Konrad Pesudovs

A Comprehensive Evaluation of the Precision (Repeatability and Reproducibility) of the Oculus Pentacam HR

PURPOSEnTo evaluate the precision (repeatability and reproducibility) of the Pentacam HR (high-resolution) tomographer (Oculus, Wetzlar, Germany) across a large range of measurement parameters.nnnMETHODSnA randomly selected healthy eye of 100 subjects was scanned twice with the Pentacam HR by one observer for each of the three measurement modes: 25-picture (1 second) scan, 50-picture (2 second) scan, and cornea fine scan (50 pictures in 1 second). The repeatability of each scan mode was assessed. One additional 25-picture scan was acquired by a second observer to test reproducibility.nnnRESULTSnOverall, the Pentacam HR had good precision, with the cornea fine scan returning the most precise results: The 25- and 50-picture scans showed similar precision. The repeatability limits, expressed as the within-subject SD × 1.96√2 of the anterior keratometry (K)1 and K2 readings with the standard 25-picture scan, were 0.25 and 0.36 D, respectively. Pachymetry maps, corneal maps, anterior chamber depth maps, corneal volume, topometric Q values and indices were also found to be precise. Poor precision was found for estimates of axis (astigmatic and progression index), pupil center pachymetry, single points on corneal maps, refractive power maps, and equivalent K readings.nnnCONCLUSIONSnMeasurements taken with the Pentacam HR are repeatable and reproducible, especially those obtained with the cornea fine scan. Although the Pentacam HR is clearly a very useful clinical and research tool, the measurement of corneal axes, pupil center pachymetry, front meridional and axial maps, refractive power maps, and equivalent K readings should be interpreted with caution.

The Psychometric Validity of the NEI VFQ-25 for Use in a Low-Vision Population

PURPOSEnTo determine the psychometric validity of the National Eye Institute-Visual Function Questionnaire (NEI VFQ-25) and its subscale structure for use in people with low vision.nnnMETHODSnTwo hundred thirty-two participants completed the NEI VFQ-25. Rasch analysis was used to test the psychometric performance of the questionnaire and each subscale. Factor models were hypothesized and tested with confirmatory factor analysis (CFA) and subsequently validated with Rasch analysis.nnnRESULTSnFor the overall scale, two rating scales had to be dichotomized and three misfitting items removed to improve fit to the Rasch model. There was evidence of multidimensionality, indicating that the scale would benefit from scale splitting. For the NEI VFQ-25 subscale structure, six of the original 12 subscales could not fit the Rasch model because of item insufficiency (fewer than two items) and the remaining six displayed poor item fit characteristics indicating that the NEI VFQ-25 does not have a viable subscale structure. CFA supported a two-factor model with visual functioning (10 items) and socioemotional (9 items) scales. Most goodness-of-fit statistics were within the recommended range of values. The factor loadings of items on their respective scales were statistically significant (P < 0.001) and ranged between 0.59 and 0.84. The two scales individually fitted the Rasch model and were found to be unidimensional with adequate psychometric characteristics.nnnCONCLUSIONSnThe native NEI VFQ-25 is a better performing instrument when split into visual functioning and socioemotional scales. These scales possess valid parameters for assessment of the impact of low vision in this population.

