Introduction
The periodic acid Schiff (PAS) stain assay is a histochemical stain used to identify carbohydrates, including glycogen, mucopolysaccharides, and basement membranes. It is a simple and inexpensive stain that is widely used in pathology laboratories to diagnose a variety of diseases and conditions, including leukemia.
When used on peripheral blood, the PAS stain can be used to identify leukemic cells that contain increased amounts of glycogen or mucopolysaccharides. This can be helpful in differentiating between different types of leukemia, such as acute myeloid leukemia and acute lymphoblastic leukemia. .
Principle of PAS stain
Periodic acid Schiff (PAS) stain is a method used to identify glycogen in tissues. PAS works by selectively oxidizing carbohydrate radicals, such as those found in glycols and related compounds, to produce dialdehydes. These dialdehydes react with Schiff’s reagent to produce a purple-magenta color. PAS stain is helpful in diagnosing acute lymphoblastic leukemia and some cases of erythroleukemia.
Method differs slightly according to the manufacturer’s protocol.
Materials
- Absolute methanol
- Schiff’s reagent
- Period acid 1% w/v with distilled water
- Methyl green 4% with distilled water
- Unstained peripheral blood smear slide
Method
- Fix slides in absolute methanol for 5 – 10 minutes.
- Rinse the slide with slow running tap water for 1 minute.
- Submerge in periodic acid for 10 minutes.
- Rinse the slide with slow running tap water.
- Wipe the back of the slide and edges with Kim wipes. Be careful not to touch the smear.
- Submerge in Schiff’s reagent for 20 minutes.
- Rinse the slide with slow running tap water for 5 minutes.
- Counterstain with methyl green for 3 minutes.
- Rinse the slide with slow running tap water.
- Wipe the back of the slide and edges with Kim wipes. Be careful not to touch the smear.
- Dry the slide using the hair dryer on the lowest speed or air dry in a tilted position.
- Mount the slide with Depex and cover the zone of morphology with a cover slip.
- This slide is now ready for viewing.
Interpretation
PAS stain is used to identify glycogen in tissues. PAS positive cells stain magenta (red) in the cytoplasm, while the nucleus stains green.
- Neutrophils are a positive control for PAS staining. They show intense, confluent granular positivity.
- In acute lymphoblastic leukemia, the lymphoblasts are stained block positivity.
- Myeloblasts contain a few small PAS-positive granules.
- Monocytes and their precursors show variable diffuse positivity.
- The reaction in megakaryocytes and platelets is variable.
- Erythroleukemia and erythroid precursors of some thalassemia are PAS positive.
- Normal erythroid precursors and red cells are negative.
