The type of test should be determined by the employer's policy and/or applicable regulation. If this is a program that falls under DOT regulations, or that purports to fall under DOT regulations, the only acceptable specimen for drug tests is urine. IF the donor (A) is selected for a random DOT test, (B) fails to provides a specimen, and (C) is determined to have a medical explanation for inability to provide a specimen, [i]THEN[i] that random DOT drug test result is deemed "cancelled" and the donor is not retested nor is the donor removed from the random pool.
If the employer wishes to test that person using a different specimen, and if it is not prohibited by law (e.g., DOT or state law), then the employer can likely go ahead and do a hair test or oral fluid test w/o much risk. Blood tests are more expensive, invasive, and complicated.
While some laboratories and training vendors offer to certify drug test specimen collectors as competent, there is no required "certification" for such tasks, nor is there consensus on how certification is achieved and what it means. As with many things in drug testing (and life), if you want someone competent, look for someone who has experience.