (95%CI: 99.4–100) in the meta-analysis report [9]. The manuals for the other RIDT kits used in Japan report their sensitivity (88%–100%) and specificity (94%–100%) [19]. The low specificity in the present study may be attributable to the characteristics of the monoclonal antibody used in the ImunoAce Flu test kit.

The performance of RIDTs is usually discussed based on the clinical data in children. The performance is significantly better in children [20, 21], because age is inversely associated with viral load. This explains the better test results in children, and leads to some doubt regarding the diagnostic value of RIDTs in elderly persons [22]. In one meta-analysis report [10], the diagnostic test accuracy for influenza was significantly decreased in adults in comparison to children or a mixed population.

However, in our study, the sensitivity of ImunoAce Flu was sufficiently high in adults, especially in elderly individuals of >65 years of age (Table 1). Another Japanese report showed that the sensitivity did not differ between children and adults [23]. In that report, the sensitivity of a Japanese RIDT, Prolast Flu AB (Mitsubishi Chemical Medience Corporation, Japan) for H1N1pdm09 was as high as 80–90% during the H1N1 pandemic of 2009. In contrast, RIDTs were reported to show low sensitivity in the detection of influenza A/H1N1pdm09 virus (40–69%) in the H1N1 pandemic of 2009 in the US [24].

The IDSA guidelines recommend that clinicians use rapid molecular assays, that is, nucleic acid amplification tests (NAAT), rather than RIDTs for the testing of outpatients in order to improve the detection of influenza virus infection [18]. According to the meta-analysis [9], the pooled sensitivity and specificity of rapid molecular tests for influenza A were 87.4% (95%CI: 71.1–95.6) and 99.0% (95%CI: 93.2–99.5), respectively, in adults. The sensitivity and specificity of a rapid molecular test, ID NOW2 (Abott Diagnostics, USA), for influenza A were reported to be 95.9% (95%CI: 89.9–98.9) and of 100% (95%CI: 98.1–100), respectively [25], which was reported from Japan, and most patients—mainly children—were tested within 48 hours after the onset of illness. In our study, in which patients were tested within 48 hours after the onset of ILI, the sensitivity of ImunoAce Flu in the detection of influenza A was as high as the sensitivity of rapid molecular tests [25]. However, the specificity of ImunoAce Flu within 48 hours after the onset of illness was 90.4% (Table 2), which was lower than the specificity of rapid molecular assays [9] [25]. Similarly, in a report from Japan [25], sensitivity and specificity of an RIDT, QUICKNAVI (Otsuka Pharmaceutical Co, Japan) were comparable to the sensitivity and specificity of a rapid molecular assay, ID NOW2. Based on the Japanese experience, RIDTs are sufficiently sensitive and highly useful if patients are tested within 48 hours after the onset of illness. Thus, in countries where an early diagnosis and treatment are possible, an RIDT is probably one of the most useful alternatives for diagnosing influenza. For example, in Israel, >50% of patients with ILI visited clinics at 0–1 days after the onset of ILI [26].

The distinction of influenza A subtypes, A/H1N1pdm and A/H3N2, has been taken seriously because of characteristic clinical manifestations. Recently, however, it has become more important, mainly because of the lower effectiveness of the influenza vaccine against A/H3N2 [27, 28]. Moreover, resistance to a polymerase inhibitor, baloxavir marboxil, is frequently reported, especially in patients with influenza A/H3N2 [4, 29]. We could not distinguish between the influenza A subtypes by traditional RIDTs or rapid molecular assays, unless we used RT-PCR. However, with the combination of ImunoAce Flu and Linjudge FluA/pdm, distinction between influenza A subtypes becomes feasible.

Our study demonstrated the high sensitivity (97.6%; 95%CI: 87.4–99.9) and specificity (92.6%; 95%CI: 82.1–97.9) of Linjudge FluA/pdm in the detection of A/H1N1pdm09 (Table 4). Thus, by testing patients consecutively with ImunoAce Flu then Linjudge FluA/pdm, we are able to diagnose whether patients had A/H1N1pdm09 infection or A/H3M2 infection within a short time. ImunoAce Flu takes up to 5 minutes to obtain results [14], while Linjudge FluA/pdm takes up to 15 minutes [15]; however, highly accurate results are usually obtained within a total of 10 minutes. Another RIDT kit that is capable of distinguishing between the influenza A subtypes had been on the market until several years ago, Clearline Influenza A/B/(H1N1) 2009 [30]; however, it has since been withdrawn due to unstable results. The combination of ImunoAce Flu and Linjudge FluA/pdm is highly useful for distinguishing between influenza A subtypes, in comparison to RT-PCR, although the sensitivity and specificity of the combination of tests is lower. Moreover, this combination of tests obtains results in much less time, and numerous samples can be tested at the same time. The most important point is its cost, which at approximately 10 US dollars per test, is much cheaper in comparison to RT-PCR.

The present study was associated with several limitations. In some severe influenza cases, including cases with viral pneumonia [31], the infectious titer in the upper respiratory tract is reported to be low. Thus, negative test results in such cases should probably be confirmed by further testing using RT-PCR or other molecular assays to improve the detection of influenza virus infection [18]. In the present study, we only analyzed the performance of ImunoAce Flu in the detection of influenza A virus infection.

In conclusion, based on the Japanese experience, RIDTs are sufficiently sensitive and highly useful for the detection of influenza A if patients are tested within 48 hours after the onset of illness.