SD-OCT instruments not interchangeable for measuring RNFL thickness

This study in the June issue of the British Journal of Ophthalmology compared parapapillary retinal nerve fiber layer (RNFL) thickness measurements taken using three different spectral-domain OCT (SD-OCT) platforms. The authors found that although measurements using the SD-OCT instruments correlated well, there were considerable differences in the absolute values. They say that the protocol for the scan and analysis software for RNFL measurements might affect the discrepancy, and these instruments should not be used interchangeably.

This study highlights that OCT measurements are to be used in combination with clinical judgment and other evaluation modalities.

The study included 126 subjects with glaucoma and 61 healthy subjects between the ages of 20 and 60 years imaged using Cirrus RTVue and 3D OCT.

Measurement with Cirrus showed significantly smaller thickness values than with RTVue (difference = 8.8 µm, P < 0.0001) and 3D OCT (difference = 8.1 µm, P < 0.0001). However, there was no difference in average RNFL thickness measurements between RTVue and 3D OCT (difference = 0.7 µm, P = 0.568). The nasal quadrant had a lower correlation than the other quadrants.

Cirrus measurements were statistically smaller in all quadrants compared with 3D OCT measurements and were smaller in all but the nasal quadrant compared with RTVue measurements. RTVue thickness measurements had higher values in the inferior quadrant and lower values in the nasal quadrant compared with the 3D OCT measurements. Also RTVue had a smaller occupied proportion of nasal quadrants (30.2 percent) and a larger proportion of inferior quadrants (32.4 percent) compared with Cirrus and 3D OCT.

The authors note that the measurement circle of RTVue might be more superior-temporally located compared with the other instruments. To identify which OCT instrument reflects the most accurate measurement of RNFL thickness, further studies combined with histological methods are needed.