This study reports intra- and interobserver variability for TMH, LLT and NIBUT measurements for individual instruments as well as between different instruments. There were no significant intraobserver differences noted in NIBUT measured using Oculus or IDRA, LLT (Lipiview), and Oculus TMH values. Between two observers, there were significant differences in LLT and TMH measurements, obtained using IDRA but not for Oculus or Lipiview. Between instruments, all the measurements (LLT, NIBUT and TMH) were significantly different for both observers.
The clinical tests for dry eye such as Schirmer and TBUT are considered unreliable. The variability in TBUT is up to 8 seconds between two visits and ICC was 0.29.4 With the advent of automated measurements, the readings are more reliable now. However, there are different diagnostic platforms available in the market and there is no study that has compared the tear film parameters across them. The intraclass coefficient of repeatability of LLT using Lipiview has been noted to be 16 nm and interclass 13 nm.3 In our study, intraclass coefficient of repeatability was 20 nm whereas interclass coefficient of repeatability was 67 nm. The sample size in study by Zhao et al. was 20 eyes compared to 60 eyes in the current study. This could be responsible for larger interobserver variation; however, the difference was not statistically significant. This variation should be kept in mind while performing clinical studies using Lipiview interferometer. The reported mean LLT values in DED patients is between 54 to 76 nm (SD,15–25) whereas 65 nm in healthy controls (33–100) that overlaps with DED patients.5 We found mean LLT values of 48.21 nm (range, 40–92 nm) and 58 nm (range, 32–99 nm) using IDRA and Lipiview respectively, which is similar to published studies.
The average TMH value in healthy subjects using K5 has been reported as 0.27 ± 0.12 mm, 0.29 mm (range, 0.26–0.34 mm) with the intraclass correlation coefficients and coefficient of variation values of 0.914 and 16.4%, respectively for inter-individual variation.6,7 For the intra-individual variation, the intraclass correlation coefficients and coefficient of variation values were 0.939 and 15.9%, respectively.6 Their study did not use Bland-Altman analysis. The intraobserver and interobserver coefficient of repeatability for K5 in the current study was 0.18 and 0.21. The values did not show significant intra or interobserver variation. The differences in mean TMH using IDRA showed more intra and interobserver variation. Hence, K5 seems more reliable for measuring TMH values. NIBUT measured with the Keratograph (software version 2.73r19) in 100 healthy personnel ranged from 0.36 s to 29.0 s, with 63% of readings being < 5 s and 85% <10 s.8 The keratograph used in the current study was the latest one, 5M. No healthy individual had value < 5 s and only 25% had value < 10 s. The reported average of NIBUT using K5 is 10.35 ± 4.2 s, 10.9 ± 3.9 s, which is comparable with our value of 11.9 ± 0.50 s.7,9 K5 showed good repeatability for NIBUT and TMH, hence can be considered as a standard for measuring tear film quality. LLT cannot be measured as numeric value with K5, hence we did not evaluate the lipid layer status using K5.
IDRA ocular surface analyzer was introduced as a comprehensive diagnostic system for tear film analysis that can measure automated LLT, TMH, NIBUT, blink rate, meibography, pupillometry and conjunctival hyperemia grading. It had advantages over Lipiview as NIBUT and TMH could also be measured. The reported normal values of NIBUT, LLT and TMH using IDRA are 10.4 ± 2.4 s, 73.4 ± 21.9 nm, and 0.289 ± 0.16 mm, respectively.10 However, repeatability values are not available for IDRA. We observed normal values to be 8.9 ± 0.34 s, 48.21 ± 1.49 nm and 0.21 ± 0.01 mm for NIBUT, LLT and TMH, respectively. The repeatability for TMH and LLT values was low, hence the same observer should be involved in performing dry eye measurements using the IDRA platform.
The limitation of this study was the number of eyes examined but 60 eyes were totally imaged thrice totaling 180 scans per patient and a total of 360 scans. The strength of this study is the evaluation of measurements across three dry eye diagnostic platforms, the results of which can be applied universally. The study findings suggest that no two dry eye diagnostic platforms can be used interchangeably for performing tear film studies. The NIBUT and LLT measurements using Oculus and Lipiview are more reliable than IDRA ocular surface analyzer, although a larger sample size would be required to validate the strength of the comparison.