Comparing Sensitivities: RT-PCR Versus Rapid Influenza Diagnostic Test

May 7, 2020
Grant M. Gallagher

It is known that the type of respiratory specimen, sampling environmental factors, and time from symptom onset impact the sensitivity of rapid influenza diagnostic tests.

Compared with reverse transcription polymerase chain reaction tests (RT-PCR), rapid influenza diagnostic tests (RIDT) detecting viral antigens only have a low to moderate degree of sensitivity. It is known that the type of respiratory specimen, sampling environmental factors, and time from symptom onset impact their sensitivity.

A team of investigators from the University of Helsinki and Helsinki University Hospital in Finland have assessed whether prior influenza vaccine administration also plays a role in rapid influenza diagnostic test sensitivity.

The research was slated to be presented at the 2020 European Congress on Clinical Microbiology and Infectious Diseases (ECCMID), but ECCMID’s in-person and virtual components were cancelled due to coronavirus disease 2019 (COVID-19). The congress organizers have, however, released a book of abstracts which were accepted for the conference.

In the 2018-2019 influenza season, a prospective test-negative design influenza vaccine effectiveness study was conducted at HUS Jorvi Hospital.

Adult inpatients who fulfilled severe acute respiratory infection criteria were recruited. Participants were swabbed for influenza. Respiratory samples were tested first via RIDT and then negative results were confirmed by RT-PCR.

Of the 69 severe acute respiratory infection cases that were tested for influenza with both RIDT and RT-PCR, 21 (30.4%) tested positive, 46 (66.7%) negative and 2 (2.9%) were ambiguous with RIDT. The sensitivity of the RIDTs in detecting influenza was ultimately only 31.3 %.

Prior influenza vaccination during the ongoing influenza season appeared to be associated with decreased sensitivity for the antigen test (odds ratio= 0.374, 95% CI 0.124, 1.068, Probability = 0.066). Among those vaccinated more than 14 days before sampling, 28 (77.8%) had false-negative RIDT results compared to 17 (56.7%) of those not vaccinated.

Patient age, genetic clade, and influenza A subtype did not have significant associations with the sensitivity of the RIDT. The team’s data suggest an association between influenza vaccination within a given season and decreased sensitivity of RIDT.

False-negative RIDT results may lead to under-detection of influenza and thus to insufficient application of infection-control and antiviral treatments. The issue could also lead to unnecessary antibiotic use. The research suggests that caution is needed when interpreting negative RIDT results.

The abstract, “Decreased sensitivity of rapid influenza diagnostic test among vaccinated adults in influenza season 2018-19,” was released May 5, 2020, as part of the ECCMID Abstract Book 2020.