Of interest, in the plasma samples from ARDS patients, levels of Ang I have been shown to be significantly increased in non-survivors (interquartile range 1990–16950 pg/mL) compared to survivors (interquartile range 730–5660 pg/mL) [92], suggesting a marked increase of renin activity in non-surviving ARDS patients. Unfortunately, no comparisons of the two ARDS groups with healthy subjects (interquartile range 35–66 pg/mL, markedly lower than both patient groups [179]) were reported; this is important to consider since the plasma concentrations of the RAS peptides may significantly differ depending on method of detection (ELISA versus vs. mass spectrometry) and, in particular, on protocol of determination (endogenous versus equilibrium Ang peptide levels [179]). Endogenous RAS peptide concentrations can be evaluated collecting and stabilizing blood samples with a RAS enzyme inhibitor cocktail. Differently, heparinized plasma samples can be incubated at 37 °C to induce ex vivo equilibrium, which unveils the presence of renin, ACE and/or ACE2 activity in the plasma (equilibrium Ang peptide levels) [179]. This last protocol usually gives significantly higher levels of the RAS peptides [179]. For example, in healthy subjects, the normal ranges (interquartile ranges) of endogenous RAS peptides, Ang I, Ang II, Ang (1–7), Ang (1–9) and Ang (1–5) were 12–24 pg/mL, 1–12 pg/mL, <2 pg/mL, <4 pg/mL and < 1 pg/mL, respectively, differently the normal range after ex vivo equilibrium of the same peptides were 35–66 pg/mL, 110–200 pg/mL, < 2 pg/mL, < 4 pg/mL and < 1 pg/mL, respectively [179], indicating the presence of renin and ACE (but no ACE2) activity (as highlighted by Ang peptide increases upon ex vivo equilibrium). Similarly, another report that evaluated Ang II and Ang (1–7) in healthy males showed normal endogenous ranges ~ 14.5 pg/mL and 0.9 pg/mL, respectively, and an Ang (1–7)/Ang II ratio ~ 0.06 ratio [180]. Moreover, Ang (1–7), Ang I, Ang (1–9) and Ang (1–7)/Ang II ratio were significantly higher in ACEI-treated hypertensive patients compared to healthy subjects [180], suggesting that inhibition of ACE predisposes to upregulation of the ACE2/Ang (1–7)/MasR pathway. Notably, two different reports that evaluated the RAS peptides in ARDS patients using the similar method of detection (high performance liquid chromatography plus mass spectrometry) showed significantly different results. One report showed that Ang II ranged (interquartile range) between 120–630 pg/mL (non-survivors) and 70–2220 pg/mL (survivors) [92]. A second report showed significantly lower values of Ang II, (interquartile range) between 40–150 ng/mL (non-survivors with placebo) and 5–20 pg/mL (survivors with placebo) (see Supplementary Data [134]), suggesting that they used different protocols of the RAS peptide determination. Nevertheless, both Ang (1–7) and Ang (1–7)/Ang II ratios found in ARDS patients of both reports [92,134] were significantly higher than healthy subjects. One report showed that Ang (1–7) ranged (interquartile range) between 2.5–10 pg/mL (see Figure 2B of [134]), Ang II between 5–20 pg/mL (see Figure 2 of [134]) and Ang (1–7)/Ang II ratio ~ 0.5. Moreover, in the report in which the elevated concentrations of Ang I and Ang II indicate an ex vivo equilibrium of the RAS peptides, the increase was even more marked. In that report, Ang (1–7) (< 1pg/mL in healthy subjects) ranged between 80–1070 pg/mL and Ang (1–7)/Ang II ratio (~ 0.06 in healthy subjects) ranged between 0.24–1.82 [92], suggesting that ACE2 activity (and ACE2 presence in plasma samples) was significantly increased in ARDS patients as compared to healthy subjects. In line with this hypothesis, there was the determination of Ang (1–9), whose production from Ang I exclusively depends on ACE2 (which also compete with ACE for the same substrate). In normal healthy subjects, it was reproducibly below the limit of quantification [179,180], differently, both in ex vivo treatments of plasma samples with human rACE2 [115,181] and in ARDS patients, it was markedly increased (Ang (1–9) ranged between 100–3080 pg/mL) [92], suggesting both that the dominating activity of ACE in plasma of healthy subjects is overcome by that of ACE2 in ARDS patients. Therefore, Ang (1–9) may represent a good surrogate marker for ACE2 activity. Similarly to Ang (1–9), there was the determination of Ang (1–5), whose production depend on both ACE2 [that produces Ang (1–7) from Ang II] and ACE [that produces Ang II from I and Ang (1–5) from Ang (1–7)]. Ang (1–5) was undetectable in healthy subjects [178,179]; however, it was significantly increased in ARDS patients (interquartile range between 50–730 pg/mL [92]), suggesting that both ACE2 and ACE activity were likely upregulated as compared with healthy subjects. The activation for both arms of the RAS needs of the contemporary activation renin, the rate-determining enzyme of the RAS. Indeed, analysis of plasma samples (from a healthy subjects), treated ex vivo with recombinant renin, showed an increase of Ang I, Ang II, Ang (1–7) and Ang (1–5), but not Ang (1–9) [115,179,181], further suggesting both the ACE2 specific activity in producing Ang (1–9) and an activation of both arms of the RAS in ARDS patients. Finally, in vivo or ex vivo administration of human rACE2 in plasma samples produced an increase not only of Ang (1–9) but also of Ang (1–5) and Ang (1–7) in both healthy subjects [178] and ARDS patients [134], indicating that all three Ang peptides might represent surrogate markers of ACE2 activity. Of interest, the mean arterial pressure of ARDS patients was markedly low and significantly lower in non–survivors (mean value 65 mmHg and more likely need vasopressor support) compared to survivors (mean value 71 mmHg) [92], which may likely be a consequence of the above described ACE2 upregulation. Interestingly, similar features were also detected in PAH and coronary atherosclerosis. In particular, a significant increase of both Ang II and Ang (1–7) peptides was observed in plasma of these patients [108,182] and plasma levels of TNF-α were significantly elevated in the critical coronary artery disease [182]. Moreover, despite a significant increase of Ang II/Ang (1–7) ratio (that suggested a therapeutic use of recombinant hACE2 in PAH patients) systolic and diastolic blood pressures were in the range of normality [108,182]. Altogether these observations suggest that Ang peptide ratios are not reliable markers for disease status in these pathologies; rather, increased plasma amounts of each RAS peptide should be careful considered.