The Ishihara Test

The Ishihara test for color blindness is a convenient and accurate method to detect total congenital color blindness and red-green blindness by assessing an individual's ability to perceive primary colors and shades of color.

The Ishihin book contains a series of polychromatic plates of primary colored dots arranged to form a numeral against a background of similar dots of contrasting colors (see Figure). Patients with normal color vision are able to read the appropriate numeral; however, patients with color vision defects read the dots either as not forming a number at all or as forming a number different from the one identified by the individual with normal color vision. The fint plate in the Ishihara book is designed to be read correctly by all individuals (those with normal vision and those exhibiting color vision deficiencies) and should be used to explain the procedure to the patient.

The book includes plates with winding colored lines for patients who are unable to read numbers, such as preschoolers and non-English-speaking people. The patient should be asked to trace the line formed by the colored dots using a cotton swab or the eraser end of a pencil. Soiled fingers can, over time, degrade the polychromatic plates and therefore the patient's finger should not be used to do the tracing.

The Ishihara test should be conducted in a quiet room illuminated by natural daylight. If this is not possible, a room lit with electric light may be used; however, the light should be adjusted to resemble the effect of natural daylight as much w possible. Using light ocher than just described, such as bright sunlight, may changc the appcarancc of shades of color on the plates, leading to inaccuratc test results

The medical assistant is responsible for performing the color vision test and recording results in the patient's chart The physician will avsess the results to determine whether the patient has a deficiency in color vision.

The Ishihara test consists of 14 color plates Plates I through 11 are used to conduct the basic test, and plates 12, 13, and 14 are used to further assess patients who exhibit a red-green color deficiency. Therefore it is not necessary to include these plates (12, 13. and 14) in the tesl of patients who exhibit normal color vision. In interpreting the results, if 10 or more plates are read correctly, the patient's color vision is considered normal. If 7 or fewer of the 11 Ishihara plates are read conectly, the patient is identified as having a color vision deficiency. It would be unusual for the medical assistant to obtain results in which the patient read 8 or 9 plates correctly. The test is structured so that a patient with a color vision defect generally does not read 8 or 9 plates correctly and the rest incorrectly.

If a defect in color vision is detected, the patient is referred for additional assessment of color vision to an ophthalmologist or optometrist, who will use more precise color vision tests. The procedure for assessing color vision using the Ishihara color plates is outlined in Procedure 6-2.