[A stochastic test for validity control of visual acuity statements]

When a patient's statement of reduced visual acuity (VA) does not correspond to the ophthalmologic findings, the validity of the statement should be checked before intensive diagnostics are initiated, because the statement may be untrue or may overstate a factual reduction in VA. A primary question is whether the patient's statement differs from his or her true VA, and if so, by how much. Especially in examinations for medical opinion, individuals may be well informed on how to check the validity of VA statements. Thus, modifications of standard test strategies are required. We examined a new test that was designed to allow probabilistic evaluation.

The optotype board has the form of a poster measuring 150 cm in height and 125 cm in width. It contains 16 lines corresponding to the logarithmic VA levels from 0.12 up to 4.0 at a distance of 5 m. Each line consists of nine Landolt-Cs with four alternative orientations. VA was determined by a 1/1 criterion for each single measurement at each of the nine optotype columns. In cases of low vision (VA <0.25), the test distance was reduced accordingly. The test was given to 100 patients (VA 0.012-2.75), 20 healthy subjects (VA >1.0) who were requested to simulate a VA of 0.1, and 13 individuals who were examined for medical opinion, nine of whom were suspected (by results of other testing) of stating nonorganic visual deterioration. The mean and the individual distribution of the nine single values were analyzed and compared with the distribution within a typical psychometric function as determined by a Monte Carlo simulation on 100,000 single measurements.

None of the 100 normally cooperative patients yielded more than seven idential single values, and only one yielded seven. The single values scattered by one to three lines. Of the 20 pseudomalingerers (examined at a distance of 2.5 m), in seven subjects (35%) we found an atypical pattern of single values (seven or nine identical values). VA results of those 10 subjects who could read the calibration column on the board (reference distance 5 m) scattered around 0.06. Results of those 10 subjects who were examined after the calibration column had been replaced with lower values (reduced by a factor of 4) scattered around 0.13. Of the nine individuals examined for medical opinion who were suspect of stating too low a VA, six (67%) showed an atypical distribution of single values.

This convenient test is a practical tool both for regular determination of visual acuity and for detection of false statements by means of an atypical psychometric function. It also allows for application of further validity checks. Intentionally false statements can be influenced by misleading calibration values.