Pneumonia creates an inflammation of the lung tissue. The edema enlarges the interstitial tissue, the exudate fills the alveoli, the inflammation crosses the visceral pleura, and fluid invades the pleural cavity. Some germs come from the airways, others from blood. The homogeneity of the distribution of the disorders partly depends on this.

Usually, fever is the main first sign. Fever with clinical respiratory signs (cough, dyspnea) evokes the pneumonia. Physical examination basically (apart from subtleties) searches for sounds suggestive of consolidation (rales mainly) and pleural effusion (loss of pleural murmur). It is usual to ask for a chest radiograph, which shows dense areas (and possibly indicates that the physical examination is not a sufficient step). CT is done sometimes for knowing more, but it is done also in countless occurrences when other diagnoses are suspected (helical CT usually). Blood gas shows hypoxia and hypocapnia. CRP and other inflammatory tests are elevated.

The diagnosis of “pneumonia” (not to deal with its origin) raises probably little problem at this step.

When the clinical presentation and basic tests are self-speaking, the diagnosis of pneumonia is done. The question of which microbe, although crucial, is not yet evoked at this step. When the physical examination is difficult, the patient has complex comorbidities, complex disorders, and factors decreasing the response to aggression, antibiotics taken earlier and masking some signs, or when the radiograph does not perfectly answer the question, or systematically, ultrasound is performed. Just note as regards pleural effusion that bedside radiographs miss up to 525 ml [1, 2]. One-third of pleural effusions in ventilated patients, which were substantial enough for a safe thoracentesis were radioccult [3].

Pneumonia generates four profiles: the B′-profile (11 % sensitive, 100 % specific), the C-profile (21 % sensitive, 99 % specific), the A/B-profile (14 % sensitive, 100 % specific), and the A-no-V-PLAPS-profile (42 % sensitive, 96 % specific). The overall accuracy is a 89 % sensitivity and a 94 % specificity. As seen, each profile is not frequent (low sensitivity), but the summation of the four profiles makes an acceptable sensitivity. Regarding the rates of 100 %, seen twice, please read Anecdotal Note 1.

For being able to compare the BLUE-protocol with the current literature, just consider that the C-profile includes consolidations of every size. The C-line is a centimetric consolidation. Smaller, it results in a thickened pleural line (Fig. 17.​4). An anterior thickened, irregular pleural line is called a “C-profile” in the BLUE-protocol. Just also consider that the A/B-profile can be understood, not only as a difference between both lungs, but also within one lung, areas with lung rockets, areas with A-lines (sometimes called spared areas in the literature).

Another point to be understood. The BLUE-diagnosis of pneumonia is done when there are interstitial signs (B′-profile, A/B-profile), alveolar signs (C-profile, PLAPS), and pleural signs (PLAPS). A pleural effusion, even small and isolated, in the sequence of the BLUE-protocol, evokes pneumonia, although the diagnoses of pneumothorax, pulmonary edema, and pulmonary embolism can all generate pleural effusions, but they were previously excluded. Searching for internal echoes is not required by the BLUE-protocol since the diagnosis of pneumonia has been done, but it can be done in the Extended BLUE-protocol for deciding the best therapy (Chap. 35).

Usually, pneumonia cannot be confused with pneumothorax, COPD, or asthma.