Reengineering the Glaucoma Quality of Life-15 Questionnaire with Rasch Analysis

PURPOSEnTo investigate, using Rasch analysis, whether the 15-item Glaucoma Quality of Life-15 (GQL-15) forms a valid scale and to optimize its psychometric properties.nnnMETHODSnOne hundred eighteen glaucoma patients (mean age, 65.7 years) completed the German-version of the GQL-15. Rasch analysis was performed to assess category function (how respondents differentiated between the response options), measurement precision (discriminative ability), unidimensionality (whether items measure a single construct), targeting (whether items are of appropriate difficulty for the sample), and differential item functioning (whether comparable subgroups respond differently to an individual item). Where any of these attributes were outside acceptable ranges, steps were taken to improve the instrument.nnnRESULTSnThe five-response categories of the GQL-15 were well differentiated by respondents, as demonstrated by ordered and well-spaced category thresholds. The GQL-15 had an excellent measurement precision but demonstrated poor targeting of item difficulty to person ability and multidimensionality, indicating that it was measuring more than one construct. Removal of six misfitting items created a nine-item unidimensional instrument with good measurement precision and no differential item functioning but poor targeting. A new name, the Glaucoma Activity Limitation (GAL-9) questionnaire, is proposed for the short version, which better reflects the construct under measurement.nnnCONCLUSIONSnThe GAL-9 has superior psychometric properties over the GQL-15. Its only limitation is poor targeting of item difficulty to person ability, which is an inevitable attribute of a vision-related activity limitation instrument for glaucoma patients, most of whom have only peripheral visual field defects and little difficulty with daily activities.

The Impact of Diabetic Retinopathy and Diabetic Macular Edema on Health-Related Quality of Life in Type 1 and Type 2 Diabetes

PURPOSEnTo assess the impact of diabetic retinopathy (DR) and diabetic macular edema (DME) on health-related quality of life (HRQoL) in type 1 and type 2 diabetes using the EuroQoL EQ-5D generic multi-attribute utility instrument (MAUI).nnnMETHODSnIn this cross-sectional study, 577 patients with diabetes were recruited from specialized eye clinics in Melbourne, Australia. Each patient underwent clinical, biochemical, and anthropometric assessments. The severity of combined DR and DME (no DR/DME; mild NPDR [nonproliferative DR (NPDR)] and/or mild DME; moderate NPDR and/or moderate DME; and vision-threatening DR (VTDR) (severe NPDR or PDR and/or severe DME) in the worse eye was calculated. EQ-5D utility measures were the main outcome. Because the distribution of the utility measures was skewed, independent associations were explored using multivariate quantile regression models (five quintiles, namely 15th, 30th, 45th, 60th, 75th) ranging from poorest to highest HRQoL.nnnRESULTSnMedian age of the participants was 66 years (range, 26-90 years). Of the 577 participants, 223 (38.7%) had no DR/DME, 35 (6.1%) had mild NPDR/DME, 127 (22.0%) had moderate NPDR/DME, and 192 (33.3%) had VTDR. In adjusted models, neither presence nor severity of DR/DME was significantly associated with any quantile of the EQ-5D. In contrast, the presence of diabetic complications (other than DR) (β = -0.153; SE = 0.052; P < 0.001), other nonocular comorbidities (β = -0.115; SE = 0.038; P < 0.01), and higher body mass index (β = -0.007; SE = 0.002; P < 0.001) were all associated with worse HRQoL.nnnCONCLUSIONSnUsing a generic MAUI, the EQ-5D, the authors found that the presence or severity of DR/DME and concomitant vision loss were not associated with any quantile of HRQoL. These findings suggest that the EQ-5D lacks sensitivity in assessing the impact of the severity of DR/DME on HRQoL parameters and that condition-specific instruments may better capture the full impact of the association.

Psychometric properties of the NEI-RQL-42 questionnaire in keratoconus.

PURPOSEnTo assess the psychometric properties of the National Eye Institute Refractive Error Quality of Life (NEI-RQL-42) questionnaire in keratoconus and compare these findings to patients with refractive error correction alone.nnnMETHODSnThe Portuguese version of the NEI-RQL-42 Quality of Life questionnaire was completed by 44 patients who had keratoconus before and after implantation of intracorneal ring segments. Rasch analysis was used to assess the use of response categories, success in measuring a single trait per subscale (unidimensionality), ability to discriminate person ability (precision), and targeting of questions to person quality of life (QoL).nnnRESULTSnRasch analysis was performed for the questionnaire subscales using stacked preoperative and postoperative data. Three subscales (Symptoms, Dependence on correction, and Suboptimal correction) contained response categories that were not used as intended. Six subscales contained misfitting items indicating multidimensionality. Eleven subscales exhibited inadequate measurement precision. Only the Near vision subscale demonstrated adequate precision with a person separation greater than 2.0. Targeting of items to person QoL was adequate in 11 of the 12 subscales with a mean item location of less than 1 logit.nnnCONCLUSIONSnOnly one NEI-RQL-42 subscale (Near vision) performed adequately in keratoconus. Targeting was better in patients with keratoconus than in patients with refractive error correction alone, but 11 of the 12 subscales remain manifestly inadequate. Better instruments exist for measuring patient-reported outcomes in keratoconus.

