Role of laboratory variables in differentiating SARS-coronavirus from other causes of community-acquired pneumonia within the first 72 h of hospitalization

The Centers for Disease Control and Prevention (CDC) recommend that SARS-coronavirus (SARS-CoV) testing be considered in epidemiologically high-risk patients hospitalized with community-acquired pneumonia ( CAP) if no alternative diagnosis is identified after 72 h. The aim of this study was to identify routine laboratory variables that might indicate the need for SARS-CoV testing. Routine hematological/biochemical variables in patients with laboratory-confirmed SARS ( 2003) were compared with those in consecutive patients hospitalized June - December 2004 with radiologically confirmed CAP. Stepwise logistic regression analyses were performed to identify discriminating variables at baseline and by day 3 of hospitalization. Nasopharyngeal aspiration and antigen detection for influenza virus and respiratory syncytial virus using an immunofluorescence assay (IFA) were routinely performed in patients with CAP. Altogether, 181 patients with CAP ( who remained undiagnosed by IFA) and 303 patients with SARS were studied. The mean intervals from symptom onset to admission were 3.1 and 2.8 days, respectively (p> 0.05). The etiological agent of CAP was identified retrospectively in only 39% of cases, the majority being bacterial pathogens. At baseline, age and absolute neutrophil count (ANC) were the only independent discriminating variables ( p< 0.0001). Using a value of < 4.4 x 10(9)/l as the cutoff for ANC, the sensitivity and specificity of ANC for discriminating SARS were 64 and 95%, respectively (AUC 0.90). By day 3 of hospitalization, age ( p< 0.0001), change in ANC (p= 0.0003), and change in bilirubin ( p= 0.0065) were discriminating variables. A model combining age < 65 years, a change in ANC of >- 3 x 10(9)/ l, and a change in bilirubin of >= 0 mmol/l had a sensitivity of 43% and a specificity of 95% for SARS ( AUC 0.90). There are only a few laboratory features ( including lymphopenia) that clearly discriminate SARS from other causes of CAP. Nevertheless, when evaluating epidemiologically high-risk patients with CAP and no immediate alternative diagnosis, a low ANC on presentation along with poor clinical and laboratory responses after 72 h of antibiotic treatment may raise the index of suspicion for SARS and indicate a need to perform SARS-CoV testing.