If you recently had a tissue sample sent to pathology, you may be wondering what happens to your tissue on its journey from your body to the laboratory. This article will explain the basic steps in the process along with the time and people required for specific steps. This explanation is a generalization and it will not reflect the process in place in all hospitals.
The amount of time required for each step is an approximation as the process is slightly different in each hospital. Also, smaller tissue specimens, such as biopsies, move through the system faster than larger tissue specimens such as resections.
Step 1. Tissue is removed from the patient and transported to the pathology laboratory to await examination.
The patient undergoes a procedure where a sample of tissue is removed from the body.
- The amount of tissue removed will depend on the reason for the procedure and the type of procedure performed.
- All tissue samples sent to pathology for examination are called specimens.
- Multiple unique patient identifiers are placed on all specimens before they are sent to pathology. Unique identifiers include: name, date of birth, and hospital identification number.
- The specimen is placed in a sealed container prior to transport. The unique patient identifiers will be attached to the container. Most hospital also include bar codes which help electronically track the specimen as it moves through the system.
- Using multiple unique identifiers is an important part of patient safety and quality assurance.
- The specimen is entered into the queue to await gross examination. Cancer specimens and small tissue specimens such as biopsies are are typically examined first before large non-cancer specimens such as resections.
Step 2. A gross examination is performed.
All specimens sent to pathology are initially examined visually (without a microscope). In pathology this is called the gross examination. During this initial examination, the type of tissue will be identified (for example “the specimen is a piece of skin”), measurements will be taken, and in some circumstances the tissue will be weighed. The tissue will then be thoroughly examined to identify any abnormal areas.
- In most hospitals, the gross examination is performed by a Pathologist’s Assistant, a laboratory professional who has been specially trained in the handling and processing of tissue for pathology. Pathologist’s Assistants will review difficult specimens directly with the Pathologist in charge of the case in order to make sure that all specimens are properly examined.
- The results of the gross examination can be found in your pathology report in the section called gross description.
- Any abnormal area of tissue will be described in the gross description. In particular, the description will include the size, colour, shape, and consistency of the abnormal area. It will also include the relationship between the abnormal area and any surrounding normal tissue. Importantly, the description will also include the relationship between the abnormal area and the cut edge of the tissue which is called the margin. Click here to learn more about the margin and why it is important.
Step 3. Tissue to be examined under the microscope is removed from the specimen and placed in blocks.
Before any tissue can be examined under the microscope, small pieces must be removed from the specimen and sent for further processing. The person performing the gross examination first identifies areas of tissue that require further examination. These areas are then cut out of the larger specimen and placed in small containers called ‘blocks’.
- The blocks are sequentially labelled with either numbers or letters. Most hospitals assign each specimen a number and each block a letter. Using this system, the blocks made from specimen 1 in a case would be called 1A, 1B, 1C, and so on.
- The person performing the gross examination will describe the contents of each block in the gross description. This allows the pathologist looking at the slides to know exactly where each piece of tissue sampled came from in the specimen.
- The number of blocks created for each specimen will depend on the size of the specimen and the complexity of the case. Most hospitals have protocols in place which provide a guide for sampling commonly encountered specimens.
- Very small specimens such as biopsies are placed entirely in a block for microscopic examination.
Step 4. The tissue is placed in a preservative called formalin.
Tissue removed form the body rapidly breaks down. Formalin is called a preservative because it prevents tissue from breaking down. Most types of specimens need to be placed in formalin before they can be viewed under a microscope.
- The amount of time a specimen spends in formalin depends on the size of the specimen and the type of tissue that was removed. Small specimens like biopsies may only need to be in formalin for several hours whereas large resections may be left in formalin for 2-3 days.
- Some large specimens are placed in formalin for several days before the gross examination. This helps ‘harden’ the tissue which makes it easier to cut and examine.
- All blocks are placed in containers full of formalin for several hours before moving on to the next step.
Step 5. The tissue is embedded with paraffin wax.
Tissue can only be examined under a microscope if it is cut into very very thin sections (about the width of a human hair). However, most types of tissue are far too delicate to be cut so thinly without falling apart. In order to make the tissue strong enough to cut, the water inside the tissue is replaced with wax in a process called embedding.
- Embedding not only makes tissue strong, it also allows it to last for years without breaking down. For that reason, pathologists can often have slides made from blocks that are many years old if new testing or treatment options become available.
- The length of the embedding process will depend on the size and type of tissue in the block. Biopsies and other small pieces of tissue can be rapidly embedded in an hour. Larger pieces of tissue, however, often require several hours before they are ready to move to the next step.
Step 6. The tissue is cut and placed on glass slides.
The tissue, now embedded in paraffin wax, is transferred to a special device called a microtome which will cut the tissue into very thin sections.
- A microtome uses a very sharp blade to cut the tissue into sections about the width of a human hair.
- After the section is cut, it is gently placed on a glass slide which is then left to dry.
- At this stage, the tissue on the slide is almost invisible.
Step 7. The tissue is stained so that it can be seen through the microscope.
The slides are now moved to a machine which applies two dyes called hematoxylin and eosin to the tissue sections. These dyes give the tissue its characteristic pink and purple colour that many people recognize as the typical microscopic slide. These dyes allow the pathologist to recognize different types of cells and to describe the changes taking place inside the tissue.
- One hematoxylin and eosin stained slide is usually made automatically for each block of tissue.
- Additional slides made me prepared for other types of tests including immunohistochemistry. These slides are usually ordered by the pathologist after the hematoxylin and eosin stained slides have been examined.
Step 8. All of the slides are sent to the pathologist for microscopic examination.
In most hospitals, slides are delivered throughout the day to the pathologists on service. Slides are usually categorized by body system (for example ‘lung’ or ‘breast’) and pathologists often specialize in one or two body systems.
- All of the slides from a single patient are called a ‘case’. Pathologists usually examine cases in the order they arrive however cases marked as urgent are often reviewed first.
- The time is takes to perform a microscopic examination of an entire case depends on many factors including the number of slides, the complexity of the case, and the experience of the pathologist. Large, complex cases may take several hours while small and relatively simple cases may be completed in minutes.
- Some cases require additional tests before they can be completed. These additional tests may increase the time it takes to complete a case by several days. The most common additional test is called immunohistochemistry.
Step 9. Your pathologist completes your report and sends it to your other doctors.
- Most pathologists complete the microscopic examination and issue a report the same day the slides are received. As part of your healthcare team, any unexpected or critical results are reported via telephone to the surgeon or specialist who ordered the test