A correlation test was chosen for the analysis of data in this study rather than the alternative statistical test of agreement between methods of clinical measurement suggested by Altman and Bland. 29 The latter technique was not appropriate for this study because the measurements of the same parameter obtained by the different techniques gave values in different (not interconvertible) units. In view of the adoption of the “correlation” approach in this study, the study strictly evaluated “association” rather than “agreement” between the measurement techniques. 29
This experiment failed to identify a significant correlate with PRT wetting. Therefore it does not offer a clear, empirical resolution of the question of what the PRT actually measures through comparison with other accepted methods of assessment of tear volume or production. Others have also not obtained a correlation between tear meniscus height and an absorption test for tear production (Schirmer test 30) in normal subjects. Scherz et al. 31 found a correlation with meniscus height and volume obtained by a fluorometer technique in normal subjects and dry eye patients, but not with the Schirmer test and tear volume. Meniscus height and kinetic Schirmer results are correlated in dry eye patients. 32 Certainly the potential for correlation is greater with a wider range of values offered by the inclusion of a dry eye group. Work on normal subjects, however, may have been expected to yield significance if a true relation existed if a large enough range of values of the parameters were included (in this study, PRT varied from 8 to 30 mm, TMH from 0.19 to 0.68 mm, and tear turnover rate from 5.75% to 52.78% per minute). A previous study incorporating dry eye patients and normal subjects did find a correlation between PRT and TMH. 33 However, their photographic technique for measuring TMH required installation of fluorescein into the eye from a strip moistened with 10 μl of saline. The addition of all, or part of, this extra fluid into an eye with between 2.74 and 7 μl 34 of preexisting tears is likely to swell the tear meniscus (80% of fluid in the eye is at this location 33) and to precipitate reflex tearing, thus rendering their results different and not comparable to this study.
The present study offers no clear evidence that the phenol red thread test measures tear volume. The comparison of the PRT wetting with volume determined with previously established methods, tear meniscus height or fluorophotometry, yields no significant correlation. It is perhaps significant that the latter two volume measures do not correlate (Table 2). Clearly these existing tests measure different aspects of tears by “meniscus swelling” (TMH) and by calculation of the tear dilution based on volume assumptions (fluorophotometric technique). Neither correlate with the “amount of residual tears” in the eye as indicated by PRT wetting length. The presumption that PRT wetting length should provide a measure of tear volume or residual tears in the lower conjunctival sac needs to be reexamined. The volume of tears in young subjects averages 7 μl. 29 The average absorption of tears by phenol red thread in 15 s has been shown to be just >0.5 μl. 20 Therefore, the proportion of total tears resident in the eye and absorbed by the phenol red thread is about 7.5%. Therefore, full elimination of tears from the eye and a consequent assessment of tear volume (or residuals) is unlikely to occur with the PRT.
The results of this study suggest that the PRT is unlikely to measure tear volume of the eye or residual tears in the lower conjunctiva. It is more likely that it measures uptake of an (small) amount of fluid residing in the eye and stimulates a low degree of reflex tearing. It is not possible to say categorically whether this reflects the whole of the basal and/or a (small) part of the reflex level of tear secretion. 38 Although the PRT is more comfortable for patients than the Schirmer test, it may not offer as valid a measurement of reflex tear facility.
Received August 3, 2000; revision received December 27, 2000.
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