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M.F. Chan, C. Onyenekwu, T. Vaz, B. Levy, C.M. Kalsow; Detection and Quantitation of Endotoxin Levels in Tears of Contact Lens Wearers and Non–Wearers . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2408.
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Studies to evaluate changes of the microbial content of the conjunctiva accompanying the use of contact lenses have been equivocal mainly because current culture techniques are not consistent and reproducible. Gram positive bacteria are present as part of the normal ocular flora. The detection of Gram negative bacteria would be of concern due to the potential for sight threatening infection. Detection of endotoxin in a tear sample could provide a simple, reproducible indication of undesirable changes in microbial content of the eye. In this study, we evaluated a method to detect and quantitate endotoxin levels in tears collected by Schirmer strip and compared the levels between wearers and non wearers of silicone hydrogel contact lenses.
Tears were collected by Schirmer strips from eyes of 20 individuals not using contact lenses (NCL) and from 20 individuals using silicone hydrogel contact lenses (CL) (PureVisionTM, Bausch & Lomb) on an extended wear basis. Endotoxin in the eluted sample was quantified by the kinetic Limulus amoebocyte lysate (LAL) assay using the Endosafe®–PTS unit. Values were calculated both for total endotoxin units (EU) recovered and as EU/µl tear recovered.
Validation studies indicated that fibers from Schirmer strips would not give false positive values, Schirmer strips would not irreversibly bind endotoxin, and endotoxin could be recovered from Schirmer strips. A normal distribution of endotoxin values was detected in tears obtained by Schirmer strip. Total EU levels for CL ranged from <0.05 – 0.98 with an average value of 0.262 ±0.184 and a median value of 0.213 EU. In NCL, total EU levels ranged from <0.05 – 2.01 with an average value of 0.274 ± 0.354 and a median value of 0.211EU. EU/µl tear for CL ranged from <0.05 – 36.11 x 10–3 with an average value of 8.02 x 10–3 ± 7.36 x 10–3 and a median value of 6.43 x 10–3 as compared to NCL in which EU/µl tear ranged from <0.05 – 73.57 x 10–3 with an average value of 7.93 x 10–3 ±13.48 x 10–3 and a median value of 6.43 x 10–3.
Quantitation of endotoxin levels in tears of both CL and NCL indicates that this rapid and quantifiable assay has potential to monitor microbial content of the conjunctiva under a variety of conditions and that silicone hydrogel contact lens wear does not appear to change the microbiota of the normal external eye.
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