Evaluation of a direct immunofluorescence assay, dot-blot enzyme immunoassay, and shell vial culture in the diagnosis of lower respiratory tract infections caused by influenza A virus.

We prospectively evaluated the efficacy of two commercial rapid methods for antigenic detection, a dot-blot enzyme immunoassay (EIA-DB) (Directigen FluA, Becton-Dickinson, USA) and a direct immunofluorescence assay (DIF) (Monofluokit Influenza A, Diagnostics Pasteur, France), compared with the shell-vial culture in the MDCK line, incubated 2 to 3 days and stained with the monoclonal antibody clone IA-52, the diagnosis of lower respiratory tract caused by Influenza A virus (IA). In the study period the presence of IA virus was detected in 59 of the 377 samples analyzed (15.7%). Only the SVC method detected all positive samples (100% sensitivity), being used as a reference method for comparison with the other techniques). The EIA-DB technique detected 50 cases (84.7%) and the DIF only 35 (59.3%). In nine (15.2%) cases the diagnosis was obtained only with the SVC method. The results of the comparison of the EIA-DB technique with SVC were: sensitivity 84.7%, specificity 100%, positive predictive value 100%, and negative predictive value 97.2%. The DIF technique gave values of 59.3%, 100%, 100%, and 92.9%, respectively. A statistically significant difference was observed between the sensitivity of the EIA-DB and the DIF method (p = 0.0001). In view of the results we recommended the use of the EIA-DB as a screening method when infection by the IA is suspected. But to obtain the maximum diagnostic yield all samples would be inoculated in a shell vial culture with the MDCK cell line.