The rule of five, commonly used to detect retinal nerve fiber layer (RNFL) changes on OCT, may not be as effective in detecting glaucoma progression as trend-based analysis, a study in Ophthalmology Glaucoma suggests.

The rule of five is a simple formula for identifying RNFL changes on OCT in which a loss of 5μm of global RNFL on a follow-up test is considered evidence of significant change when compared with baseline. The rule is based on short-term test-retest OCT variability and is often used in clinical practice.

The investigation included 300 eyes of 210 glaucoma subjects who were followed for an average of about five years over approximately 11 visits.

The researchers performed trend-based analysis of global RNFL thickness over time. The specificity of the trend-based analysis was matched to the rule of five results to allow meaningful comparison of the “hit rate,” or the proportion of glaucoma eyes categorized as progressing at each time point.

After five years, the simple rule of five identified 37.5% of eyes as progressing at a specificity of 81.1%. At the same specificity, the hit rate for trend-based analysis was significantly greater at 62.9%.

If the rule of five had to be repeatable on a consecutive test, specificity improved to 93.4%, but the hit rate decreased to 21%. At this higher specificity, trend-based analysis still had a significantly greater hit rate at 47.4%.

As a result of these findings, the researchers suggest trend-based analysis should be the preferred method to assess OCT RNFL change over time.