Periodic Acid-Schiff (PAS) Staining 50 FAQs and 30 MCQs
Periodic Acid-Schiff (PAS) Staining 50 FAQs
What is PAS staining used for?
Detecting carbohydrates (glycogen, mucin, fungal cell walls) in tissues.
What structures stain positive with PAS?
Glycogen, mucin, basement membranes, fungal hyphae, and glycoproteins.
Why is PAS staining important in pathology?
Diagnoses glycogen storage diseases, fungal infections, and mucin-secreting tumors.
What is the principle of PAS staining?
Periodic acid oxidizes 1,2-glycols to aldehydes, which react with Schiff’s reagent to form a magenta color.
Can PAS be used on frozen sections?
Yes, it works on both paraffin-embedded and frozen tissues.
How do you prepare 0.5% periodic acid solution?
Dissolve 0.5g periodic acid in 100mL distilled water.
How to test Schiff’s reagent quality?
Add Schiff’s reagent to 10% formalin; good reagent turns red-purple immediately.
What indicates deteriorated Schiff’s reagent?
Delayed deep blue-purple color change.
How should Schiff’s reagent be stored?
Refrigerated (2–8°C) and tightly capped.
Can periodic acid solution be reused?
Yes, but fresh preparation is preferred for consistency.
What is the oxidation time in periodic acid?
Typically 5–10 minutes at room temperature.
How long should slides stay in Schiff’s reagent?
15 minutes (turns light pink).
Why is a tap water wash used after Schiff’s reagent?
Enhances color (dark pink) and removes unbound reagent.
What is the purpose of hematoxylin counterstaining?
Highlights nuclei (blue) for contrast.
Can PAS staining be accelerated?
Yes, microwave protocols reduce steps to seconds.
Why is PAS staining faint?
Over-oxidation, old Schiff’s reagent, or insufficient aldehyde formation.
What causes high background staining?
Inadequate washing or over-oxidation.
How to fix uneven PAS staining?
Ensure consistent reagent application and timing.
Why do nuclei stain pink instead of blue?
Hematoxylin counterstain was skipped or underused.
What if no staining occurs?
Check reagent expiration, oxidation step, or tissue fixation.
How does PAS help diagnose fungal infections?
Fungal cell walls (e.g., Candida, Aspergillus) stain magenta.
Why use PAS for glycogen storage diseases?
Glycogen stains pink; diastase digestion confirms specificity.
How is PAS used in renal pathology?
Highlights basement membranes (e.g., glomerular diseases).
Can PAS detect mucin in adenocarcinoma?
Yes, mucin-secreting tumors stain positive.
What autoimmune diseases correlate with PAS staining?
Bullous pemphigoid, lupus (98% match with DIF patterns).
What is diastase digestion in PAS?
α-amylase removes glycogen; residual staining indicates non-glycogen carbs.
When is microwave PAS staining preferred?
For rapid results (e.g., intraoperative consultations).
How does chromic acid differ from periodic acid?
Chromic acid reduces background staining (better for fungi).
Can PAS be combined with other stains?
Yes, e.g., PAS-D (diastase) or PAS-Alcian blue for mucins.
What fixatives are best for glycogen preservation?
Carnoy’s fluid or ethanol-based fixatives.
PAS vs. H&E: When to use PAS?
PAS highlights carbs; H&E shows general morphology.
PAS vs. mucicarmine: Which detects mucin better?
Mucicarmine is more specific for epithelial mucins.
Why use PAS instead of immunofluorescence (DIF)?
PAS is cheaper and accessible where DIF isn’t available.
Does PAS work on blood smears?
Yes, for diagnosing erythroleukemia or lymphoblastic leukemia.
How does PAS compare to Grocott’s methenamine silver (GMS)?
GMS is more sensitive for fungi; PAS is quicker.
What’s the role of formalin-ethanol fixative?
Preserves glycogen and improves staining quality.
Why use Scott’s tap water substitute?
Enhances nuclear (hematoxylin) bluing.
Can PAS stain glycolipids?
