Reliability Indices in Visual Field Testing

Over the past four decades, automated visual field testing has become an important standardized and quantitative clinical tool for detecting and monitoring progression of glaucoma. However, the usefulness of automated perimetry as a test for visual function and clinical decision making is dependent on whether the patient performs the test properly. Here, we will address the reliability measures commonly used in interpreting results from the broadly used Humphrey Field Analyzer (HFA) test.

The development of the Swedish Interactive Threshold Algorithm (SITA) for Humphrey perimetry cut testing time in half as compared to the older full threshold strategy while improving reliability due to a reduction in patient fatigue.1 SITA still presents “catch trials” during testing to estimate the rate of fixation losses, false positives, and false negatives. Fixation losses (FLs) are identified when patients respond positively to a stimulus presented in the blind spot that they should not be able to see when properly fixating on the central target. False positives (FPs) occur when patients respond positively even when no stimulus is presented. Finally, false negatives (FNs) occur when a suprathreshold stimulus presented in a previously tested location is not detected by the patient. Although FNs can be an indication of advanced visual field loss itself due to the variability of threshold values in such eyes,2 a high FN rate in eyes with minimal visual field damage suggests poor patient attention during testing.

The HFA software sets cutoffs at 20% for FLs and at 33% for FPs and FNs to indicate an unreliable test. However, these cutoffs were selected by testing normal, ocular hypertensive, and early glaucoma patients and determining who met or exceeded these cutoffs (0.5% for FPs and FNs and 19-35% for FLs) rather than whether the reliability indices impact visual loss measurements themselves. Moreover, using these standard cutoff measures leads to a binary categorization of visual fields as reliable or not.

More recent studies have explored the degree to which FL, FP, and FN rates contribute to visual field testing reliability by quantitatively calculating their impacts on mean deviation (MD). In a large retrospective study of over 10,000 HVF 24-2 SITA fields in 909 patients, FLs were found to have little impact on MD measurements (Figure 1, red line).3 In contrast, FPs lead to a greater than expected mean deviation, and FNs lead to lower than expected MDs. While the impact of FNs on MD was minimal when rates were less than 20%, FPs impacted MD even when rates were below 20% (Figure 1, black and blue lines). These findings are in general agreement with other smaller studies that have explored the impact of reliability indices in 24-24 and 30-2 tests5 or on mean sensitivity6. Although not commonly included as a reliability index, testing duration also has a moderate impact on mean deviation (with a one minute increase in testing leading to 0.35 to 0.4dB decrease in MD)3.

 

Figure 1. Effect of fixation losses (red line), false positives (blue line), and false negatives (black line) on mean deviation (MD) error as determined by the difference between measured and predicted MD. Adapted from Yohannan, J. et al. Evidence-based Criteria for Assessment of Visual Field Reliability. Ophthalmology 124, 1612–1620 (2017).

Although one should evaluate reliability on a continuum, the following evidence-based criteria can be used as an updated guide for interpreting reliability:

  1. FLs do not meaningfully impact reliability.
  2. Any level of FPs decreases reliability but particularly when FPs are greater than 20% or occur in advanced disease.
  3. FNs have less impact on reliability than FPs do but should be considered when they are greater than 35% in advanced disease or 25% in mild disease.
  4. Large increases in test duration (more than 2-3 minutes) can indicate poor reliability.

 

References

  1. Hudson, C., Wild, J. M. & O’Neill, E. C. Fatigue effects during a single session of automated static threshold perimetry. Invest Ophth Vis Sci 35, 268–80 (1994).
  2. Bengtsson, B. & Heijl, A. False-negative responses in glaucoma perimetry: indicators of patient performance or test reliability? Invest Ophth Vis Sci 41, 2201–4 (2000).
  3. Yohannan, J. et al. Evidence-based Criteria for Assessment of Visual Field Reliability. Ophthalmology 124, 1612–1620 (2017).
  4. Newkirk, M. R., Gardiner, S. K., Demirel, S. & Johnson, C. A. Assessment of False Positives with the Humphrey Field Analyzer II Perimeter with the SITA Algorithm. Invest Ophth Vis Sci 47, 4632–4637 (2006).
  5. Montolio, F. G. J., Wesselink, C., Gordijn, M. & Jansonius, N. M. Factors That Influence Standard Automated Perimetry Test Results in Glaucoma: Test Reliability, Technician Experience, Time of Day, and SeasonFactors That Influence Perimetry Test Results. Invest Ophth Vis Sci 53, 7010–7017 (2012).
  6. Lee, M., Zulauf, M. & Caprioli, J. The Influence of Patient Reliability on Visual Field Outcome. Am J Ophthalmol 117, 756–761 (1994).

 

By,
Qing Wang, MD, PhD
Jithin Yohannan, MD, MPH

Wilmer Eye Institute
Glaucoma Center of Excellence
Johns Hopkins University
Baltimore, MD, USA

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