Table 1.

Calculations of sensitivity, specificity and predictive values

Positive
True positive, that is, visual impairment present and referred	74
False negative, that is, visual impairment present but not referred	8
Negative
False positive, that is, visual impairment not present but referred	5
True negative, that is, visual impairment not present and not referred	29
Output
Sensitivity (true positive/true positive+false negative)	90.24% (95% CI 81.68% to 95.69%)
Specificity (true negative/false positive+true negative)	85.29% (95% CI 68.94% to 95.05%)
Positive predictive value (true positive/false positive+true positive)	93.67% (95% CI 86.78% to 97.09%)
Negative predictive value (true negative/false negative+true negative)	78.38% (95% CI 64.91% to 87.66%)