In many cases the cause of death is evident at the time of autopsy. In these cases the death certificate is generated immediately. In other cases the cause of death may not be apparent from anatomic (grossly visible) findings. In these instances the pathologist must await additional studies. For example, in some cases the pathologist may realize that the person has an infection (pneumonia for example) but he may wait for the results of cultures to find which microorganism(s) is involved. In other instances drug toxicity may be suspected and so the pathologist must await results from the toxicology laboratory. In some instances, the cause of death may be readily apparent at the time of autopsy, but the manner of death is not known. An example of the latter is a person dying in a house fire from smoke inhalation and burns. We can see soot in the airway, and we can see burns. However, we cannot tell from autopsy what started the fire. That answer must come from the arson investigation (which is outside the purview of the medical examiner). As manner of death (homicide, suicide or accident) will then hinge on that determination, we must await conclusion of the investigation. Other examples of the latter are head injuries where nobody saw the traumatic incident, and some close range gunshot wounds (where gunshot residue analysis, range determination, firearms examination, and police investigation may all be necessary to arrive at a determination between suicide, homicide or accident).
The need for toxicology studies is one of the most frequent reasons for delay in death certificate completion. Toxicology in the medical examiner setting is different than drug testing that is done in the hospitals. Hospitals will simply do screening tests for types and categories of drugs ("opiates" for example). Because clinicians have the advantage of a living patient with symptoms to evaluate, they do not need to know quantities (if a sedative toxicity is suspected from clinical symptoms and opiates are detected on a drug screen, then the person is treated for opiate toxicity). The medical examiner however will need to know which specific drug(s) within the drug class or category (for example, whether "opiates" is morphine, heroin, hydrocodone, etc) are involved. Also, the medical examiner will need to know the quantity of those drugs in multiple biological samples (blood, liver, gastric contents, etc) in order to put the death into perspective. In other words, postmortem levels must be interpreted in light of tissue distribution/redistribution of drugs, gastric quantity and so forth in order to know 1) is this a toxic level? and 2) is this an accident or suicide? Furthermore, in order for accredited forensic toxicology laboratory results to be considered legally valid the results must be confirmed by two types of tests. Toxicology tests will take longer if multiple drugs are involved, if unusual drugs are involved, or if the person is decomposed.
Finally, one must consider the possibility that the pathologist's initial suspicions are not confirmed. For example, if the pathologist suspects drug toxicity but the initial toxicology report reveals only low levels of drugs or no drugs at all. In these cases, the pathologist may submit additional tissues to reevaluate specific parts of the heart muscle or brain, while at the same time consulting with the toxicologist to pursue testing for more unusual or exotic drugs that weren't originally analyzed, while at the same time asking the police to return to the scene of death to ascertain more information, or perhaps to interview additional witnesses. Each death is different, and the findings in each case are different. Additional follow-up is determined as results are obtained. Obviously in such multi-step investigations the process may be lengthy, and the requisite time necessary may not be entirely under the control of the pathologist.