Necropsy Submissions: How to Maximize Diagnostic Yield
Tips for Submission · All species
A necropsy is only as informative as the tissue you submit. The pathologist works with what arrives in the container — and by the time tissue reaches the laboratory, decisions you made in the first minutes after death have already determined how much diagnostic information remains recoverable. This post is a practical guide to getting the most out of every necropsy submission, written for the veterinarian performing the procedure.
The first decision: refrigerate or fix?
The single most consequential decision in necropsy submission is how you handle the body or tissues in the interval between death and laboratory receipt. Autolysis begins immediately after death and proceeds faster than most clinicians appreciate — particularly in small animals, animals that died in warm environments, and animals with GI rupture or sepsis, where endogenous bacteria accelerate tissue breakdown from within.
The rule is straightforward: if you are submitting fresh tissue for histopathology, refrigerate — do not freeze — as quickly as possible. Freezing damages cellular architecture through ice crystal formation and significantly degrades tissue quality for routine histopathology — though freeze artifact is preferable to advanced autolysis when those are the only two options. Refrigeration slows autolysis meaningfully and buys time without compromising morphology. For whole small animal carcasses, refrigeration within one to two hours of death preserves diagnostic quality significantly better than room temperature storage, even if the carcass is submitted the same day. To accelerate cooling, wetting down the external surface of the carcass before refrigeration helps dissipate body heat more quickly.
Formalin fixation is the alternative for tissues you are collecting yourself rather than shipping a whole carcass. Fixed tissue is stable at room temperature, does not require refrigeration during transit, and provides the best morphologic preservation for histopathology. The tradeoff is that once tissue is fixed, most ancillary testing — culture, PCR on fresh tissue, flow cytometry, electron microscopy — is no longer possible. This makes the fix-or-fresh decision consequential when the diagnostic question is broad or when infectious etiology is on the differential list.
Whole carcass vs. tissue selection: what to submit
For small patients — cats, small dogs, rabbits, rodents, birds, reptiles — submitting the intact refrigerated carcass is generally preferable to selective tissue collection. It allows for a complete systematic examination, observation of gross lesion distribution, and sampling of tissues based on what is actually present rather than what was anticipated. If you are uncertain about technique or tissue selection at any point during the procedure, a brief call or video consultation before or during the necropsy is worth more than a follow-up question after tissues have already been fixed. Unexpected findings are common at necropsy, and the tissue that answers the diagnostic question is not always the tissue that seemed most relevant before the procedure.
For large patients where whole carcass submission is impractical, tissue selection becomes critical. The minimum tissue set for a complete histopathologic survey in a dog or cat should include: brain (at minimum one coronal section through the cerebrum and a section of cerebellum), spinal cord if neurologic signs were present, heart (transverse section through both ventricles and the interventricular septum), lung (multiple lobes, including any grossly abnormal areas), liver (multiple sections including portal areas), kidney (cortex and medulla from both kidneys if possible), spleen, lymph node (regional to the primary lesion if applicable, plus a mesenteric node), stomach and small intestine (full thickness), large intestine, adrenal glands, and any grossly abnormal tissue regardless of suspected relevance.
The adrenal glands deserve particular mention. They are small, easy to overlook, and diagnostically important in a wide range of systemic diseases — from Addison's disease to lymphoma infiltration to adrenocortical tumors. Assess them grossly before deciding whether to submit: small, uniform, unremarkable adrenal glands in an otherwise well-sampled case may not require submission. However, if they appear enlarged, irregular in shape, hemorrhagic, or discolored beyond what autolysis would explain, submit them without hesitation.
Gross examination before you cut: what to document
Before making any incisions, take two to three minutes to document the external examination. Body condition score, evidence of trauma, mucosal color, hydration status, presence of icterus, and the condition of the coat or feathers are all findings that the pathologist cannot assess once the carcass has been opened. Photograph the intact animal from multiple angles if possible — a phone photograph takes seconds and is far more informative than a written description.
As you open the body cavities, note and photograph the volume and character of any free fluid — pleural effusion, pericardial effusion, ascites — before suctioning or disturbing it. Record the volume, or at minimum an estimate if precise measurement is not practical. The appearance of the serosal surfaces, the position and relationship of organs, and the presence of adhesions or masses are observations that cannot be reconstructed from tissue sections. Write these findings down in real time rather than from memory.
For each organ, note size relative to expected, color, texture, and any focal lesions — their number, distribution, size, color, and relationship to surrounding tissue. A liver that is diffusely pale yellow tells a different story than one with multifocal white nodules, and the pathologist interpreting the histologic sections will use your gross description to contextualize what they see under the microscope.
Tissue collection: size, orientation, and handling
Tissue sections submitted for histopathology should be approximately 0.5 to 1 cm in thickness — thin enough to allow adequate formalin penetration within the standard fixation window, but thick enough to provide representative tissue depth. Sections that are too thick fix incompletely at the center, producing autolytic artifact in the interior of the section even when the surface appears well-preserved.
For paired organs — kidneys, adrenal glands, lungs — sample both sides whenever possible. Unilateral lesions are common in renal, adrenal, and pulmonary disease, and submitting only one kidney from an animal with unilateral renal carcinoma produces a report that may read as unremarkable on the submitted tissue — that is, if the submitted tissue itself contains no abnormalities.
