Laboratory Devices, Part 3

The histology section of a clinical lab is the area where tissue is prepared and studied to determine its cell structure. In this section you will find tissue processors, slide strainer, paraffin heaters, microtomes, microscopes, and water baths. If the area is not well ventilated, the smell of the processing chemicals can be very strong. Odor is also an indication that something is not operating correctly, either with the chemical dispensing or draining.

Tissue Processor
Tissue to be studied is first bathed one or more times with a fixative such as formalin. This preserves the natural state of the cellular components so they can withstand cleaning, dehydration, and embedding. The specimen can be either stationary with fixative pumped into and out of the chamber, or automatic (rotating systems), in which the specimen is automatically moved from one bath to another. Each bath lasts from 1 to 4 hours. On rotating systems, cross contaminations can occur if the central valve is not aligned. Ensuring that the central valve is aligned is critical and should be done on every preventive maintenance (PM) cycle.

Both systems are potential fire hazards and the wiring also should be carefully inspected at each PM cycle.

The next step is dehydration in a series of as many as six ethanol solutions, ranging from 70% to 100% in concentration, lasting as long as 1 hour each. Next the specimen is “cleared” twice with toluene or chloroform for 30 minutes to 3 hours each (a major source of odor in the lab).

The next step is the immersion in a paraffin bath for up to 4 hours, after which time the specimen is removed and placed into a mold where more paraffin is added to make a block that is of consistent size and that can be sliced on the microtomes.

This is a long process, often done during off hours when no one is in the lab, so the timing system and temperature controls must be calibrated and properly working to assure good specimens. The ideal temperature for the baths is 60° C; a temperature of more than 65° C will cause tissue destruction and scorching of the paraffin. If a specimen is being prepared for examination under an electron microscope, plastics are substituted for paraffin.

Common problems, rotating systems
•    sticking in the up position as specimens are moved from one bath to another
•    temperature variations
•    spills and drips
•    timer, especially if mechanical
•    odors

Common problems, stationary systems
•    pumps not filling or draining properly
•    temperature variations

Common problems, paraffin dispenser
•    temperature variations
•    paraffin buildup in dispensing valve

Microtomes
Most histology laboratories will have two types of microtomes: rotary units and cryostat microtomes for doing frozen sections. Each cuts tissue into slices 0.2 µm–50 µm thick. The very thin slices are not often used, as they are difficult to make and, in most cases, have limited clinical value.

Once the specimen is clamped in place, it is advanced on the feed pawl until its edge is in the cutting zone. The knife is moved in an up-down stroke by turning a flywheel. As the knife descends, a specimen slice is made; as the knife ascends the specimen is drawn back so the knife edge does not touch the specimen. At the top of the ascension stroke, the specimen is moved forward to be in position for the next descending stroke of the knife. This process is repeated until there are enough slices for the study. On some units the knife and feed pawl are motor driven, but most are manual or mechanical linkages.

The technologist removes each slice by hand and floats the slice on a heated water bath. The tissue itself is not handled; only the paraffin surrounding the specimen is picked up by tweezers and placed in the bath.

Cryostat
A cryostat is a rotary microtome placed in a refrigerated cabinet—typically to -30° C—that uses frozen-tissue specimens instead of prepared specimens from a tissue processor. The slices are generally several micrometers thicker and must be viewed quickly. Frozen sections are completed in minutes, rather than hours or days, to get a gross reading. Frozen sections are used when a quick determination has to be made, typically while the patient is on the operating table. A portion of the specimen will also be sent through the full process for a definitive diagnosis.

Ice buildup in cryostats is a common problem, especially in hot/humid climates. The slide door needs to be closed securely and checked for a tight fit during the PM process. If ice does build up, the unit has to be shut down and the ice melted. Some facilities turn the unit off for a weekend with the door open. A quicker way is to use a heat gun. The unit must be completely dry before it is restarted.

Common problems
•    feed pawl not advancing correctly
•    blade not cutting straight down, concave face on specimen block
•    loose blade
•    bad blade

Common problems, cryostat units
    •    all the above plus:
    •    defogger not working on viewing port
    •    temperature variation
    •    low temperature lubricant not used on mechanisms.

Slide Stainer
The technician will select several of the floating slices from the water bath by slipping a glass slide underneath the specimen. The specimen is allowed to air dry for a short time before undergoing the staining process.

The slides are stained with one or more dyes to help define tissue edges, nucleus, collagen, or elastin. It is important to remove excess stain from the slides. The staining and removal of excess stain is done automatically on linear or rotating units.

The rotating units can look very similar to the rotating tissue processor, as there are a series of “pots” containing stain and solvents into which the slides are dipped. Since the slides are vertical, specimens may slide off during the dipping process. There is a linear version in which individual or small numbers of slides are dipped into stains or solvents, but it is for low-volume applications.

The most common unit is a linear system that is similar to a conveyor belt. Slides are placed on the transport and moved from one station to the next. This allows for a greater volume of slides to be processed, and more slides can be added at any time. Vibrations from other devices can cause problems. Putting rubber or cloth cushions under the feet of the unit will prevent frequent jams.

Some units may have a drying station where hot air is gently blown over the slides to remove any residual moisture. The key word is gently, as too much force will affect the quality of the slides.

Slides will have glass “slip covers” put on them to protect the specimens. If the glass is not squarely placed on the slide or the racks are not aligned, jamming and resultant breakage can occur. To clean out broken glass don’t use your hands. Use tweezers for the large pieces and damp cotton swabs for the small pieces.

Because of the solvents used, these devices should either be in a specially vented area, in a fume hood, or have an air-handling system with a filter.

Common problems
•    rotating unit same as with rotating tissue prep unit

Common problems on linear units
•    broken slides in transport gears
•    varying belt speed
•    with all units odor is a major problem.

Microscopes
In histology, pathology, hematology, and other sections of clinical laboratories, microscopes are critical devices. While these units rarely fail, they do require cleaning and lubrication on a regular basis.

Microscopes have a light source, either external or built in, that is used to bottom light the slide-mounted specimen. It may be a direct light beam or focused via a mirror, while others may use fiber optics to transmit the light. On some units there is a diaphragm that will control the size of the light beam to the specimen. There may be an intensity control on the light source that can fail. There also may be a filter between the light and the specimen that removes certain wavelengths of light from the spectrum. Bulbs should only be replaced with their exact replacements. Different bulbs may have different light output, filament voltages, or heat or light spectrums that could affect the reading of the specimen.

If mechanical positioning adjustments do not operate smoothly, it is an indication that either too much or too little lubricant has been used. Clean off any excess lubricant, especially if it has dried and is “clumping,” using a soft cloth dampened with alcohol. Do not use any solvents that may leave a residue. Also be careful not to leave lint on any surface.

Common problems
•    light bulb
•    spills
•    scratches

Optics should be cleaned with an alcohol-dampened, lint-free cloth. It is a good idea to blow off any dust with canned air before cleaning. Be very careful not to scratch the optics.

David Harrington, PhD, is director of staff development and training at Technology in Medicine, Holliston, Mass.

Contributing to this article is Lance Barr, the Technology in Medicine BMET at Wythe County Hospital, Wytheville, Va.