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 Thornton¹ 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.

Clinical Application

Although dark adaptation testing has the advantage of being an in vivo test, and therefore directly related to function, it is somewhat less specific. Box 26-1 lists conditions that may give abnormal dark adaptation results with a normal vitamin A status.

Procedure

Interpretation

The RDAT time depends largely on the individual setting. However, the difference in the RDAT time between normal and vitamin A deficient individuals is significant, and therefore standardization is easily achieved. The normal values depend on the age of the subject, with older subjects having an increased RDAT time. An increased RDAT time may also indicate a zinc deficiency.