Assigning a screening diagnosis when you don't know why a test was ordered may get your claims paid but it is not correct coding. In addition, I would recommend again this practice because it could cause a payment error or incorrect patient responsibility for plans that do not cover this as screening benefit if in fact the test was not actually a screening.
All diagnostic tests, as a requirement for coverage, must have an order and a rationale documented in the patient's record. If the organization sending these to you does not provide that information, the correct fix, though perhaps not always practical or easy, would be to work with them to get a process set up to get that information to you or give you access to the patient's records so that you can look this up.