Direct immunofluorescence assay performance in diagnosis of the Influenza A(H1N1) virus

By 25 April 2009, less than one month after the first human with Influenza A(H1N1) virus was detected in Mexico, the disease had already spread to more than 40 countries, with over 10 000 cases reported. Due to its unpredictability, this type of virus requires appropriate, reliable, and safe diagnostic methods that are also accessible to clinical laboratories. Through the analysis of 291 samples taken from patients with suspected Influenza A(H1N1) virus infection in Neuquen, Argentina, this study compares the two diagnostic methods used simultaneously: direct immunofluorescence assay (DFA) and real-time polymerase chain reaction (RT-PCR). DFA had a sensitivity of 44.4%, a specificity of 99.6%, a positive predictive value of 95.2%, and a negative predictive value of 90.7%. Positive results obtained with this method can be considered true positives. A negative result does not rule out the presence of the virus. In this case, the sample should be examined by RT-PCR. Out of a total of 291 samples, there were 45 positive results with RT-PCR and 21 positive results with DFA.