Lung ultrasound (LUS) in combination with a biomarker has not yet been studied. The investigators propose a clinical trial where the primary aims are: 1. To assess whether an algorithm with LUS and procalcitonin (PCT) may be useful for diagnosing bacterial pneumonia; 2. To analyse the sensitivity and specificity of LUS vs chest radiograph (CXR).

Objectives: The general objective is to analyze whether a new algorithm approach of severe pneumonia in pediatric intensive unit (PICU) improves the quality of care. Objectives: 1- To analyze the sensitivity and specificity of lung ultrasound (LUS) compared respect to the chest radiography (CXR) in severe pneumonia and if LUS discriminates between bacterial and viral one. 2- If there is a lower CXR indication. 3- If is possible to reduce the dose of irradiation and costs associated with CXR. 4- To analyze the interobserver agreement of the LUS. 5- determine whether a diagnosis of pneumonia algorithm using LUS and procalcitonin may be useful in directing the indication of antibiotic therapy and / or the duration thereof.

Methodology: clinical, prospective, controlled, randomized, blinded intervention and 3-year trial. Inclusion of children under 18 years, with severe pneumonia, who enter (PICU). Experimental Group 1: pediatrician researcher (PR) will conduct a LUS at admission time, as a first test of lung image; in Group 2, CXR is conduced as first image. Patients will be classified into 3 subgroups, in both branches: a) if PCT is <1 ng /L and LUS is not suggestive of bacterial pneumonia, no antibiotic will be prescribed; b) if LUS is suggestive of bacterial pneumonia, regardless of the PCT, it will be recommended to start antibiotic therapy; c) If the ultrasound is suggestive of bacterial pneumonia but PCT is> 1 ng / L, antibiotic therapy will be recommended to cover other causes of infection. The same attitude will be performed, but depending on the CXR instead of the LUS, in group 2. Clinical variables, complementary tests and evolution data will be collected. Statistics by SPPS® 20.0. Informed consent will be requested and the study will be conducted according to the Helsinki Declaration guidelines and Good Clinical Practice.

Inclusion Criteria:

• children under 18
• severe pneumonia criteria
• admitted at PICU
• informed consent signed

Exclusion Criteria:

• previous respiratory disease (cystic fibrosis and/or immunosuppression)
• Nosocomial pneumonia development while in charge for community pneumonia.
• Researcher pediatrician has valuated the chest X ray before the PICU admission
• Included in other clinical trial