In general I have to say No, we can not depend that products will show correct if we don´t have some verification or calibration done for the actual product. The manufacture for Medical devices must provide how often we as Healthcare organizations must re-calibrate(not adjust) and verify the actual product.  Regarding consumer products that is used for follow up on personal medical data, for example, blood pressure, bioelectric impedance scales, and others, In my opinion I think the Healthcare organizations must guide the consumer if they can depend or not depend on this kind of Equipments. Pls. see my abstract from the World Congress on Medical Physics and  Biomedical Engineering in Beijing, 2012.

Recalibration of devices needs to be standardized. Though the device could be FDA/CE certified, many a times we end up with wrong readouts from these devices. This could be checked by verifying the readouts against known standardized parameters. Also, in the device manual, we need to include frequency of calibration., ie., once every 50 measurements or so, which is dependent on the field of study. In general the answer to your question is YES, but it depends heavily on the age and quality of make of the device. 

To my opinion, the "sign" of answer "YES" or "NOT" determines by aims of diagnostics. In case of medical service the requirements is very high and, correspondingly, they must be supported by regulatory approval and recalibration. In research practice (f.e. in biomechanics) some devices mentioned above may be useful because allow us to obtain prelimenary but valuable information.  

thank you for all above answers but  i am some non-profit organization are using digital devices in rural areas for assessing vital signs and BP  and eye examination so how these data useful in diagnosis 

Some times the medical devices gives just funny results from actual. Then how can we blindly depend on the device?