Handle tissue with the least traumatic instrument available. Serrated forceps crush tissue at the contact point and produce artifact that can obscure or mimic lesions, particularly in lymph nodes, spleen, and intestinal mucosa. Use smooth-tipped forceps or handle tissue by its edges. Do not squeeze, stretch, or compress tissue sections before or after placing them in formalin.
Intestinal segments should be opened along the mesenteric border and pinned flat on a piece of cardboard or foam before immersion in formalin. Unfixed intestinal rolls are one of the most common sources of inadequate sections in necropsy submissions — the tissue scrolls on itself during fixation, the mucosa is compressed against the serosa, and meaningful histologic assessment of mucosal architecture becomes impossible. Two minutes of pinning prevents this entirely.
Brain should be fixed whole whenever possible rather than sectioned fresh. Fresh brain is extremely fragile and sections poorly — the gyri collapse, the tissue tears, and sectioning artifact is introduced that mimics or obscures lesions. A whole brain in an adequate volume of formalin (ten parts formalin to one part tissue by volume) fixes uniformly and sections cleanly after 48 to 72 hours. If shipping time means the brain will be in formalin for longer before processing, that is preferable to fresh sectioning.
Fresh tissue for ancillary testing: what to collect and when
The decision to collect fresh tissue should be made before you begin the necropsy, not after the formalin container is already open. Once tissue has been placed in formalin, it cannot be used for bacterial or fungal culture, fresh tissue PCR, toxicology, or electron microscopy. If any of these tests are on your differential list, collect fresh samples first.
For infectious disease workup, collect fresh lung, liver, spleen, and kidney in sterile containers for culture. Brain should be collected fresh for PCR if encephalitis or viral neurologic disease is suspected. Intestinal content — not just intestinal wall tissue — should be collected in a separate sterile container if enteric infectious disease is the primary question.
For toxicology, the tissues and samples of interest depend on the suspected toxin but generally include: liver (the primary site of toxin metabolism), kidney (renal toxins), stomach content (ingested toxins), and whole blood or serum collected before death if available. If heavy metal toxicity is a concern, bone — particularly rib — is the appropriate tissue for lead and other metals with skeletal deposition. Discuss specific tissue requirements with the laboratory before submission if the suspected toxin is unusual.
Vitreous humor from the eye is a remarkably stable and diagnostically underutilized sample for several parameters — electrolytes, glucose, urea nitrogen, and certain toxins — that remain measurable postmortem far longer than serum values. In cases where antemortem bloodwork was not obtained and metabolic or toxic cause of death is possible, vitreous collection takes seconds and provides information unavailable from any other postmortem source.
Formalin volume and container selection
The ten-to-one formalin-to-tissue ratio is a standard that is frequently violated in practice and consistently matters for tissue quality. Submitting five large tissue sections in a 60 mL container of formalin does not fix tissue adequately — the formalin is rapidly consumed by the tissue and fixation stalls. Use a container large enough to hold the required formalin volume with room for the tissue to be fully immersed without compression.
Separate containers for different body regions — CNS tissues, thoracic organs, abdominal organs, and any grossly abnormal tissue — allows the pathologist to process and section related tissues together and reduces the risk of tissue identification errors. Label each container clearly on the lid and the side — lids become separated, and a label only on the lid is a label waiting to be lost.
The submission history: what the pathologist cannot infer
The necropsy report is only as complete as the clinical context provided. For each submission, include: species, breed, age, sex, and reproductive status; body weight and condition score at death; duration and nature of clinical signs; all treatments administered with doses and duration; vaccination and deworming history where relevant; housing and diet; known or suspected exposures (toxins, infectious contacts, environmental hazards); and your primary differential diagnosis.
If the animal died under anesthesia, note the agents used and the circumstances of death. Anesthetic agents and their metabolites produce histologic findings — hepatic and renal changes in particular — that can be misinterpreted without this context. If cardiopulmonary resuscitation was performed, note this as well, since CPR-associated rib fractures, pulmonary hemorrhage, and hepatic laceration are findings that require clinical context to interpret correctly.
Quick reference: necropsy submission checklist
| Step | What to do |
|---|---|
| Immediately after death | Refrigerate carcass or begin tissue collection — do not leave at room temperature |
| Before opening | Photograph externally, document BCS, mucosal color, hydration, any external lesions |
| Body cavities | Note and photograph free fluid before disturbing; record volume or estimate; document serosal surfaces |
| Fresh tissue first | Collect culture, PCR, tox samples before opening formalin containers |
| Tissue sections | 0.5–1 cm thick; both sides for paired organs; pin intestinal segments flat |
| Brain | Fix whole — do not section fresh |
| Vitreous humor | Collect from at least one eye if metabolic or toxic cause suspected |
| Formalin ratio | 10:1 formalin to tissue by volume; use large enough containers |
| Labeling | Label container lid AND side; separate containers by body region |
| Submission history | Include full signalment, clinical signs, treatments, differentials, circumstances of death |
The necropsy is the last diagnostic opportunity for that patient. The information it can yield depends entirely on how it is performed and what is submitted. For the clinician, a complete necropsy answers the diagnostic question that could not be resolved in life. For the owner of a household pet, it can provide closure — a concrete explanation for an illness that may have felt inexplicable and a way to make sense of a loss. For a herd or multi-animal household, it may prevent the next death. These steps are not complicated. They are habits, and the ones worth building.
Eric Snook, DVM, PhD, DACVP — Vetopathy. Questions about tissue submission protocols for specific cases or species? Reach out before you begin — a two-minute conversation before the procedure is worth more than a follow-up call after.