Is processing speed a valid cognitive endophenotype for bipolar disorder?

Objectives: The current study investigated whether a single brief cognitive assessment, processing speed, could be considered as a valid endophenotype for bipolar disorder (BD). Methods: Processing speed was assessed using the Digit Symbol Test (DST) in 53 euthymic BD probands (BD-P), 50 unaffected first-degree relatives (UFDR) and 60 unrelated healthy controls (HC). Results: Euthymic BD-P and the UFDR were significantly more impaired on DST performance even after controlling for demography and current mood symptoms (effect sizes 0.89 and 0.52). Clinically significant performance impairment was present in about 30% BD-P and 25% UFDR. Limitations: Pharmacotherapy was not controlled for. Conclusions: Processing speed, as measured with the DST, is a brief reliable measure that could be used in clinical assessments of at risk populations. Our findings support the hypothesis that processing speed may be a valid endophenotype, highly specific for differentiating both euthymic BD-P and UFDR, from HC. (C) 2012 Elsevier B.V. All rights reserved.