Postmortem examination provides great opportunities for studying disease process and correlating clinical problems to pathologic lesions in vivo. In recent years, there has been tremendous improvement in imaging technology; however, postmortem examination is arguably still considered to be the most efficient and cost effective technique of investigating disease pathogenesis. Below are some important tips for performing a necropsy in your own clinic, as well as for preparing and submitting specimens to a veterinary diagnostic laboratory.
For best results, the necropsy should be performed immediately after the animal expires. If it is impossible to perform the necropsy immediately, keep the carcass in a refrigerator or on ice in a cooler to slow postmortem decomposition. Samples refrigerated for up to 2–3 days may still be used for routine diagnostic purposes.
Do not freeze the carcass unless you have to wait for necropsy longer than 2–3 days. Freezing artifact significantly hampers microscopic evaluation. However, if a viral or toxic etiology is strongly suspected, freezing the carcass would be more beneficial than keeping it in a refrigerator for several days as laboratory testing will likely be more fruitful than microscopic examination due to post-mortem decomposition.
Always keep biosecurity measures in mind. Protect yourself with adequate personal protective equipment and secure the area where the necropsy is being performed. Disinfect all tools and surfaces after the necropsy and be sure to properly dispose of biological waste according to local laws and regulations.
Record all findings, especially on a possible legal case, and make a complete necropsy report. Include identifying marks such as tattoos, tags, and coat colors. Take plenty of photos before and during the necropsy. You can make your own necropsy report form; however, the following essential data should be included:
Animal's ID (name, tag number, tattoo, microchip number)
Owner's ID (name, address, phone number)
Species, breed, sex, age, weight
Date and time of death
Died/euthanized (method of euthanasia)
Clinical history (medical record can be attached)
Laboratory findings (if any)
Presumptive gross diagnosis
When describing lesions, you should be as accurate as possible concerning the location, size, color, shape, number or extent (% of the organ affected), and consistency. Do not use definitive diagnostic words such as hemorrhage, inflammation, edema, pneumonia, etc. It is better to use conventional, objective, non-interpretive language such as 'orange-colored', 'cauliflower-like', 'creamy', 'nutmeg-like', etc. Interpretation of gross lesions requires considerable experience and training. Therefore, incorrect use of definitive diagnostic terms in the necropsy report could result in misdiagnosis.
Be consistent with your necropsy procedure every time. There are several different ways to dissect the carcass. For example, some schools prefer to start with the respiratory system, but some schools examine digestive system first. Find the most effective method for yourself and stick with it. Complete the examination on one system and then move onto the next system. No matter the order in which you perform the necropsy, if you perform it the same way every time, you decrease your chances of skipping important organ systems.
Weigh and measure important organs, especially the liver and heart.
Open the entire lengths of the small and large intestines, particularly if you suspect gastrointestinal disease. Beware that intestinal wall of small animals is thick, especially in cats. Therefore, luminal changes often cannot be observed through the intestinal wall.
Be familiar with common, non-lesional postmortem changes:
Hypostatic congestion (livor mortis): Blood pools by gravity on the down-side of the animal
Hemoglobin imbibition: Dull-red discoloration of tissue due to lysis of red blood cells
Bile imbibition: Yellow to green discoloration of tissue adjacent to the gallbladder
Pseudomelanosis: Red-tan to gray or nearly black discoloration of tissue due to autolysis
Putrefaction: Severe softening and accumulation of gas in the tissues due to autolysis
Tissue sampling and submission for histopathology:
10% neutral buffered formalin (10:1 formalin to tissue ratio)
Less than 0.5cm in thickness
Use appropriately sealed containers. Check for leaks before shipping.
Fill out a submission form with a complete, detailed history including differential diagnoses and clinical concerns regarding the patient's disease
Include photos, clinical records, or laboratory results, if possible
Tissue sampling for toxicology:
Freeze 6 specimens: liver, kidney, stomach contents, urine, body fat, and brain
Freeze food and water specimens
Tissue sampling for bacteriology:
Use a sterile swab if the specimen is fluid, an abscess, or necrotic tissue on the surface of an organ
Submitting a sample of fresh tissue (no smaller than 2x2x2cm) is acceptable and, in some cases, preferred.
Do not freeze
Tissue sampling for molecular biology (PCR):
Take samples from the lesion itself (no random sampling)
If multiple specimens from different organs are collected, use two bags: one for gastrointestinal samples and the other for non-gastrointestinal samples. Intestinal specimens are processed separately in many labs because intestinal enzymes often inhibit the routine PCR reaction.