Wellness health screenings: Are they legal and can you base premiums on the results?

Q. We are considering a wellness/preventive care program for our employees. We are looking at offering a health test and blood testing. The company would receive an aggregate risk assessment from the results, allowing us to put wellness programs together in our risk area. Are there any legal concerns in requiring employees to go for the testing? If an employee didn't want to participate, can we charge them a percentage of their medical premium for not participating? – Peggy, Idaho