“…This heterogeneity might be due to differences in (1) the study population (more mature infants, higher use of antenatal steroids, variation in the definition of RDS, diverse level of care) or (2) methods for sampling, processing, and analyzing data. For instance, the studies on SMT considered different MB sizes (from 10 to 25 μm), various parameters for defining the cut-off (absolute number, proportion, or average diameter of MB for a given sampling area) [18,19,[21][22][23][24]; sometimes the analysis was performed by an operator [18,19,23,24], sometimes by a computerized program [21,22]; some studies used fresh [19,24] while others frozen samples [18,[21][22][23]. However, above all, one of the study's significant limitations was the lack of a proper gold standard.…”