Purpose To evaluate the utility of ring ratios in detecting hydroxychloroquine (HCQ) related retinal toxicity using the 103-hexagon multifocal electroretinogram (mfERG).

Design Retrospective cross-sectional study.

Methods 23 patients taking HCQ were consecutively evaluated for retinal toxicity and divided into those without (HCQ-non-toxic group) and with documented visual field loss (HCQ-toxic group). A control patient group without retinal disease and not on HCQ was used for comparison. 103-hexagon P₁ mfERG amplitude response densities were analysed by averaging the 103 responses into six (age-corrected) concentric rings (R₁–R₆), calculating standard ring ratios (R₁:R₂–R₁:R₆) and R₅ ring ratios (R₅:R₁–R₅:R₆). Receiver operating characteristic curves were used to compare these tests for detecting toxicity.

Results Relative to HCQ-non-toxic and control groups, the HCQ-toxic group showed generalised reduction of the 103-hexagon mfERG absolute responses most prominent in the foveal/pericentral regions. R₅ ring ratios were superior to standard ring ratios in discriminating the HCQ-toxic from the HCQ-non-toxic and control groups and were approximately equivalent to pericentral absolute ring responses in detecting HCQ retinal toxicity by receiver operating characteristic criteria, with R₅:R₄ and R₅:R₃ ratios performing best. However, R₅ ring ratios revealed improved sensitivity over absolute ring responses (89% vs 73%) at a 95% specificity threshold.

Conclusions Ring ratio analysis using the R₅ ring response as the ‘internal reference ring’ appeared equivalent to pericentral absolute ring responses in detecting HCQ retinal toxicity, and possibly superior at clinically desirable specificity thresholds. R₅ ring ratios did not require age correction, a potential clinical advantage over absolute ring responses.