# Study of Diagnostic Accuracy: Fundus Photography vs. Optical Coherence Tomography
## 3 Results
### Diagnostic Accuracy Values and Interobserver Agreement between Ophthalmologists and Optometrists  
To calculate the diagnostic precision of both instruments, FP and OCT, a 2 \(\times\) 2 confusion matrix was created (Appendix A). The reference values, true positives, and true negatives were defined as cases diagnosed by the group of ophthalmologists, which was the gold standard in this study. To calculate the predictive values (sensibility, specificity...) and the Kappa coefficient, we used the 2 \(\times\) 2 confusion matrix with contingency tables (Tables 2 and 3). Figure 7: (**A**) FP non-mydriatic imaging; (**B**) report by OCT 3D macula. For the statistical analysis, the sample was divided into posterior poles without abnormalities (healthy) and posterior poles with abnormalities (including those that are referable and urgently referable). Not evaluable cases were excluded [21; 22; 23]. The sample was reduced from 1334 to 974 in the FP-based assessment and from 1334 to 1287 in the assessment based on OCT and FP due to the poor image quality caused by the opacity of the cornea or of the ocular media, mitotic pupils, or a lack of patient cooperation. A series of statistical measures were used to provide safe and reliable results, with the aim of ensuring that optometrists can provide high-quality and valid assessments in their daily practice. As a result, these methods can be used to confirm the presence or absence of posterior pole abnormalities [24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. Table 4 shows the calculated values of sensitivity, specificity, positive and negative predictive values (PPV, NPV), the likelihood ratios, and the kappa coefficient [27; 28; 29; 30; 31; 32; 33; 34]. The results show that, when comparing the assessment of the optometrist with that of the ophthalmologist (gold standard), based on retinography, the sensitivity is markedly higher than the specificity, with 0.981, CI 0.97-0.99, versus 0.378, CI 0.33-0.43. The same is true of sensitivity and specificity for OCT, but with a less marked difference: 0.956, CI 0.94-0.97, versus 0.527, CI 0.49-0.57. This is due to an increase in false positives in the assessments conducted by the optometrist, that is, the optometrist tends to judge more cases to exhibit alternations when the ophthalmologist does not consider this to be the case. The presence of high sensitivity values compared to the relatively low specificity values is affirmed by the likelihood ratio values. After performing the contrast test to see whether there were differences between the results obtained using FP and those obtained using FP together with OCT, we confirmed that there are statistically significant differences, since the \(p\)-value obtained was 0.01. According to the classification of [PERSON] and [PERSON] when comparing the optometrist and ophthalmologist evaluations in each block, we found that there was fair (0.39, CI: \begin{table}
\begin{tabular}{c c c} \hline \hline
**Variable (C1 95\%)** & **FP** & **OCT + FP** \\ \hline Accuracy & 0.73 (0.70–0.76) & 0.76 (0.74–0.79) \\ Sensitivity \% & 98 (0.97–0.99) & 96 (0.94–0.97) \\ Specificity \% & 38 (0.33–0.43) & 53 (0.49–0.57) \\ PPV & 0.69 (0.66–0.72) & 0.71 (0.69–0.74) \\ NPV & 0.93 (0.88–0.97) & 0.91 (0.87–0.94) \\ Likelihood (+) & 1.58 (1.46–1.70) & 2.02 (1.85–2.20) \\ Likelihood (\(-\)) & 0.05 (0.03–0.09) & 0.08 (0.06–0.12) \\ Kappa coefficient & 0.39 (0.34–0.45) & 0.50 (0.46–0.55) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Summary of diagnostic accuracy results. Diagnostic analysis of non-mydriatic fundus images vs. OCT reports from patients in a retinal screening; comparison between ophthalmologists (gold standard) and optometrists (95% confidence interval: CI 95%). \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Positive** & **Negative** \\ \hline Positive & 561 & 250 \\ Negative & 11 & 152 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Confusion matrix used to define the optometrist’s performance. Comparison based on FP and OCT.