Psychometric properties of visual functioning index using Rasch analysis

Acta Ophthalmol. 2010: 88: 797–803

Guttman Scale analysis of the Distance Vision Scale

PURPOSEnThe Distance Vision Scale (DVS) is a self-assessment of visual acuity (VA). Like VA testing in which letter reading becomes progressively difficult through the test, DVS questions have a hierarchy of difficulty (Guttman scale). The aims were to determine whether the DVS fulfills Guttman scaling criteria and to test the relationship between DVS score and VA in a cataract population.nnnMETHODSnResponses of 113 participants to the DVS were subjected to Guttman analysis. Standard criteria of scalability were evaluated that included the coefficient of reproducibility (CR), minimal marginal reproducibility (MMR), and coefficient of scalability (CS). The relationship between total item score and binocular visual acuity was determined.nnnRESULTSnFive participants were excluded because of missing data. Regularity in the banding pattern of the scalogram of the 108 participants was suggestive of a deterministic Guttman scale. Analyses showed that DVS satisfies the criteria for classification as a valid unidimensional and cumulative scale, as CS (0.93), CR (0.99), and MMR (0.85) values fall within the desired range. The statistically significant correlation between the total item score and binocular VA was 0.24.nnnCONCLUSIONSnThe DVS fits the Guttman scale, supporting the deterministic model underlying the scale. It correlates poorly with VA, suggesting it taps aspects of visual performance and other issues beyond high-contrast VA. The DVS could be used as an outcome measure to evaluate change over time and could be used to set achievable treatment objectives because of its hierarchical properties.

Establishing levels of indications for cataract surgery: combining clinical and questionnaire data into a measure of cataract impact.

PURPOSEnTo develop a model for establishing indications for cataract surgery that incorporates clinical and questionnaire data on a single linear scale using Rasch analysis.nnnMETHODSnIn this prospective study, 293 preoperative cataract surgery patients (mean age, 72.8±10 years; age range, 33-98 years; 174 female, 119 male; 49% with ocular comorbidity) completed two questionnaires, and visual acuity was measured in each eye. A cataract impact model was developed using Rasch analysis incorporating questionnaire scores and visual acuity. Participants were ranked from 1 to 293 based on the order in which they presented (first in first out [FIFO]) and then were ranked based on the cataract impact model. The main outcome measure was the number of participants moving 49 (16.7% change) rank positions, which represented a likelihood to change priority category.nnnRESULTSnThe cataract impact model was unidimensional (fit statistics within 0.66-1.68) and had adequate precision (person separation of 2.58), and the components were well targeted to the population (0.05 logits between the mean item difficulty and person ability). Two hundred twenty-seven (77.5%) patients moved by at least 49 rank positions.nnnCONCLUSIONSnIt is possible to combine clinical and questionnaire data and rank patients on a single linear scale. This approach modifies the ranking that occurs with the FIFO model and can be used for prioritizing patients for surgical intervention. More sophisticated models incorporating more clinical information may provide a better measure of the cataract impact latent trait.

