Red-dot card test of the paracentral field as a screening test for optic nerve disease in onchocerciasis.

A new screening test for optic nerve pathology is described, consisting of a series of four red targets presented at an angle of 12 degrees in the paracentral field above and below the horizontal meridian. Nonperception and desaturation of the targets are recorded. Inter-observer variability studies found a kappa value = 0.8. A total of 6831 individuals aged > or = 5 years in communities that were mesoendemic for savanna onchocerciasis in Kaduna State, northern Nigeria, were screened using the test. Of the participants 22% were unable to complete the test; almost two-thirds of these (62%) were aged 5-8 years. After exclusion of those visually impaired or blind according to WHO criteria and those unable to complete the test, the test showed a sensitivity of 40% and a specificity of 98% for optic nerve disease when inability to visualize one or more targets was used as the definition of test failure. The sensitivity increased to 54% with a specificity of 96% when the criterion for failure included desaturation of one or more targets. These values compare favourably with those for other available screening methods. The test took 1-2 minutes to perform and was readily accepted by patients and nurses.