To assess the accuracy of a test, its outcome must be compared with an independently established gold standard test. The gold standard test is the time honoured diagnostic test that is considered to be the current standard or definitive test for the disease in question. An ideal gold standard test has a sensitivity of 100% (it identifies all individuals with the disease) and a specificity of 100% (it does not falsely identify someone with a condition that does not have the condition). In practice, of course, there are no ideal gold standard tests, and one tries to use a test that is as close as possible to the ideal test. Results on sensitivity and specificity of a new test depend critically on how closely the gold standard approaches the ideal state.
Unfortunately the use of the word 'gold' is often misunderstood to mean the 'perfect' or ideal diagnostic test. This misunderstanding is deepened by the use of the term 'golden standard' by some medical researchers. It is exemplified by a letter to the BMJ by Duggan 
who called for the abolition of the term 'gold standard' on the basis that the phrase smacks of dogma. Versi countered this by pointing out that the gold standard is not the perfect test but merely the best available test, and gave the Oxford English Dictionary definition which states that it is a "measure to which others conform or by which the accuracy of others is judged ; a thing serving as a basis for comparison." In fact some medical journals have now discontinued use of the term gold standard and instead insist that the term criterion standard be used instead.
Gold standard tests for some diseases may not be used in the clinical practice of medicine at all because the procedure is too invasive or is only applicable after death. For example, the gold standard test for Alzheimer's requires a biopsy on brain tissue - which can only be carried out post-mortem. Such a test would still, however, be used as the standard against which other tests are assessed. As new diagnostic methods become available, the "gold standard" test may change over time. For instance, there is intensive research underway to come up with a new gold standard test for Alzheimer's and other forms of dementia which may utilize chemical markers of the disease, or imaging from brain scans.
