This is an interesting ‘pilot’ study regarding the increasing use of CGM systems and their stability during flight as being an important issue.

In order to enlighten this issue, 4 marketed CGM devices were worn by a total 20 subjects on the ground and also during flight while including fasting and non-fasting periods.

As result, there seem to be a clear outcome in favour of the Dexcom G7 and Abbott Libre 3 systems compared to Medtronic Simplera and Sinocare iCan i3.

However, as already stated in the final paragraph by the authors, there are many limitations and shortcomings that hinder a clear statement or conclusion belonging to the results of this work.

So, these findings present preliminary state with need for much more comprehensive controlled trials.

From methodological point of view, there remain many points to be cleared:

• How was the 4 CGM systems distributed with regard to their insertion sites? In a totally randomized manner, or relative equally distributed among, for instance, right and/or left side of abdomen, dominant and/or non-dominant arms?

• As the used systems have different life times varying from 7 to 10,5 and up to 14-15 days and as it is a known issue that CGM systems tend to reflect more precise outcome during the middle period of this time, it is important to know during which period of these timeframes the study was conducted?

• Were all measurements per subject done at one session, i.e., during a round-trip on the same day, or have we more sessions for each subject?

• Were the measurements of the groundphase performed prior to start or end of the flight at Vienna or Rejkjavik airports, or separately in controlled settings?

• Were the subjects placed in a regularly scheduled flight or have we one or more organized study flights?

• Were daytime and seasonal effects eliminated, i.e., the outbound and return flights planned for the same timeframes at a day and the entire study finished during a stable daytime/nighttime period?

• Were the meals provided standardized or had the subjects free choice?

• In a 4-4,5 hours flight time, how precise the fasting and non-fasting periods could be delineated?

From reflecting the results, the following shortcomings should be highlighted:

• With a BG-measurement interval of 30 min using the comparator device, at most 8-9 measurements could be available during one flight session per subject. This fact reflects a strong limitation for distinguishing among the ascending, descending and cruising periods of the flight as well as the fasting and non-fasting periods. Why these measurement were not planned more closely?

• It is obvious that the possible availability for comparison of sufficient pairs of values during ascending and descending periods of flight is extremely limited. Thus, I suggest not to emphasizing those values in the main text and let them solely in Table 1.

• In addition, I’m somewhat confused with the number of pairs given in Table 1. The sums of pairs from ascending, descending and cruising periods is marked lower than the given numbers for the overall in-flight periods.

Moreover, the number of pairs during overall in-flight periods is comparable with the pairs for in-flight fasting and in-flight non-fasting periods whereas it would be expected to reflect nearly a double count.

• In order give the potential reader a clearer sight, and to avoid repeated presentation of the data and a superflous number of tables and figures, I suggest to combine figures 1-4 and 5-8 in 4 panels each (a to d). Then, the numeric values in Table 3 should be embedded into these figures.

• I wonder whether the variation co-efficient for the performance of the sensors was estimated or not?

• Did no adverse events occur? Because the number of subjects during different study periods vary from 17 to 20 (see tables), it looks like some problems happened?

Discussion and interpretation:

• Have the authors any idea, why Dexcom G7 and Abbott Libre 3 resulted in better MARD values during flight when compared with the respective values at ground?

• Do we know at which level exactly (altitude and partial oxygen pressure) the cabin pressure of the study-flights were stabilized? This is important for comparing the present results with earlier studies performed in hypobaric chambers.

• Although not directly related to flight conditions, including some other head-to-head studies, if available, with these sensors into the discussion section would enrich the scientific background, because nearly half of the values obtained from the present work were performed at ground.

One minor note:

• Closing paranthesis in the first row and last column of Table 2 is inverted.

Taken together, as being a pilot study with numerous points to be elucidated, however, reflecting interesting outcome for an important field of real life, switching the submission from a full-length original article into a brief report should be taken into consideration.