Chapter 26 Rapid Dark Adaptation Test

Introduction

The earliest sign of vitamin A deficiency is a decrease in dark adaptation, or poor night vision. Serum retinol levels are not sensitive enough to reliably identify mild deficiency states. A dark adaptation test has the advantage of more reliably identifying marginal vitamin A deficiency; however, classic dark adaptation testing is a cumbersome and time-consuming process (usually taking 45 minutes or longer).

A rapid dark adaptation test (RDAT) was described by Thornton ¹ and evaluated by Vinton and Russell.² This rapid (6 minutes) test has significant clinical utility. The basis for the test is the measurement of the time of the so-called Purkinje shift. This term refers to the shifting of peak retinal wavelength sensitivity from the red toward the blue end of the visual spectrum during the transition from day (cone-mediated) vision to night (rod-mediated) vision. When color vision is nonfunctional, this shift causes the intensity, not the color, of blue to appear brighter than red under dim lighting.