What is a special stain?

A special stain is a specialized test pathologists use to examine tissue samples more closely. While the most common stain used by pathologists is called hematoxylin and eosin (H&E), sometimes additional details are needed that this standard stain cannot reveal. Special stains involve applying specific coloured dyes to tissue samples, highlighting particular cells, microorganisms, or structures, making them easier for pathologists to identify under a microscope.

Why do pathologists use special stains?

Pathologists use special stains to answer specific questions about a tissue sample. For example, certain bacteria, fungi, or proteins might not be visible with a routine H&E stain. Special stains can reveal these hidden details by turning these components different colours, allowing for accurate diagnosis of infections, inflammatory conditions, or cancers.

What are some common types of special stains?

Hundreds of special stains are available, each designed for a specific purpose. Some commonly used special stains include:

• PAS/D stain: Used to highlight fungal organisms and certain substances in tissues like mucin and glycogen (a type of sugar storage). This stain helps pathologists diagnose infections and identify certain tumours.

• Masson trichrome stain: Highlights scar tissue called fibrosis. Pathologists use this stain to identify tissue scarring from chronic inflammation or injury.

• Mucicarmine stain: This stain specifically stains mucin, a substance produced by certain cells. This stain is particularly helpful in diagnosing cancers like adenocarcinoma, where mucin is commonly produced.

• Ziehl-Neelsen stain: Used primarily to detect the bacteria that cause tuberculosis (TB), though it can also highlight other types of microorganisms.

• Elastic stain: This stain highlights elastic fibers, which are special proteins normally found around blood vessels and lining the lungs. It helps pathologists determine if elastic fibers have been damaged or disrupted by diseases like cancer or inflammation.

• Congo red stain: Used to detect abnormal protein deposits known as amyloid. Amyloid deposits are associated with conditions called amyloidosis and can also be found in certain chronic inflammatory diseases and some cancers.

• Grocott (GMS) stain: This stain specifically highlights fungal organisms by staining them black. It helps pathologists diagnose fungal infections, especially those that are difficult to see clearly with standard stains.

• Reticulin stain: Highlights reticulin fibers, which are delicate protein fibers found in various organs such as the liver, spleen, and bone marrow. Pathologists use this stain to evaluate the structural framework of these organs, particularly in diagnosing liver diseases and certain blood or bone marrow conditions.

• Warthin-Starry stain: Used to identify specific bacteria that are difficult to detect with other staining methods, such as Helicobacter pylori (associated with stomach ulcers and gastritis) and organisms causing cat-scratch disease. It stains these bacteria black, making them easier to visualize clearly under the microscope.

• Von Kossa stain: Used to identify calcium deposits within tissues. This stain is helpful for pathologists when diagnosing conditions that involve abnormal calcification, such as certain kidney disorders, blood vessel diseases, and some types of tumours.

Pathologists select the appropriate special stain based on the clinical question or suspicion they are investigating, which greatly assists in making an accurate diagnosis and guiding your care.

How do pathologists choose which special stain to use?

The choice of special stain depends on the clinical question the pathologist needs to answer. For instance:

• If a fungal infection is suspected, a PAS/D stain might be chosen.

• If cancer cells producing mucin are suspected, mucicarmine might be selected.

• If tuberculosis is a concern, Ziehl-Neelsen would likely be used.

Pathologists decide which special stains are most appropriate based on the patient’s clinical history, symptoms, and the initial findings from the standard H&E stain.

Does the use of special stains affect my diagnosis?

Yes, special stains can be essential for accurate diagnosis. Special stains help pathologists pinpoint the exact nature of a disease by highlighting specific microorganisms, proteins, or cellular components. This accuracy guides doctors in choosing the best possible treatment for you.

Questions to ask your doctor

If your doctor mentions that special stains were used on your tissue sample, you might want to ask:

• Why was a special stain necessary for my diagnosis?

• What specific question was the pathologist trying to answer?

• How do the results from the special stain affect my treatment or care?

• Do I need additional testing based on these special stain results?

Understanding special stains and their role in your diagnosis can help you stay informed and actively participate in your healthcare.