**3.3. Diagnosis of CAP**

Patient with CAP mainly complaints of cough, fever, shivers, pleuritic chest pain of abrupt onset. Dyspnea may occur when the involved pulmonary sector is large. Sputum production is not rare, of purulent aspect or blood stained. Physical examination may reveal a condensation syndrome with focal crackles. It is of great importance to consider also extrathoracic signs (mouth, nose, ear).

History should be extended to demographic and behavioral considerations (age, gender, style and site of living) and comorbidities. CRB65, which is a clinical score for assessment of CAP is recommended in many guidelines. It includes data about confusion, respiratory rate, blood pressure, and age ≥ 65 years. A CRB65n score of 0 seems a patient at low risk of death and not normally requiring hospitalization.

Additional investigations are required to guide the diagnosis setting, such as inflammatory markers (leukocytosis and differential counts, ESR, CRP) and etiological identification of pathogens through conventional microbiological analyses of samples (sputum, nasotracheal aspirates, bronchoalveolar lavage), the latter could be positive in only 15% of cases. The quality of the specimen to be examined by gram stain is a prerequisite, and the sputum should contain more than 10–25 polymorphonuclear cells by microscopic screening to be suitable. Culturebased techniques are widely used for the diagnosis of pneumonia. That is the case of blood, sputum cultures, or endotracheal aspirates, which can allow pathogen identification and antibiotic sensitivity tests. Urinary antigen tests are helpful for *S. pneumoniae* and *L. pneumophila*. Molecular diagnosis tools are currently more and more used for diagnosis of infectious diseases. CRP, an acute-phase protein during acute inflammation has not proven to be accurate in differential diagnosis between bacterial or viral infections; it is nevertheless an indicator of response to the treatment. Other biomarkers are being developed, such as procalcitonin, a prohormone elevated in response to bacterial infections. Studies using this biomarker have not provided sufficient probes for large clinical use [51]. Otherwise, both biomarkers are useful in the assessment of response to treatment and suitability of the antibiotic prescribed.

Serological tests find applicability in the diagnosis of pneumonia due to atypical pathogens such as Chlamydia, Mycoplasma, and viral infections, not easy to identify by current culturebased techniques.

Molecular techniques are now developed ensuring an early diagnosis and sensitivity tests for resistance to treatment. Dried blood spots have been used for bacterial identification and have proven to be a useful and easily accessible tool for molecular diagnosis in poor resources countries [46]. Urinary antigen tests are available for *S. pneumoniae* and *L. pneumophila*. Bronchoscopic aspirations are more relevant than sputum samples culture to minimize the likelihood of commensal flora.

Chest radiograph demonstrating a peripheral airspace consolidation pattern is relevant in the diagnosis of pneumonia. Lung shadowing of lobar pattern associated to air bronchogram is common, while centrilobular and peribronchiolar opacity are defining the bronchopneumonia pattern. The lower lobes are most commonly affected. Recent literature illustrates the relevance of lung ultrasound in the diagnosis of CAP. A multicenter study in 14 European centers has shown a sensitivity of 93.4% and a specificity of 97.7% in the confirmation of the diagnosis [52].

The differential diagnosis should consider all illness expressed with dyspnea such as pulmonary embolism, COPD exacerbations, bronchiectasis, exacerbation of fibrosis, or with cough and associated fever such as acute bronchitis.
