• This prospective study found that retinal nerve fiber layer (RNFL) thickness measurements by Cirrus SD-OCT and the Heidelberg retinal tomograph 3 (HRT3) were strongly correlated but significantly different.

    Participants included 85 glaucoma and 88 normal subjects. One eye from each individual was selected randomly for imaging by SD-OCT and HRT3. The investigators took measurements at the four quadrants around the optic disc.

    Although well-correlated, the measurement differences were substantial and proportional to RNFL thickness. HRT3 measurements were significantly larger than Cirrus measurements. The agreement of categorical classification ranged between poor and fair.

    The authors write that three reasons may explain these results. First, HRT3 and Cirrus OCT measure the RNFL differently. Cirrus OCT measures RNFL more directly by identifying the highly reflective layer of tissue on the retinal surface, while HRT3 estimates the RNFL thickness, assuming that the outer limit of this layer begins at a certain depth (reference plane) arbitrarily located 50 μm below a narrow sector of the parapapillary temporal retina.

    Second, the place at which the RNFL is measured varies between instruments. For the HRT3, the precise location depends on the operator’s placement of the contour line at the optic disc edge after image acquisition, whereas the Cirrus OCT measures RNFL at a circumference of 3.4 mm in diameter centered on the disc.

    Finally, the two instruments use different classification systems. The ethnicity breakdown of the Cirrus RNFL normative database includes Caucasians, Asians, African-Americans, Hispanics, Indians and people of mixed ethnicity, while the HRT3 software database only includes Caucasians, Africans and Indians.

    They conclude that the normative diagnostic classification of the two technologies may not agree. Further research is required.