This is an important question and can, in fact, make the difference between rapid diagnosis and no diagnosis. If insufficient or incorrect specimens are received, even the best effort on the part of the diagnostic laboratory cannot produce a diagnosis. Talk to the laboratory veterinarian who will be handling the specimen when it arrives and solicit his or her suggestions regarding type and quantity of materials to send. This is also an opportunity to communicate first-hand important details of the history, clinical signs, and other pertinent information you will be including on the submission form. In the case of abortions, the fetal membranes are also necessary for diagnosis as they may contain the only clues as to the cause.
Time is the enemy in the case of a dead animal.
Natural processes begin to degrade the body tissues at the instant of death and progress relentlessly with time. Eventually, if too much time passes, the tissue is useless as diagnostic material. Not only are lesions obscured with time, bacteria in the gut of the animal begin to proliferate after death, leave the intestinal tract, and invade other organs. In fact, they are important contributors to the process of postmortem decomposition of the body. These postmortem invaders complicate interpretation of bacterial cultures, which are undertaken to identify the cause of the infectious diseases, such as pneumonia or diarrhea.