03 Specimen Processing
Description
This article is from the The Biopsy Report: A Patient's Guide, by Ed Uthman uthman@neosoft.com with numerous contributions by others.
03 Specimen Processing
After the specimen is removed from the patient, it is processed in
one or both of two major ways:
1. Histologic sections. This involves preparation of stained,
thin (less than 5 micrometers, or 0.005 millimeters) slices
mounted on a glass slide, under a very thin pane of glass
called a coverslip. There are two major techniques for
preparation of histologic sections:
a. Permanent sections. This technique gives the
best quality of specimen for examination, at the
expense of time. The fresh specimen is immersed in a
fluid called a fixative for several hours (the
necessary time dependent on the size of the specimen).
The fixative, typically formalin (a 10% solution of
formaldehyde gas in buffered water), causes the
proteins in the cells to denature and become hard and
"fixed." Adequate fixation is probably the most
important technical aspect of biopsy processing.
The fixed specimen is then placed in a machine that
automatically goes through an elaborate overnight cycle
that removes all the water from the specimen and
replaces it with paraffin wax. The next morning, a
technical professional, called a histologic technician,
or "histotech," removes the paraffin-impregnated
specimen and "embeds" it in a larger bloc of molten
paraffin. This is allowed to solidify by chilling and
is set in a cutting machine, called a microtome. The
histotech uses the microtome to cut thin sections of
the paraffin block containing the biopsy specimen.
These delicate sections are floated out on a water bath
and picked up on a glass slide.
The the paraffin is dissolved from the tissue on the
slide. With a series of solvents, water is restored to
the sections, and they are stained in a mixture of
dyes. The most common dyes used are hematoxylin, a
natural product of the heartwood of the logwood tree,
Haematoxylon campechianum, which is native to Central
America, and eosin, an artifcial aniline dye. The stain
combination, casually referred to by pathologists as "H
and E" yields pink, orange, and blue sections that make
it easier for us to distinguish different parts of
cells. Typically, the nucleus of cells stains dark
blue, while the cytoplasm stains pink or orange.
b. Frozen sections. This technique allows one to
examine histologic sections within a few minutes of
removing the specimen from the patient, but the price
paid is that the quality of the sections is not nearly
as good as those of the permanent section. Still, a
skilled pathologist and a knowledgeable surgeon can
work together to use the frozen section's rapid
availability to the patient's great benefit.
2. Smears. The specimen is a liquid, or small solid chunks
suspended in liquid. This material is smeared on a microscope
slide and is either allowed to dry in air or is "fixed" by
spraying or immersion in a liquid. The fixed smears are then
stained, coverslipped, and examined under the microscope.
Like the frozen section, smear preparations can be examined within a
few minutes of the time the biopsy was obtained. This is especially
useful in FNA procedures (see above), in which a radiologist is
using ultrasound or CT scan to find the area to be biopsied. He or
she can make one "pass" with the needle and immediately give the
specimen to the pathologist, who can within a few minutes determine
if a diagnostic specimen was obtained. The procedure can be
terminated at that point, sparing the patient the discomfort and
inconvenience of repeated sticks.
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