Vapor pressure osmometry: minimum sample microvolumes.

PURPOSE: Vapor pressure osmometers are currently designed to handle sample volumes as small as 2.0 microliters (microl) but smaller sample volumes are desirable in tear fluid studies. We determined the minimum sample size required for adequate validity and repeatability of osmolality measurements. METHODS: A standard saline solution (290 mmol/kg) was sampled by a variable, calibrated pipette. Forty samples were processed with a Wescor 5520 vapor pressure osmometer at each of the following volumes: 2.0, 1.6, 1.2, 1.0, 0.8, 0.7, 0.6 and 0.5 microl. Prior to each test series, the instrument was calibrated with the identical volume of 290 mmol/kg saline. Relevant descriptive statistics were computed and an analysis of variance (ANOVA) was performed on the resulting data. RESULTS: The mean osmolalities of the eight 40-sample groups ranged from 288.42 to 290.68 mmol/kg and were not significantly different from 290 mmol/ kg or each other (p>0.05). The standard deviations were inversely correlated with the sample volumes, gradually increased to approximately 1% as sample volume was reduced to 0.8 microl, then more rapidly increased as the sample volume was lowered still further. CONCLUSION: Sample microvolumes as small as 0.8 microl can be collected for accurate and repeatable results with the Wescor 5520 vapor pressure osmometer when a standard deviation of approximately 1% is acceptable. Microvolumes from 0.7 to 0.5 microl may also be used if the expanded spread of data can be offset by multiple repeated readings. Using a 2.0 microl sample volume, the ultimate accuracy and repeatability of the Wescor vapor pressure osmometer was +/-2% at 290 mmol/kg: 99%, of all readings (+/-3 standard deviations) should fall within +/-6 mmol/kg of the true value.

Corneal scarring in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study: baseline prevalence and repeatability of detection.

PURPOSE: The multicenter Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a prospective, observational study of 1,209 keratoconus patients. We report on the prevalence of corneal scarring in these patients. We also report on the test-retest repeatability of corneal scar documentation at the slit-lamp biomicroscope by trained clinicians and by masked photograph readers and on the scarring-status agreement at baseline between clinicians and photograph readers. METHODS: Clinicians and masked photograph readers graded each cornea as to scarring status. Patients were examined by clinicians, and their corneas were photographed at baseline (2,297 nongrafted eyes of 1,209 patients) and at a repeated visit (258 nongrafted eyes of 138 patients). These photographs were evaluated by the masked readers at the CLEK Photography Reading Center (CPRC). Clinicians reported corneal scars in right eyes at baseline as "definitely not present" in 53.9%, "probably not present" in 8.4%, "probably present" in 8.2%, and "definitely present" in 29.4% of patients. A weighted kappa statistic of 0.83 (95% confidence interval from 0.78 to 0.88) indicates that agreement is excellent between baseline and repeated assessments for the presence of a corneal scar by clinicians. RESULTS: Agreement is very good between baseline and repeated photograph-reader assessments for the presence of a scar, with a weighted kappa of 0.77 (95% confidence interval, 0.72-0.82). The kappa statistic comparing photograph-reader scarring assessments with clinician results was 0.69 (95% confidence interval, 0.66-0.71). CONCLUSION: The data also suggest better agreement between clinicians and readers when Vogt's striae and corneal nerves were observed. The data also suggest better agreement when corneal staining was not observed by the photograph readers. The CLEK Study protocol for determining the presence of scars is highly repeatable.

The repeatability of tear mucus ferning grading.

PURPOSE: Rolando's classification system for tear mucus ferning patterns subjectively assigns grades based on the size and spacing between ferns. Grade 1 and 2 patterns are considered "normal," whereas grade 3 and 4 patterns are often associated with keratoconjunctivitis sicca. This study was designed to examine the intraobserver and interobserver repeatability of Rolando's system. METHODS: Photographic slides (N = 418) of portions of tear ferning patterns were randomly assembled, numbered, and graded by two investigators (I1 and I2). I1 graded the slides twice; the slides were remixed under masked conditions between the first and second runs. I2 graded the slides once, independent of I1. RESULTS: For the four-grade system, intraobserver agreement was 85.41% (simple kappa = 0.75; 95% confidence interval ]CI] = 0.69-0.82). Interobserver agreement was 80.62% (kappa = 0.67; CI = 0.60-0.74) and 86.12% (kappa = 0.75; CI = 0.68-0.83) for the first and second runs of I1, respectively. Analysis of the ability to simply classify normal (grade 1 and 2) from abnormal (grade 3 and 4) patterns revealed intraobserver repeatability of 94.50% (kappa = 0.76; CI = 0.67-0.86). Interobserver agreement was 92.10% (kappa = 0.65; CI = 0.56-0.75) and 94.26% (kappa = 0.71; CI = 0.62-0.81) for the first and second runs, respectively. CONCLUSION: Based on the high rate of agreement between intraobserver and interobserver trials, Rolando's grading system appears to be an easy and consistent method for the classification of tear ferning patterns.

Distribution of astigmatism among Sioux Indians in South Dakota.

BACKGROUND: Previous studies have reported a greater prevalence of high astigmatism and vision disorders among Native Americans than occurs in the general U.S. population. The majority of these studies, however, have focused on Native American schoolchildren. This study assessed the distribution of astigmatism in a general clinic population at the Rosebud Indian Reservation in South Dakota. METHODS: From examination of 174 patients, demographic information, visual acuities, refractive error, binocular vision status, and eye health were recorded. RESULTS: The data showed more high astigmatism than would be expected in a general U.S. population. Three or more diopters of astigmatism were found in 9.2% of the right eyes and 10.8% of the left eyes in the study population. High astigmatism was noted less frequently among those ages 40 years and older (3.3% right eyes, 6.3% left eyes). Most of the astigmatism was with-the-rule, but a shift toward against-the-rule and oblique astigmatism was found in older individuals. No significant gender differences were noted. CONCLUSIONS: The results suggest an ethnic difference in astigmatism and emphasize thee need for continued and expanded vision services for Native American populations.

Low vision aids and the presbyope.

When light rays emerge divergent from a low vision device, a presbyopic patient requires a near addition to focus the image. The magnitude of the divergence and the near addition power used by a presbyope can substantially alter the magnification effects of a device. This study examines the effects of different emerging divergences and near addition powers on the magnification of low vision devices. The emerging divergence of 54 fixed focus stand magnifiers was measured using trial lenses and a telescope. The magnifiers represented a range of powers and manufacturers. The emerging divergence was not predictable. It could not be correlated to magnifier power. Although mathematical formulas exist to calculate the effect of the divergence and near addition on magnification, tables were developed to simplify this determination.