Despite a formal sample size calculation is not warranted, based on the available data [5, 11, 19] , we previewed a convenience sample size, as follows: 50-60 preterm infants affected by iRDS, with at least 20 receiving a second surfactant dose; for ARDS and IRRF groups 20 patients will be also convenient, while 10 control neonates or infants will be considered. These sample size have been checked for power regarding the correlation between sPLA2 activity and two selected clinical variables (using Power and Precision demo rel. 3.2 [68] ). Given α-error of 0.05 and a correlation coefficient (r) ≥ 0.6 and ≥ 0.85 [5, 11, 19] for the respiratory compliance and the PaO 2 /FiO 2 ratio, respectively, the power resulted > 80% in both cases.