## 4 Discussion  
The tear film is formed by three layers, the mucin layer, which is the inner one covering the ocular surface; the aqueous layer, which provides lubricity, some nutrients, and antimicrobial proteins; and the lipid layer, which prevents evaporation of the aqueous layer [12; 13]. A dysfunction of the tear film layers is related to DED, which has been categorized into two types: evaporative dry eye, caused by a deficient lipid layer that produces an excessive tear evaporation in the presence of normal tear secretion, and aqueous deficient, due to an abnormal lacimal secretory function [14]. Tear film evaluation, including the measurements of NIBUT and TMH, among others, has been established as an essential procedure to the DED diagnosis [1; 2; 6]. The Medmont E300 and the Visionix VX120+ systems have previously demonstrated their consistency achieving good repeatability levels in the assessment of dry eye parameters [7; 8]. However, it is unknown whether both instruments provide analogous outcomes for NIBUT and TMH. Therefore, the present study investigated the agreement between both devices in these assessments. We found that the E300 and the VX120+ systems do not provide interchangeable outcomes for NIBUT, comparing the NIBUT measured by the E300 with first NIBUT and 50%NIBUT of the VX120+, or for TMH. In our study, NIBUT provided by the E300 system was significantly higher than both NIBUT obtained with the VX120+, the first NIBUT, and the 50%NIBUT. Particularly, the E300 system established a NIBUT value of more than 8 and 3 s in median than the first NIBUT and the 50%NIBUT, respectively. Considering that our sample involved healthy subjects, the expected NIBUT should be approximately 10 s or higher [15]. However, most of our NIBUT results measured with VX120+ were below this cut-off, suggesting that the VX120+ underestimated the real NIBUT value. This finding has also been reported with another automatic device [3; 16]. In addition, according to the Passing-Bablok analyses, most of the values obtained with the E300 were substantially higher than the first NIBUT (Figure 2). However, regarding the 50%NIBUT, our results show that the VX120+ estimates higher values in subjects with low NIBUT times (approximately below 5 s) and underestimates in subjects with high NIBUT times (approximately over 9 s). Surprisingly, no correlations were found between NIBUT measurements. Nonetheless, this poor agreement is in concordance with previous studies that also obtained a high variability in NIBUT measurements among different devices [3; 17; 18]. The abovementioned differences in the NIBUT measurements could be the consequence of various aspects related to how each device performs the measurement. First, the exposure air flow around the ocular surface could influence NIBUT outcomes [19]. Given that the E300 platform uses a small-cone to project the Placido-disks placed very close to the patient's eye, the air flow may be reduced. On the contrary, the VX120+ system uses a large-cone, which allows having a considerably higher distance to perform the assessment. Second, the algorithms and thresholds used by both devices to calculate the NIBUT are different: whereas the E300 considers the time to reach a TFSQ area of 5% in two consecutive measurements, the VX120+ detects the time until the appearance of the first tear break-up (first NIBUT) and the time until the 50% of the area presents distortion (50%NIBUT). Finally, the Placido-disks illumination is specific to the design of each device. Thus, all these issues may explain the poor agreement found in this research between devices in the NIBUT measurements. Regarding the TMH outcomes, the values obtained using the VX120+ were generally higher (approximately 0.03 mm) than using the E300, being significant when both eyes were analyzed. Nonetheless, the values obtained from both devices are like those previously described for healthy subjects [20; 21; 22].