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Objective: Performance and symptom validity tests (PVTs and SVTs) measure the credibility of the assessment results. Cognitive impairment and apathy potentially interfere with validity test performance and may thus lead to an incorrect (i.e., false-positive) classification of the patient's scores as non-credible. The study aimed at examining the false-positive rate of three validity tests in patients with cognitive impairment and apathy. Methods: A cross-sectional, comparative study was performed in 56 patients with dementia, 41 patients with mild cognitive impairment, and 41 patients with Parkinson's disease. Two PVTs – the Test of Memory Malingering (TOMM) and the Dot Counting Test (DCT) – and one SVT – the Structured Inventory of Malingered Symptomatology (SIMS) – were administered. Apathy was measured with the Apathy Evaluation Scale, and severity of cognitive impairment with the Mini Mental State Examination. Results: The failure rate was 13.7% for the TOMM, 23.8% for the DCT, and 12.5% for the SIMS. Of the patients with data on all three tests (n = 105), 13.5% failed one test, 2.9% failed two tests, and none failed all three. Failing the PVTs was associated with cognitive impairment, but not with apathy. Failing the SVT was related to apathy, but not to cognitive impairment. Conclusions: In patients with cognitive impairment or apathy, failing one validity test is not uncommon. Validity tests are differentially sensitive to cognitive impairment and apathy. However, the rule that at least two validity tests should be failed to identify non-credibility seemed to ensure a high percentage of correct classification of credibility. [ABSTRACT FROM AUTHOR]