ABSTRACT

Objectives:

The present study was conducted to develop an instrument for measuring adults’ glaucoma knowledge levels and to establish the instrument’s validity and reliability.

Materials and Methods:

The study group consisted of 811 persons aged 40-80 years who presented to primary health care institutions and did not have a glaucoma diagnosis. A 27-item questionnaire measuring level of glaucoma knowledge was created by the study team. Following expert consultation, it was structurally evaluated. The difficulty index and discrimination index were calculated for each item. Factor analysis was used to determine construct validity, Cronbach’s alpha internal consistency coefficient and item-total correlations were calculated to determine reliability. Confirmatory factor analysis was used to assess the extent to which the factor structure of the scale fit. We analysed correlation with the National Eye Health Education Program (NEHEP) Eye-Q scale in order to evaluate the validity of the scale.

Results:

The final glaucoma knowledge level questionnaire comprised 10 items in one dimension. The discrimination index and difficulty index ranged between 0.28 to 0.65 and 33 to 61%, respectively. According to factor analysis, the Kaiser-Meyer-Olkin score was 0.760 and Bartlett’s test indicated p<0.001. Confirmatory factor analysis showed acceptable scale fit and fit indices. Validity assessment revealed a positive correlation between the total score of the items of the NEHEP scale and glaucoma knowledge level questionnaire score (r=0.522; p<0.001). Scores were higher in participants who were aged 40-64, living in the city, had education level of high school or above and had previous eye examination or intraocular pressure measurement.

Conclusion:

The glaucoma knowledge level questionnaire has the distinction of being the first valid and reliable scale for assessing level of glaucoma knowledge in Turkey.

Introduction

Glaucoma is an optic neuropathy characterized by loss of retinal ganglion cells, optic nerve atrophy and visual field loss.¹ This global public health problem is the most common cause of blindness in the world after cataract.² It is estimated that glaucoma affects more than 60 million people in the world and that this number will exceed 100 million by the year 2040. Because early glaucoma is usually asymptomatic, many people are unaware of the disease until the onset of vision loss.^3,4 Early diagnosis can prevent glaucoma-related blindness and its adverse effects on quality of life.⁵ It is estimated that about 90% of glaucoma-related blindness can be prevented with early and appropriate treatment.⁶

Timely eye examinations and appropriate treatment are critical to reduce visual impairment and blindness caused by glaucoma. However, many people in developing countries do not have regular and timely eye examinations due to a lack of knowledge and awareness about glaucoma-related blindness.⁶ As glaucoma does not cause obvious symptoms such as pain, many people do not undergo screening for early diagnosis.² Glaucoma awareness is especially low in rural areas and poor communities.⁶ Some authors have reported that awareness of glaucoma is insufficient even in Western societies.^7,8 In addition to the early recognition of asymptomatic patients, the treatment of diagnosed patients is also an important link in controlling glaucoma. Therefore, knowledge and awareness about glaucoma must be increased among both the general population and glaucoma patients. Patient education has also been shown to improve treatment adherence.¹

Although tools have been used in studies conducted on various populations to determine the level of knowledge regarding glaucoma and related risk factors, there are no valid and reliable tools for the Turkish population.

This study was conducted to develop a scale that assesses the knowledge level of Turkish adults about glaucoma and to ensure the validity and reliability of this scale.

Materials and Methods

Results

Discussion

The aim of this study was to develop a scale to measure the level of glaucoma knowledge in a community-based sample and to test the validity and reliability of the scale. In order to determine how effectively scale items assess knowledge, they must be evaluated based on item discrimination and difficulty indices. For this scale, item discrimination index values ranged from 0.28 to 0.65 and difficulty index values ranged from 33% to 61%. An item discrimination index of 0.2 or higher is considered acceptable and indicative that the item can distinguish between the unknowledgeable and the knowledgeable.¹⁴ None of the previously developed glaucoma scales were tested for item discrimination and difficulty indices.

For a reliable scale, the Cronbach’s alpha value should be at least 0.70.¹⁵ The Cronbach’s alpha value of our scale was 0.69, which was considered adequate. Previously developed glaucoma knowledge scales had lower Cronbach’s alpha values. In fact, although the NEHEP scale is the most widely accepted scale for measuring level of glaucoma knowledge, its Cronbach’s alpha value is 0.59.¹⁶ Therefore, we believe our scale is reliable. Removal of single items from the scale did not result in a significant increase in the Cronbach’s alpha value, indicating good consistency between the scale items.

CFA was done to ascertain whether the model of the 10-item, one-dimensional GKLQ developed with an EFA was confirmed. The first value to be examined in CFA is the p value. This value indicates the significance of the difference between the expected covariance matrix and the observed covariance matrices (c²). Naturally, a nonsignificant p value is desired. However, it is also normal for the p value to be significant due to a large sample size. In this study, a significant p value was tolerated and alternative fit indices were evaluated.¹⁷ It is reported that the RMSEA value must be below 0.08 and the GFI and AGFI values must be higher than 0.90 in order for the model fit to be regarded as acceptable.¹³ For this scale, CFA yielded values of 0.082 for RMSEA, 0.95 for GFI and 0.92 for AGFI. These values were evaluated according to fit indices and it was determined that all were at an acceptable level for model fit. Consequently, we consider this evidence that the factor construct resulting from the EFA is strongly confirmed.

We consider the one-dimensional nature of the scale and the small number of items as appropriate for the purpose of the study. There is still no ideal scale for measuring levels of glaucoma knowledge. The NEHEP scale has gained more acceptance compared to other scales. Based on item analyses, three of the items in the NEHEP scale (“Glaucoma is more common among people with glaucoma in their family.”, “The risk of having glaucoma is higher among people over 60 years of age.”, “Glaucoma can be controlled.”) remained in the scale. We believe that the inclusion of items pertaining to the risk factors and treatability of glaucoma in our scale will result in wider acceptance.

Evaluation of GKLQ scores according to sociodemographic characteristics showed that scores were higher among people less than 65 years of age, those living in urban areas, those with education level of high school or higher and those with good income level. These findings are consistent with studies reporting that young age and good socioeconomic and education level are factors that increase knowledge and awareness of glaucoma.^18,19,20 In addition, the participants in our study group who had previously undergone eye examinations and ocular pressure measurement scored higher on the scale. This supports the reliability of the scale.

Conclusion

The scale created in this study is not designed to investigate all aspects of glaucoma knowledge. However, the GKLQ is the first scale for determining glaucoma knowledge in Turkey that has been tested for validity and reliability. While previously published tools assessing glaucoma knowledge generally targeted glaucoma patients, the GKLQ is designed as a simple and quick measurement tool that can also be applied to the general population. The reliability of the scale in specific groups needs to be tested and the scale requires further research and development.