October 2019

Tissue Fixation

By Rachel Corn, RVT

There are many questions about submitting fixed tissue for histopathology. How much 10% formalin do I need? What sample size is best for fixation? What type and size of container should I use? Here are a few pointers to clear up some of the confusion.

First let’s talk about the paperwork. All of the submission forms for the Kansas State Veterinary Diagnostic Laboratory can be found on our website. You can use the general submission form or a species-specific form. Make sure you fill out your submission form as completely as possible including the veterinarian and clinic information, the owner’s information, patient information including the breed and species, and a complete history.

Next, let’s discuss your sample. Make sure that if you submit more than one tissue, they are clearly identified. This can be done by placing them in separate containers and labeling each container, placing a certain number of sutures in the sample or submitting small masses in different colored cassettes and describing these labels on the submission form. Note that shape, color, and size of the mass or tissue may change during the fixation process; therefore using these descriptions are not always appropriate.

Having enough formalin for the sample to fix the tissue properly is another problem commonly noticed in the lab. There should be 20 times as much formalin as tissue in each container. Packing large tissues into a small container does not provide enough volume for the formalin, and the structure of the tissue can be artificially modified by the tight environment. Make sure that the mouth of the container is large enough to remove the sample in one piece after fixation. If your veterinarian desires the margins evaluated, the tissue sample needs to be submitted whole, but if you must cut into a skin mass, cut the skin side, not the surgical margin. It is also very helpful if you wrap it in gauze to hold it in its original shape. This helps the laboratory technicians and pathologists know what the mass looked like at the time of removal.

Placing the tissue immediately after removal into formalin is very important to prevent autolysis and maintain tissue quality. Prevent freezing as this causes the tissue to crack and can render the sample unusable. Formalin penetrates tissues at 0.5mm/hour, so the larger the sample the longer time is required for proper fixation.

Proper packaging is also important for tissue submission. All fixed tissue should be placed in a plastic container that seals securely; screw-top containers are ideal. The containers should be clearly marked with animal and owner information and sample identity if more than one sample is submitted per animal. The plastic container should then be wrapped in absorbent material and enclosed in a sealed bag. This bag should then be placed in an appropriate shipping container. We also suggest placing the box inside a UPS or FedEx over-pack. If you would like to simplify the packaging and shipping process, ask us about our free biopsy supplies. The supplies include a formalin filled container, the proper external packaging and a pre-paid UPS shipping label.

Please contact client care at 1-866-512-5650 or clientcare@vet.k-state.edu.

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