In healthcare as it stands currently we are used to tests and results being returned in a particularly expeditious manner.  It also isn’t uncommon for medical students, residents and other healthcare professionals to have a complete absence of exposure to a pathology lab. Given these expectations and a lack of awareness, I am sure at some point in their career a Pathologist may have had to explain why the sample is still not ready a few hours after it gets dropped off at the lab.  

Unlike Radiology where imaging modalities are digitized in real time and available to read nearly instantly.  Unlike radiologic studies, pathologic and specifically histologic interpretation has intrinsic rate limiting steps that are requisite to producing a finished slide that is ready for interpretation by a Pathologist.  This process involves numerous steps from fixation to staining which we will discuss further in a moment.

During tissue processing, fixation is an essential step which helps facilitate preservation of tissue and cellular structures and prevents decomposition as they are when they are fixed.  Tissue fixation also allows for the ability to run specialized tests including immunostains, FISH or other molecular studies..  Lastly it also allows for long term storage of the samples for review at later dates even years later.  

Tissue fixation is often the first place where you can truly see the rate limiting steps in the world of pathology.  The most commonly used fixation agent is Formalin, though other agents exist, and convey their own benefits in particular use cases.  Formalin itself has been well understood to penetrate tissue at a rate of approximately 1 mm per hour.  So as you can see if you have a tissue that is 4mm thick, this step alone would contribute approximately 4 hours to the time it takes before it reaches a finished slide.

After tissue fixation, the next step is known as tissue processing.  This step involves dehydrating the fixed tissue and replacing it with a support medium to provide rigidity without damaging the tissue.  The steps of this process include dehydration with a series of ethanol solutions, removing the ethanol solution in a process known as clearing with xylene and infiltration with a support medium, usually a paraffin wax.   Once these steps are complete the tissue is then oriented for ideal microscopic visualization and surrounded with additional support material to form a tissue block.  While this step is largely automated, it still requires time and we have yet to create a slide.

With a tissue block, the next step involves microtomy.   This process involves creating very thinly sliced tissues approximately 4 microns thick using a small blade and a specialized machine.   The end product is a very thin ribbon that is then placed in a warm water bath.  The warm water bath is useful in that it can help relieve compression induced by the microtome. Once in a bath, the slides are brought to the ribbon and the tissue is finally on the slide.  Once the tissue is on the slide, it is allowed to dry, and then, only once dry the tissue is ready to be stained, usually with hematoxylin and eosin known as H & E.

As you can see there are numerous steps involved simply in the process of creating a slide that is ready for a pathologist’s interpretation.   These steps will likely take the vast majority of time on the first day a sample is received by a lab, and make it unlikely the finished product reaches the pathologist’s desk on the same day.  Once on the Pathologists desk, there may be a need for additional stains or deeper cuts which can further increase the time to the release of a pathology report.   Hopefully now you can understand why your friendly pathologist may not have the answers available immediately at their fingertips.

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