Detecting glaucoma progression is challenging, and now a study suggests that two commonly used OCT metrics—retinal nerve fiber layer (RNFL) and retinal ganglion cell (RGL) plus inner plexiform layer thicknesses—have inherent flaws that miss early glaucomatous damage that’s otherwise visible on probability/deviation maps.

The researchers included eyes from 76 patients and 28 controls who had at least two OCT scans at least one year apart. They reported that all eyes had a 24-2 mean deviation ≥-6dB.

OCT identified 31 of the 76 patient eyes as progressors and the rest of the study participants as non-progressors. RNFL and RGC plus inner plexiform layer thickness measurements showed false negatives 48% and 29% of the time, respectively. These two OCT global measures also falsely identified (false positives) 22.2% of the 45 non-progressor glaucomatous eyes and 25% of the 28 controls as progressing. The researchers identified segmentation and centering issues in addition to local damage as the causes of the errors.

The researchers concluded that OCT global metrics turn up false positives and negatives because of inherent segmentation and centering problems in the scanning. These scans may also miss local damage, resulting in false negatives. “These problems are difficult, if not impossible, to correct and raise concerns about the advisability of using global circumpapillary RNFL and global RGC plus inner plexiform layer for detecting progression,” they concluded in their paper.