Diagnostic Ultrasound
Limited evidence supports not routinely using ultrasound for the diagnosis of carpal tunnel syndrome.

There were five high quality (Naranjo, 2007; Moran, 2009; Ziswiler, 2005; Wong, 2004; Claes, 2013) and seven moderate quality studies (Abdel Ghaffar, 2012; Dejaco, 2013; Fowler, 2014; Hashemi, 2009; Moghtaderi, 2012; Nakamichi, 2002; Pastare, 2009) evaluating ultrasound for the diagnosis of CTS compared with EDS as the reference standard. These studies showed conflicting results regarding the utility of ultrasound (US) as either a rule in or rule out test in the diagnosis of CTS. In general, there was variation between the studies for the cut-off value for making the diagnosis or for exclusion of CTS. The ideal location for measuring the cross-sectional area (CSA) of the median nerve for indicating the diagnosis of CTS also varied between studies.  There is a general agreement that a CSA greater than 12-13 mm is strongly correlated with EDS.  As a rule out study for CTS, there is a strong correlation with CSA below 8 mm. One moderate quality (Abdel Ghaffar, 2012) and one low quality study (Mallouhi, 2006) suggest that a US measurement of nerve hypervascularity may have a strong association as a rule out study for CTS.