LETTER TO THE EDITOR Non-specific laboratory test indicators of severity in hospitalized adults with swine influenza (H1N1) pneumonia B. A. Cunha & U. Syed & S. Strollo Received: 14 October 2009 / Accepted: 19 September 2010 / Published online: 22 October 2010 # Springer-Verlag 2010 Introduction New York was at the epicenter of the “herald wave” of the swine influenza (H1N1) pandemic in the spring of 2009 [1, 2]. Our hospital, Winthrop-University Hospital (WUH), like other hospitals in the area, were inundated with patients with influenza-like illnesses (ILIs) presenting themselves to our Emergency Department (ED) for testing and clinical evaluation. In the majority of patients, the swine influenza (H1N1) was a mild ILI not severe enough to warrant hospitalization. However, 25 adult patients were ill enough to be admitted with definite/probable swine influenza (H1N1) pneumonia during the “herald wave” of the pandemic. Methods The diagnoses of swine influenza (H1N1) was problematic and either made on the basis of laboratory confirmation, i.e., a rapid influenza diagnostic test (RIDTs) and/or a positive RT- PCR for H1N1 [3]. Because of restricted RT-PCR testing by the Health Department in July of 2009, the CDC reclassified cases into one diagnostic category, i.e., probable/definite swine influenza (H1N1) [4]. At Winthrop-University Hospi- tal, probable diagnosis was based on the swine influenza diagnostic triad, i.e., an ILI with a temperature of >102°F, severe myalgias plus three of four otherwise unexplained laboratory abnormalities, i.e., relative lymphopenia, elevated serum transaminases (SGOT/SGPT) or an elevated creati- nine phosphokinase (CPK) [5]. Of the 25 hospitalized adults with swine influenza (H1N1) pneumonia, three were considered severe and required ventilatory support; two thirds of the severe cases were immunocompetent adults, one had HIV. Two died from swine influenza (H1N1) pneumonia [6, 7]. None of our patients with swine influenza (H1N1) pneumonia presented with or subsequently developed bacterial pneumonia. Unlike in other studies, bacterial pneumonia was not a severity factor in our experience [8– 10]. The swine influenza diagnostic triad correctly identified swine influenza H1N1 patients from those admitted with mimics of swine influenza (H1N1) pneu- monia during the pandemic [11]. Twenty two out of 25 of the remaining patients were classified as non-severe, i.e., not requiring ventilatory support, recovered, and were eventually discharged. Results We compared non-specific laboratory tests in our cohort of 25 adult hospitalized patients with probable/definite swine influenza (H1N1) pneumonia to determine if there were non-specific laboratory predictors of clinical severity. The presence of otherwise unexplained relative lympho- penia were a key determining diagnostic marker for hospitalized adults with swine influenza (H1N1) pneumo- nia [12]. In addition to relative lymphopenia, other non- B. A. Cunha (*) : U. Syed : S. Strollo Infectious Disease Division, Winthrop-University Hospital, Mineola, NY 11501, USA e-mail: llusardi@winthrop.org B. A. Cunha : U. Syed : S. Strollo State University of New York School of Medicine, Stony Brook, NY, USA Eur J Clin Microbiol Infect Dis (2010) 29:1583–1588 DOI 10.1007/s10096-010-1069-x