Rapid clinical diagnosis in fatal swine influenza (H1N1) pneumonia in an adult with negative rapid influenza diagnostic tests (RIDTs): Diagnostic swine influenza triad

Background

The “herald wave” of the H1N1 pandemic spread from Mexico to the United States in spring 2009. Initially, the epicenter of H1N1 in the United States was in the New York area. Our hospital, like others, was inundated with large numbers of patients who presented at the Emergency Department (ED) with influenza-like illnesses (ILIs) for swine influenza testing and evaluation.

Methods

The Winthrop-University Hospital ED used rapid influenza (QuickVue A/B) tests to screen for H1N1 infection. Patients who were rapid influenza A test-positive were also reverse transcription-polymerase chain reaction (RT-PCR) positive for H1N1. In our ED, 30% of patients with ILIs and possible H1N1 pneumonia had negative rapid influenza A screening tests. Because H1N1 RT-PCR testing was restricted, there was no laboratory test to confirm or rule out H1N1. Other rapid influenza diagnostic tests (RIDTs), e.g., the respiratory fluorescent antibody (FA) viral panel test, were used to identify H1N1 patients with negative RIDTs.

Results

Unfortunately, there was not a good correlation between RIDT results and RT-PCR results. There was a critical need to develop a clinical syndromic approach for diagnosing hospitalized adults with probable H1N1 pneumonia with negative RIDTs. Early in the pandemic, the Winthrop-University Hospital Infectious Disease Division developed a diagnostic weighted point score system to diagnose H1N1 pneumonia clinically in RIDT-negative adults. The point score system worked well, but was time-consuming. As the “herald wave” of the pandemic progressed, our ED staff needed a rapid, simplified method to diagnose probable H1N1 pneumonia in hospitalized adults with negative RIDTs. A rapid and simplified diagnosis was based on the diagnostic weighted point score system, which we simplified into a triad of key, nonspecific laboratory indicators. In adults hospitalized with an ILI, a fever >102°F with severe myalgias, and a chest x-ray without focal segmental/lobar infiltrates, the presence of three indicators, i.e., otherwise unexplained relative lymphopenia, elevated serum transaminases, and an elevated creatinine phosphokinase, constituted the diagnostic swine influenza triad. The Infectious Disease Division's diagnostic swine flu triad was used effectively as the pandemic progressed, and was not only useful in correctly diagnosing probable H1N1 pneumonia in hospitalized adults with negative RIDTs, but was also in ruling out mimics of swine influenza, e.g., exacerbations of chronic bronchitis, asthma, or congestive heart failure, as well as bacterial community-acquired pneumonias (CAPs), e.g., legionnaire's disease.

Conclusion

Clinicians can use the Winthrop-University Hospital Infectious Disease Division's Diagnostic swine influenza triad to make a rapid clinical diagnosis of probable H1N1 pneumonia in hospitalized adult patients with negative RIDTs.