Procalcitonin (PCT) levels for ruling-out bacterial coinfection in ICU patients with influenza: A CHAID decision-tree analysis

J Infect. 2016 Feb;72(2):143-51. doi: 10.1016/j.jinf.2015.11.007. Epub 2015 Dec 15.


Objectives: To define which variables upon ICU admission could be related to the presence of coinfection using CHAID (Chi-squared Automatic Interaction Detection) analysis.

Methods: A secondary analysis from a prospective, multicentre, observational study (2009-2014) in ICU patients with confirmed A(H1N1)pdm09 infection. We assessed the potential of biomarkers and clinical variables upon admission to the ICU for coinfection diagnosis using CHAID analysis. Performance of cut-off points obtained was determined on the basis of the binominal distributions of the true (+) and true (-) results.

Results: Of the 972 patients included, 196 (20.3%) had coinfection. Procalcitonin (PCT; ng/mL 2.4 vs. 0.5, p < 0.001), but not C-reactive protein (CRP; mg/dL 25 vs. 38.5; p = 0.62) was higher in patients with coinfection. In CHAID analyses, PCT was the most important variable for coinfection. PCT <0.29 ng/mL showed high sensitivity (Se = 88.2%), low Sp (33.2%) and high negative predictive value (NPV = 91.9%). The absence of shock improved classification capacity. Thus, for PCT <0.29 ng/mL, the Se was 84%, the Sp 43% and an NPV of 94% with a post-test probability of coinfection of only 6%.

Conclusion: PCT has a high negative predictive value (94%) and lower PCT levels seems to be a good tool for excluding coinfection, particularly for patients without shock.

Keywords: CHAID analysis; Community-acquired pneumonia; Influenza A(H1N1)pmd; Procalcitonin; Prognosis; Respiratory coinfection; Septic shock.

Publication types

  • Multicenter Study
  • Observational Study

MeSH terms

  • Adult
  • Bacterial Infections / diagnosis*
  • Bacterial Infections / pathology*
  • Biomarkers / blood
  • Calcitonin / blood*
  • Calcitonin Gene-Related Peptide
  • Coinfection / diagnosis*
  • Coinfection / pathology*
  • Decision Trees
  • Female
  • Humans
  • Influenza, Human / complications*
  • Influenza, Human / pathology*
  • Intensive Care Units
  • Male
  • Middle Aged
  • Predictive Value of Tests
  • Prospective Studies
  • Protein Precursors / blood*
  • Sensitivity and Specificity


  • Biomarkers
  • CALCA protein, human
  • Protein Precursors
  • Calcitonin
  • Calcitonin Gene-Related Peptide