Page 580 - The Veterinary Laboratory and Field Manual 3rd Edition

Necropsy guidelines 517

tion prior to use as improperly buffered formalin with a small amount of formalin or formalin-
increases the formation of artefactual pigments soaked paper towel to keep the sample moist.
in the tissue which can complicate examination This negates the need to send large volumes
of tissue sections. of hazardous formalin through the mail and is
Formalin penetrates tissue by diffusion at a especially useful if the samples are very large.
rate of approximately 1 mm/h. Tissue samples • In cold weather, there is a risk that the for-
placed in formalin should be of an appropriate malin may freeze during transport to the
size to allow formalin penetration and fixation. laboratory. This will produce catastrophic
freeze–thaw artefact in affected tissues mak-
ing examination of tissues very difficult or
Guidelines for sampling tissue into
fixative impossible. In these cases, it is recommended
to fix the samples in formalin for 24 h and
• Tissue sections should be no more than 1 cm then transfer to 70% ethanol for transporta-
thick. tion. Ethanol has a lower freezing point and
• Ratio of formalin to tissue volume should be this will reduce the risk of freeze–thaw arte-
no less than 10 : 1. fact developing.
• Open all sections of GI tract to expose muco- • All containers should be labelled appropri-
sal surface prior to immersion in fixative. ately:
• If sampling a diffuse lesion or taking routine
sections of a grossly normal organ try to take • ID of case and tissues submitted and date
a section which will be representative of tis- • warning labels to notify that sample con-
sue architecture or anatomy. tains 10% neutral buffered formalin
• If sampling a focal or mass lesion within an • labels should meet requirements on label-
organ take a sample which includes the bor- ling of hazardous chemicals in your area.
der between normal and abnormal tissue as
this can be the most diagnostically useful.
Microbiology

Packaging formalin-fixed tissue for Sampling
shipping
Sections of organs containing suspect lesions
• Samples should be placed in a straight sided, should be sampled and submitted for ancillary
screw-topped container. testing. Samples should be taken into clean or
• Plastic containers are lighter and much less ideally sterile bags or containers. Ideally samples
likely to break compared with glass and are should be refrigerated during shipping to the
therefore more suitable. laboratory as freezing can reduce the sensitivity
• Remember that fixation will cause tissues to of culture-based techniques. In addition, lesions
become much stiffer and if a narrow-necked can be swabbed with appropriate microbiological
container is used samples will become swabs which can then be submitted for testing.
trapped inside after fixation. Ideally a section of lesional tissue along with a
• Containers should be placed with a quantity swab taken from the lesion should be submitted
of absorbable material (for example, paper for testing.
towel) inside a sealable plastic bag. Sampling of GI tract involves using string to
• Alternatively, the sample can be fixed for tie off a short (5–10 cm long) section of bowel
24–48 h and then shipped in a sealable bag with contents. This can then be excised and

Vet Lab.indb 517 26/03/2019 10:26