The plus + 1.00 DS is a taught at optometry school and a test used during subjective refraction. This blur check is performed towards the end of subjective refraction to determine if a young patient with accommodative capacity has been over-minused or under-plused, ie accommodating during the course of their refraction. The blur test is usually performed after the best sphere and cylinder have been determined and before any binocular balancing and binocular addition.
The procedure is:
- Occlude one eye the left eye.
- Place a +1.00 DS trial lens over the right eye.
- If the original visual acuity was 6/4 (the average for a young patient) it should be reduced by four lines to about 6/12 with the +1.00 DS lens.
- If the visual acuity is better than 6/12 with the +1.00 DS lens, it suggests the patient has been over-minussed and is accommodating.
- Note, the four-line loss of visual acuity with the +1.00 DS lens is an average and it may be between 2-7 lines.
With such a large variation in visual acuity reduction, the question that comes to my mind, is: Is it worthwhile conducting this test? If the visual acuity only drops by 2 lines is this normal for the patient or have they been over-minused. While this is theoretically interesting and warrants discussion at optometry school such wide variability is clinically confusing and the test should not be included in optometry school clinical curricula.
An excellent research paper, dedicated to what some would consider a mundane topic, highlights the problems with the +1.00 DS blur test and offers good explanations for the variability.
The authors state that the large range of normal values for the +1.00 DS blur test is not due to unreliable measurements, but probably due to variability in ocular optics or perceptual interpretation of reduced quality retinal images. These vary from person to person. Some people are just better at interpreting blurred letters than others.
In addition, larger reductions in visual acuity with the +1.00 DS test appeared to be partly due to large pupils, while smaller reductions in VA are associated with small pupils.
Also, average visual acuity levels with a +1.00 DS blur depend on the original visual acuity. If the original visual acuity is about 6/4, then visual acuity will blur to about 6/12+ with a +1.00 DS. However, note that 6/9 or even 6/6 visual acuity with the +1.00 DS may not necessarily indicate that the patient has been over-minused or under-plussed. Such values would, however, indicate that the spherical endpoint of the refractive correction should be rechecked.
There are two many factors affecting the results from the +1.00 DS blur test and there are all beyond the control of the practitioner. It’s a poor test that is, in my experience, used by only a few practitioners. It should be dropped from the curricula used at optometry school.