Generally speaking, NIDA labs are accurate. Despite what you might hear on the net, urinalysis, if done correctly, is a very accurate scientific procedure. NIDA keeps it's labs in check by sending positive and negative double-blind samples. If the lab results are wrong, NIDA may take away the labs certification. Some labs use cheap alternative methods to reduce expenses.
Many human errors occur in labs and cause inaccurate results.
Some are careless or irresponsible errors, and some errors are accidents. The only lab you should be concerned with is the one that is testing you. Only Federal jobs require NIDA standards. Some employers use the RIA, and some use the hair test. They may or may not supervise the subject.