The medical assistant's role in vision screening tests
Interpreting snellen results
- The patient always stands 20 feet from the chart.
- Each result is a record of how well the patient can see compared with normal vision.
- Example: A patient with a 20/40 reading can see that line correctly standing at 20 feet, but an individual with normal vision can see the same line correctly at 40 feet, so the patient’s vision is not as acute as someone with normal vision.
- Example: A patient with a 20/15 reading can see that line accurately standing at 20 feet, but a person with normal vision must stand at 15 feet to have the same vision, meaning the patient’s vision is better than someone with normal vision.
Near visual acuity
Near visual acuity can be tested with the near vision acuity chart. This test is given to screen for presbyopia or hyperopia. If the patient wears corrective lenses, they should be worn during the test. The size of the type on the card varies from a 66-point font (newspaper headlines) to a 3-point font (medicine bottle labels). The test should be given in a well-lit room, with the patient holding the card approximately 14 to 16 inches away. As with the Snellen examination, the near visual acuity test is given for each eye, starting with the right eye. The eye not being tested should be covered with an occluder but left open. The patient should be monitored for indications of difficulty, such as squinting or tearing. The patient reads the card, starting at the top until reaching the smallest print that can be read. The medical assistant should document the number at which the patient had no more than two errors for each eye and the two eyes together, whether corrective lenses were worn, and any signs of eyestrain.
Ishihara color vision test
Defects in color vision are classified as congenital or acquired. Congenital defects are caused by an inherited color vision defect and are found most often in males. Acquired defects are caused by eye injury or disease. The Ishihara test is a simple, convenient, and accurate procedure that detects total color blindness, in addition to the red-green blindness prevalent in congenital blindness. The test assesses the perception of primary colors and shades of colors.
The test booklet contains polychromatic plates made of colored dots in numeric patterns. The numbers are one color, and the background dots are a different color. Patients with average visual acuity can read the number within the dot matrix without difficulty. Patients with color vision defects are unable to read the number, or they see a totally different number. A section of plates is included that contains colored line trails through a background of dots. These plates are designed to be used with children and adults who are unable to read numbers. In this situation, the patient uses a finger to follow the dotted trail through the picture.
The test should be administered in a quiet room that is well illuminated by sunlight, not by artificial lighting. If this cannot be done, the best situation possible is created by adjusting lights to resemble the effect of natural daylight. The test uses 14 color plates. The basic test consists of plates 1 through 11. Plates 12 through 14 are used if the patient appears to be having difficulty with red-green differentiation. The medical assistant records the number of plates read correctly. If the score is 10 or higher, the patient is within the average range. If the score is 7 or lower, the patient is suspected of having a color deficiency, and the ophthalmologist performs additional assessment tests using more precise color vision testing equipment.