Many people are confused about the purpose of the Healthcare Worker Certificate (a.k.a VisaScreen) and the need to keep it valid. The Healthcare Worker Certificate is a requirement of entry into the US. The purpose of the Healthcare Worker Certificate is to make sure your education is at least equal to a US education in your field; that you can read and write English; and that you have never had a professional license taken from you for disciplinary reasons.
If you are entering the US to work in one of the positions which require a Healthcare Worker Certificate, you must have a valid certificate each time you enter the US, or each time you file an extension of your current work visa, or when you file an application to switch from a temporary work visa to a green card. Healthcare Worker Certificates are valid for five years from the date on which they were issued.
Interestingly, you do not have to have a valid Healthcare Worker Certificate to remain in the US once you have entered on a temporary work visa or green card. You only have to have a valid Healthcare Worker Certificate when you enter, change or extend status, etc. Your Healthcare Worker Certificate could technically be valid for only one day after you entered, and you would be fine without renewing it until it was time to enter again, or extend/change status, etc.
That said, renewing it is a good idea. You never know when you will need to travel on a n emergency basis. Also, you should renew it within a year after it expires because the agency that issued it to you will destroy all of your documents if a year passes without it being renewed, which will require a whole lot more work for you, getting your educational documents, etc. resubmitted when the time does come to renew the Healthcare Worker Certificate.