Having established the importance of visual fields assessment, most readers regularly undertaking the procedure, by whatever instrument, will also be aware of its limitations. Modern semi-automated techniques include reliability indices which may accurately measure how well a patient is maintaining fixation throughout a test, count the number of times a stimulus is falsely reported (a false positive) and the number of times a previously seen stimulus is now apparently missed (a false negative) as some way of overcoming user error due to lost attention or over-enthusiastic response. However, one fact has as yet not been fully addressed. Fields assessment is a subjective test and, as such, is only as reliable as the patient responding. This is the reason why one should never refer on the basis of one field assessment. A field defect is only considered significant if it is repeatable. Furthermore, there are some patients for whom fields assessment is not suitable. The obvious example would be the very young (most practitioners consider eight years the starting age for possible fields assessment, though obviously children's ability varies) but there are also challenges in assessing those with a learning or developmental impairment. The very elderly may also find a more prolonged test (full threshold testing is appropriate for those at risk of early field loss and this is a longer and more challenging test) and the result is often a loss of specificity (too many normals show apparent defects due to loss of attention or confused response) or an excessive use of valuable clinic time, compounded if repeat testing is needed.
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