Validation of the National Eye Institute Visual Function Questionnaire-25 (NEI VFQ-25) in Age-Related Macular Degeneration

This letter is in reference to the article by Orr et al. in the May 2011 issue. The title implies that the article provides validation of the NEI VFQ-25 for use in neovascular age-related macular degeneration (AMD); however, the evidence presented provides, at best, only very weak support for the validity of the NEI VFQ-25. The authors report only three aspects of validity: convergence (testing the association between NEI VFQ-25 scores and visual performance measures), known-groups (comparing mean questionnaire scores between subgroups stratified by visual performance results), and internal consistency (demonstrating that a series of items measure a common theme). They demonstrate no (R 0.00) to moderate (R 0.31) relationships between NEI VFQ-25 scores and visual acuity (VA), contrast sensitivity, and reading speed. They also demonstrate that people with VA better than 20/32 have better NEI VFQ-25 scores than those with VA worse than 20/100. Finally, they tested internal consistency with Cronbach’s , which is the weakest of the suite of methods for testing internal consistency (versus factor analysis and principal components analysis of Rasch residuals). Although Cronbach’s is easy to calculate, it is confounded by questionnaire length and may fail to identify the multidimensionality made evident by more sophisticated techniques. Indeed, several studies have shown that the NEI VFQ-25 in its original format is a multidimensional scale, and investigators have recommended reorganizing the items into two scales and abandoning most of the subscales. Taken together, the results of Orr et al. provide some of the absolute minimum requirements for a questionnaire, but are a long way from suggesting “Validation of the NEI VFQ-25 in AMD.” The study did not show whether the NEI VFQ-25 is valid in the functioning of the response categories, measurement precision, unidimensionality, targeting of the scale to the study population, and differential item functioning (whether measurement properties are stable across populations). Indeed, the NEI VFQ-25 has been shown to lack validity for a number of these properties. This is important because these psychometric properties have more serious implications for measurement validity than those presented by Orr et al. Therefore, sending the message that the NEI VFQ-25 has been validated for use in AMD by this study appears inappropriate and carries the danger of encouraging use of the instrument despite published evidence questioning its validity. We also suggest that any paper that describes the validation of an instrument be more comprehensive, ideally including modern psychometric methods such as item-response theory models or Rasch analysis, given the deep insight that these provide into the psychometric properties of an instrument, and comparison of the findings against established frameworks of questionnaire quality. The other important advantage of Rasch analysis is that it enables interval level estimates from ordinal questionnaire responses. The reduction of measurement noise provided by interval scaling can halve the sample size needed for outcome measurement, with substantial potential cost savings for major clinical trials such as those from which the data in Orr et al. were taken. Jyoti Khadka Colm McAlinden Konrad Pesudovs

Translation, cultural adaptation, and Rasch analysis of the visual function (VF-14) questionnaire

PURPOSEnTo translate, culturally adapt, and validate the original and previously validated shorter versions of the Visual Function Index (VF-14) questionnaire in a Chinese population.nnnMETHODSnThe VF-14 was completed by patients with cataract. The analysis was carried out in three phases: phase I, testing whether the VF-14 and its valid shorter versions,VF-8R and VF-11R, form valid scales in Chinese settings using Rasch analysis; phase II, developing completely new Chinese versions of the VF-14; phase III, testing whether the previously validated shorter versions of the VF-14 could be applied in a Chinese population. This was tested by assessing the agreement between the new Chinese (developed in phase II) and the previously validated shorter versions of the VF-14 using Bland-Altman plots.nnnRESULTSnA total of 456 patients (median age, 70 years; range, 40-92 years; females, 58%) completed the Chinese translated version of the VF-14. The VF-14 and the VF-11R demonstrated good Rasch based psychometric properties when a grossly misfitting item was removed. The VF-8R formed a valid scale without any modification. The scores of the VF-11R and the Chinese shorter version (VF-11RChin) showed very good agreement, with a mean difference of -0.18 logits and 95% limits of agreement between 0.11 and -0.47.nnnCONCLUSIONSnThe Chinese translated VF-14, VF-11R, and VF-8R were valid and could be applied to assess cataract outcomes in Chinese settings. The existing shorter version had good agreement with the new Chinese version, which signifies that there was no need to develop a different version of the VF-14 in China.