I have sent certified letters to employer,mro,and lab about error in testing. What is my next step? Do I need to hire a lawyer?

You will need a lawyer... because the results of the test will stand unless you can provide irrefutable proof it was in error.

Plus you stand to lose too much to wing it on your own.

Unless you have ironclad evidence concerning the error, yes, hire a lawyer as this most likely affects your livelihood big time.
Was it a procedural error that you can prove with documents?