How Can Pathology Labs Better Protect Patient Safety?


Patient tissue samples can contaminate each other on their way to the histology lab, a study at one hospital is reported that experts say reveals long-standing problems in pathology processing.

Experiments included the practice of packing cassettes of tissue samples on racks in a shared formalin bath for transport. The movement of fluid flowing between and through the cassettes appears to push out small tissue fragments that end up in other samples, even when using protective packaging, the authors, pathologists at the University of Chicago Medical Center, report.

“The general approach to shipping and transporting tissue in a humid environment presents a significant risk of contamination that is casually accepted in most practices,” lead author Timothy Karll, MD, told Medscape Medical News. “And as we have shown, even the use of occlusive packaging material within the cassettes themselves is not guaranteed to prevent contamination.”


He concludes by calling for change: “Practical methods and technological innovation are needed to reduce this risk.”

The study was published this month in the American Journal of Clinical Pathology.

Finding the source of pollution

The research began shortly after the hospital construction project separated the reception area from the histology lab, placing them at opposite ends of the medical campus.

As a temporary solution, the lab chose the hospital’s pneumatic tubing system to transport the dissected tissue to minimize personnel movement between the two locations.

A mysterious source of increased pollution began to haunt the pathological department. Extraneous pieces of tissue from other patients’ samples fell onto pathologists’ slides at an alarmingly high rate. Fortunately, no patients were harmed because the pathologists were able to immediately identify and report these contaminations.


To find out what went wrong, the researchers designed an experiment to determine if the piping system was the culprit. After excluding other processing steps as foci of contamination, the de-identified samples, which are known to be common contaminants, were subjected to the usual conditions of fixation in formalin, shaking with pneumatic tube and processing.

Steps were taken to minimize contamination throughout the process, including handling samples on two different tables and placing wipes in protective packaging, including mesh wipes, lens paper and sponges. As a control, part of the fabric was not placed in a protective package. Samples were transported to histology, as usual, through a system of pneumatic tubes. After processing, formalin samples were collected from formalin storage bins, shipping containers, and processor waste containers, and then placed in a centrifuge in preparation for cytology.

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Pathologists found that 14.9% of the experimental tissue samples were contaminated with tissue carriers, including those in protective packaging. Cytological preparations showed viable tumor cells both in the formalin storage containers and in the spent processor fluid, which affected not only the tubing system.


The pathology laboratory has noticed a decrease in contamination after the study and now sends the cassettes, removed from the liquid, but still wet, by courier to histology.

The authors write that it is not only laboratory workers using pneumatic tube systems that should be concerned. Mixing the cassettes in any liquid is dangerous. Another form of agitation involves courier transportation of cassettes, which is used in many laboratories. The study also mentions that contamination can be a problem for fabric manufacturers.

“It seems likely that as long as the cassettes are being processed next to each other on racks with no intervening filters, contamination of the processor retort cannot be ruled out,” Carl said.


Manifesto to change archaic processes

According to Richard Zarbo, MD, chairman of the Division of Pathology and Laboratory Medicine at Henry Ford Hospital, Detroit, Michigan, the study reopened a century-old dilemma, who wrote the accompanying editorial.

Zarbo has been on a pollution reduction crusade since a 1994 study, when he and colleague Gordon Gephardt, MD, recorded the presence of contaminants in 2.9% of 57,083 glass slides from 275 laboratories. In a retrospective review, they reported that 12.7% of tissue contaminants were found to be neoplastic and 0.4% posed a serious threat to patient care.

Zarbo cites outdated slide preparation processes as the main culprit. Slides for microscopic examination in pathology have been prepared in the same way for more than a century.

The dissected tissue samples are first placed in slotted hollow plastic containers called cassettes. The cassettes are soaked in a formalin bath for several hours, then loaded into a machine called a processor, which automatically dehydrates the tissue by gradually immersing it in alcohols and solvents to saturate it with paraffin as a final step. The paraffin-impregnated tissue is then embedded with additional paraffin to cut into ultra-thin sections, which are placed on a glass slide.

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The danger of infection is present at every stage of this process – during the initial preparation and subsequent fixation, processing, pouring and cutting.

To eliminate pollutants, radical changes must occur, writes Zarbo in his editorial. “Radical means doing things differently rather than putting a band-aid on existing separate modes of operation,” he told Medscape Medical News.

Current processes need serious revision, he believes. “The new process should be seamless, as automated as possible, with built-in quality control checks,” he said. “This will require innovative technologies to continue to provide the basis for tissue-based pathological interpretation. It will also require the definition of a standardized human interaction with this technology to minimize human error in the new process.”

For example, after a patient safety event in his department, Zarbo purchased 1,000 single-use reusable forceps with each cassette during insertion. Forceps are disinfected at night.

Clinical Consequences

Zarbo advises clinicians to “trust but verify” pathology findings. The amount of contaminants may be small overall, but for an individual patient, even one such misdiagnosis due to a contaminant is too many.

He continued, “So, for the presenting surgeon or procedural specialist, if you get a pathological diagnosis that just doesn’t make sense, ask why. Ask to reconsider the case for this possibility.

“If you are a pathologist, think twice before making a correct diagnosis based on a tiny piece of tissue that does not fit with the rest of the tissue or medical history. Solve your suspicions with a molecular profile and if your lab can’t perform this test, send it to one that can microprep and confirm the identity.

“And if you are a patient doubting your unexpected diagnosis that doesn’t make sense,” Zarbo emphasized, “don’t be afraid to go to your doctor and revisit the diagnostic material to make sure your biopsy diagnosis isn’t timely.” to misleading pollution. All participants will be grateful to you.”

Am Jay Clean Patol. Published in the July 2022 issue. Abstract, Editorial

Alice Gray is a pathologist’s assistant who also despises soiling slides.

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