A visascreen is needed for those in the medical field (medical technologists, nurses, physiotherapists, etc,). You basically have to present your credentials, schooling, licenses, etc. to prove you have the qualifications to do the job in the U.S. An agency in the US (usually the CGFNS will evaluate everything and issue you with the visascreen certificate valid for 5 years if you qualify). You also have to take a English proficiency exam if from a country whose instruction is not in English.You must have the certificate to apply for Adjustment of Status and for H-1B and soon Trade Nafta Visas also.