What is the periodic acid Schiff plus diastase (PAS-D) special stain?
Periodic acid Schiff plus diastase (also known as PAS-D) is a special stain test that is primarily used by pathologists to determine if a substance inside of a cell is a type of sugar called glycogen or a type of protein called mucin. When a PAS-D-stained slide is examined under the microscope, intracellular mucin (mucin inside of the cell) appears bright pink or red. In contrast, glycogen (which can look very similar to mucin on the routine hematoxylin and eosin-stained slide) appears white. Intracellular mucin is commonly found in a type of cancer called adenocarcinoma and this test can help pathologists tell the difference between adenocarcinoma and other types of cancer that do not show intracellular mucin.
Other uses for PAS-D include:
- Fungal infections: This test makes it easy for pathologists to see fungal micro-organisms such as Candida and Pneumocystis because the PAS-D makes them look bright pink or red when examined under the microscope.
- Whipple’s disease: Whipple’s disease is caused by infection with the bacteria T. whipplei. Pathologists will perform a PAS-D test on tissue samples from the small bowel in patients who may have Whipple’s disease. This test makes it easier to see specialized immune cells called histiocytes which are found in large numbers in Whipple’s disease.
- Liver disease: The PAS-D test can be used to highlight an abnormal protein called alpha-1-anti-trypsin in the liver. Alpha-1-anti-trypsin globules are seen in a disease called alpha-1-anti-trypsin deficiency, which affects both the liver and the lungs.
- Zymogen granules: Zymogen granules are a specialized piece of cellular machinery that are normally found in salivary gland-type tissue. The tumour cells in a type of salivary gland cancer called acinic cell carcinoma will contain lots of zymogen granules and pathologists often perform this test to confirm the diagnosis.
This test is often performed at the same time as a periodic acid Schiff (PAS) stained slide. Unlike PAS-D, the PAS test does not include diastase. Pathologists perform these two tests together to look for glycogen inside a cell. These tests work well together because glycogen appears pink on the PAS stained slide but clear on the PAS-D stained slide.