Basically, the BLUE-protocol is a protocol. Yet it was designed for being a flexible one. Some protocols are possibly built for exempting doctors to think, but this one requires to keep on being a doctor. It is permanently “piloted.” In some situations, it will just confirm an obvious diagnosis. In others, it will confidently invalidate a diagnosis which looked the likely one.

For being perfectly understood (and anticipating remarks), the BLUE-protocol should be considered as an “intellectual exercise,” a tool just designed for using the minimal bunch of data for the maximal accuracy, when used alone. Countless articles are now using lung ultrasound, and many propose various algorithms including echocardiography and other items, advocating a “multiorgan approach.” This is not the spirit of our protocol: it associates these various items but does not include them (the difference is substantial). Comparing these studies with our approach would therefore make little sense. Regarding, for instance, the heart, see at the end of this chapter and at the end of Chap. 24 that we did not “forget” it (it is known that searchers may sometimes be absent minded, but up to forgetting the heart, there is a substantial step!); we just deleted it from our data. The accuracy of “BLUE-protocol plus echocardiography” is anyway featuring at end of this chapter, and we invite the readers to make an opinion.

Same remark for all clinical signs. Some academicians reproached to the BLUE-protocol to forget these precious signs [10] (don’t miss the discussion at the end of Ref. [10]). The clinical signs are, ironically, in the center of the Extended BLUE-protocol, for an improved accuracy (Chap. 35). The BLUE-profiles are here, available, up to this respected physician to integrate his favorite clinical data at will in his clinical approach.

The BLUE-protocol was conceived in an observational study in a Parisian university-affiliated teaching hospital. We performed ultrasonography on admission, in the climax of dyspnea, on serial patients with acute respiratory failure. Acute respiratory failure was defined based on clinical criteria requiring admission to the ICU.

The gold standard was the final diagnosis considered in the hospitalization report, made by a medical ICU team (expert panel) who did not take into account the lung ultrasound data and used traditional approaches. Uncertain diagnoses, multiple diagnoses, and rare causes (frequency <3 %) were excluded (see Chap. 21).

After years necessary for the publication of the preliminary background (mostly lung terminology), we were able to propose the analysis of three items at the lung area – with dichotomous answer, collected at standardized points (upper and lower BLUE-points, PLAPS-point).

We added an adapted venous analysis (indicated in 54 % of cases). Note that the venous analysis takes the major time (2 min of a 3 min examination), which is nonetheless short, since we use a simple machine with fast start-up, the same microconvex probe, time-saving maneuvers, only one setting, and a contact product which allows major time savings. This will be repeated again, intentionally.

A work of a “profiler,” based on analysis of hundreds of pre-data, was done during 7 years. From this observational work, the profiles of the BLUE-protocol were defined (Fig. 20.1).

There are eight profiles. The anterior analysis, which initiates the BLUE-protocol, can describe six situations. Five of them conclude the protocol: the B-, B′-, A/B-, C-, and A′-profile. One of them is the A-profile (Video 10.​1). The A-profile designates an anterior chest wall with predominant A-lines and lung sliding. The A-profile opens to three more profiles (A-DVT, A-no-V-PLAPS, and nude profile).

Here are the profiles, which were assimilated to specific diseases. The term “profile” assumes an association of signs (two), plus a location (Fig. 20.2).

Once these profiles were predefined as written, the study could begin. We then assessed the concordance between profiles and diseases.

We specify the precise language used in the BLUE-protocol for enabling other teams to reproduce our results.

When the first of the four anterior BLUE-points shows lung sliding with A-lines, labeling it a “quarter of A-profile” indicates that the user has understood that the “A-profile” is defined on the four anterior points. We prefer to read that a given patient had “four quarters of B-profile” (i.e., a B-profile, clearly expressed).

One of the four anterior points with a lung consolidation, even minute (C-line), makes a C-profile.

One isolated B-line visible at all four anterior BLUE-points: this is such a rare pattern that we do not know its clinical relevance. We should temporarily consider this profile as an A-profile.

Some profiles should not generate too much troubles (Fig. 20.3).

A quarter of B-profile visible on three of the four anterior BLUE-points makes sensu stricto “three-quarters of a B-profile.” It should probably be linked to a B-profile.

A quarter of B-profile at the right upper BLUE-point with a quarter of B-profile at the left lower BLUE-point and a quarter of B-profile at the right upper BLUE-point with a quarter of B-profile at the left upper BLUE-point are rare profiles, rare enough for not having been seen in the BLUE-protocol. We think wise and logical to link such profiles to an irregular A/B-profile (much more than a B-profile) – suggesting pneumonia/ARDS.

Two-quarters of B-profile at the two lower BLUE-points: this profile, seen in 5 % of cases of hemodynamic pulmonary edema, should probably be considered as an irregular B-profile (patient under beginning of depletive therapy?).

Three-quarters of A-profile with one-quarter of B-profile must be assimilated to an A-profile. This is usually a pneumonia, which will be recognized using the long sequence of the BLUE-protocol: no venous thrombosis and a PLAPS usually present: “A”-no-V-PLAPS-profile.