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Performance validity tests (PVTs) are not widely used beyond medico-legal contexts in the UK. A UK survey suggests clinicians have reservations about their accuracy in clinical settings. This study sought to explore the validity of PVTs in an acute adult neuropsychology setting and to establish a potential “false positive” (FP) base rate. Failures on the Medical Symptom Validity Test (MSVT) in a consecutive clinical series of 405 patients were evaluated systematically and allocated to groups depending on clinical context. All failures were checked against the test’s “dementia profile”. Of the 405 participants, 329 passed the MSVT (81.2%), while 76 participants (18.8%) failed based on standard criteria. A 5.2% rate of potentially ‘unexplained’ failures was found. Other reasons for failure were classified as:presumed malingered neurocognitive dysfunction(4.6%),dementia/significant cognitive impairment(3.7%),technical/visual problems(1.8%), and“unexplained failure” with contributory factors(2.4%).These results suggest test specificity between 0.95 and 0.90. Most of the clinically significantly impaired patients matched the dementia profile (86.7%). Our results support the sensitivity, but not the specificity, of the dementia profile. However, approximately 1 in 20 patients failed the MSVT despite an otherwise unremarkable neuropsychological presentation; moreover, mood and pain may affect MSVT performance. Clinical implications for interpreting test scores are discussed. [ABSTRACT FROM AUTHOR]