Procalcitonin and C-reactive protein in severe 2009 H1N1 influenza infection

To examine whether, in an adult intensive care unit (ICU), procalcitonin or C-reactive protein (CRP) levels discriminated between 2009 H1N1 influenza infection and community-acquired pneumonia of bacterial origin.

A retrospective observational study performed at an Australian hospital over a 4-month winter period during the 2009 H1N1 influenza pandemic. Levels on admission of procalcitonin and CRP were compared between patients admitted to the ICU with community-acquired pneumonia of bacterial and 2009 H1N1 origin.

Compared to those with bacterial or mixed infection (n = 9), patients with 2009 H1N1 infection (n = 16) were significantly more likely to have bilateral chest X-ray infiltrates, lower APACHE scores, more prolonged lengths of stay in ICU and lower white cell count, procalcitonin and CRP levels. Using a cutoff of >0.8 ng/ml, the sensitivity and specificity of procalcitonin for detection of patients with bacterial/mixed infection were 100 and 62%, respectively. A CRP cutoff of >200 mg/l best identified patients with bacterial/mixed infection (sensitivity 100%, specificity 87.5%). In combination, procalcitonin levels >0.8 ng/ml and CRP >200 mg/l had optimal sensitivity (100%), specificity (94%), negative predictive value (100%) and positive predictive value (90%). Receiver-operating characteristic curve analysis suggested the diagnostic accuracy of procalcitonin may be inferior to CRP in this setting.

Procalcitonin measurement potentially assists in the discrimination between severe lower respiratory tract infections of bacterial and 2009 H1N1 origin, although less effectively than CRP. Low values, particularly when combined with low CRP levels, suggested bacterial infection, alone or in combination with influenza, was unlikely.