You'd need to check with the laboratory that's going to be doing the test, to find out what sample they want, how they want it collected, and when/how to ship it.
Some labs do IHC(immunohistochemistry) for BVD and require an ear notch - they may or may not prefer them fresh(dry), in saline, or in 10% formalin.
We do the BVD antigen-capture ELISA at the lab where I'm employed - can be done on ear notches(our virology section prefers them 'dry' in individual tubes/whirlpaks/ziplocs, or on serum or whole blood. For calves under 3 months of age, the ear notch is preferred, but above 3 months, PI animals have tons of BVD virus in all body fluids/secretions, and it's easy to pick it up in the serum - and there's less 'contamination' to deal with in that clean serum sample.
Thus far, I've only sampled stillborn or aborted calves, and any cows that might die for whatever reason, but we had a heifer deliver a big full-term fetus, dead, on her due date, that was BVD positive, so...a whole-herd BVD test will be on the calendar here in a couple of weeks when everything gets its Spirovac L5 booster.