These rare patients were advocated by some to be the most difficult, so their exclusion was advocated as creating a bias [1]. Why? Why is the exclusion of these patients not an issue? Simply because “rare diagnoses” does not mean “difficult” diagnoses. Massive pleural effusion is the best example. No need for multicentric randomized studies for understanding the interest of ultrasound there. No need for BLUE-protocol. The diagnosis is easy using usual tools, including traditional ultrasound.

The BLUE-protocol has incorporated 97 % of the patients seen in the ER (or pre-hospital medicine) and eventually admitted to the ICU of our parisian hospital. The multiple diagnoses of the 3 % remaining patients were not considered, in order to keep our protocol simple, fit for use: daily problems were prioritized. Those daily patients had pneumonia, pulmonary edema, COPD, asthma, pulmonary embolism, and pneumothorax. The 3 % remaining causes were [11]:

If the protocol includes not 300 but 3,000 or 30,000 patients, the list would be enriched by countless but rare diseases: acute gastric dilatation, pneumoniae linked to drugs, sterile aspiration pneumonia, phrenic palsy, Guillain-Barré syndrome, extended causes of chronic interstitial disease (histiocytosis X, sarcoïdosis and other alveolar proteinosis, etc.), acute hypovolemia, metabolic dyspnea, etc. Ask to experts for a comprehensive list. Most of these diseases will be accessible to the Extended BLUE-protocol (Chap. 35). For assessing ultrasound for one given rarity, years of large-scale multicentric studies will be necessary for gathering enough patients. The BLUE-protocol favors the real life.

Interestingly, each of the rare diagnoses had a profile among the eight of the BLUE-protocol. Let us see these main rare causes.

Regarding chronic interstitial diseases, the B-profile is linked to a lung disease using various tools. The simplest is disease history, when the disease is known (most of the cases). During the first episode (an occurrence far lower than 1.4 %), simple tools from the Extended-BLUE-protocol, mainly the simple cardiac sonography, will find right heart anomalies together with the left heart normality, immediately linking this interstitial syndrome to a pulmonary origin.

Massive atelectasis yields numerous standardized signs, as discussed in Chap. 35.

Tracheal stenosis had a nude profile, following the logic: this is the main profile of asthma and COPD, i.e., obstruction (as is tracheal stenosis). The characteristic clinical signs should make ultrasound of lesser relevance, although an anterior location of granuloma (usual location) can be found using ultrasound.

We can consider infinite combinations, such as hemodynamic pulmonary edema due to myocarditis complicating an infectious pneumonia. These patients will likely have the appropriate B-profile.

Thoracic disorders occurring in children and neonates are detailed in Chap. 32.

The case of the diaphragm. The BLUE-protocol was reproached not to include it [9]. First, the diaphragm is included: detecting an abolished lung sliding means a motionless cupola. Second, we wanted to keep our decision tree as simple as can be. Bilateral causes, although having originated the birth of intensive care in 1954, are now an extinct cause. Would it even be seen, the therapy is purely symptomatic. Read Anecdotal Note 1 which explains why the diaphragm was not included. Read also the section on diaphragm in the chapter dealing with noncritical ultrasound (Chap. 36).

To say it differently, the BLUE-protocol works always, even when it is not used. When rare diagnoses are suspected by the initial approach, the Extended-BLUE-protocol will be used, with increased ultrasound potential. The native BLUE-protocol makes nothing but adding decisive points to the usual management. Used this way, we are accustomed to work with the correct diagnosis.