Yes, if they contain oxidizable 1,2-glycols.
What’s the optimal tissue section thickness?
4–5 microns for paraffin sections.
How long can PAS-stained slides be stored?
Months to years if mounted properly
Does PAS stain all carbohydrates?
No, only those with 1,2-glycols or amino-hydroxy groups.
Can PAS detect bacterial capsules?
Some (e.g., Actinomyces), but not all.
Why is PAS less sensitive for water-soluble carbs?
Leaching during fixation/washing.
What’s the false-negative rate for PAS in fungi?
Low, but GMS may be needed for sparse organisms.
Can PAS replace DIF in autoimmune diagnostics?
Not fully, but it’s a useful adjunct (98% correlation).
Is periodic acid hazardous?
Yes, corrosive; use gloves and eye protection.
How to dispose of Schiff’s reagent waste?
Follow local regulations for chemical disposal.
Can PAS reagents expire?
Yes, Schiff’s reagent degrades over time.
What’s the shelf life of periodic acid solution?
Stable for months if stored properly.
Why avoid picric acid fixatives for glycogen?
Prolongs diastase digestion time.
Periodic Acid-Schiff (PAS) Staining 30 MCQs
- What is the primary purpose of PAS staining?
a) Detect lipids
b) Identify carbohydrates (glycogen, mucin, fungi)✔
c) Stain nucleic acids
d) Highlight collagen fibers - Which chemical group is oxidized by periodic acid in PAS staining?
a) 1,2-glycols✔
b) Sulfhydryl groups
c) Phosphate groups
d) Carboxyl groups - What color do PAS-positive structures typically stain?
a) Blue
b) Green
c) Magenta/red✔
d) Yellow - Which of the following is NOT PAS-positive?
a) Glycogen
b) Fungal cell walls
c) Collagen (non-reticular)✔
d) Mucin - What is the role of Schiff’s reagent in PAS staining?
a) Oxidize carbohydrates
b) Bind to aldehydes to form a colored product✔
c) Counterstain nuclei
d) Digest glycogen
- How long should tissue sections be treated with periodic acid?
a) 1 minute
b) 5–10 minutes✔
c) 30 minutes
d) 1 hour - What is used to counterstain nuclei in PAS staining?
a) Eosin
b) Hematoxylin✔
c) Methylene blue
d) Alcian blue - How do you test the quality of Schiff’s reagent?
a) Mix with ethanol—should turn blue
b) Add to formalin—good reagent turns red-purple immediately✔
c) Heat to 60°C—should precipitate
d) Expose to light—should fluoresce - Which fixative is BEST for preserving glycogen for PAS staining?
a) 10% neutral buffered formalin
b) Carnoy’s fluid✔
c) Bouin’s solution
d) Zenker’s fixative - What is the purpose of diastase digestion in PAS staining?
a) Enhance fungal staining
b) Remove glycogen to confirm its presence✔
c) Increase mucin visibility
d) Reduce background staining
- PAS staining is most useful for diagnosing:
a) Amyloidosis
b) Fungal infections✔
c) Iron overload
d) Melanoma - In renal pathology, PAS helps visualize:
a) Glomerular basement membranes✔
b) Tubular lipid deposits
c) Immune complexes
d) Hemoglobin casts - Which condition would show PAS-positive diastase-resistant material?
a) Glycogen storage disease
b) Mucin-producing adenocarcinoma✔
c) Fungal infection
d) Amyloidosis - PAS staining correlates strongly with which diagnostic technique for autoimmune diseases?
a) Gram staining
b) Direct immunofluorescence (DIF)✔
c) Ziehl-Neelsen stain
d) Masson’s trichrome - Which structure would NOT stain with PAS?
a) Russell bodies
b) Thyroid colloid
c) Elastic fibers✔
d) Glycogen
- Faint PAS staining may result from:
a) Over-oxidation✔
b) Fresh Schiff’s reagent
c) Short periodic acid exposure
d) Excessive hematoxylin - High background staining in PAS can be reduced by:
a) Using chromic acid instead of periodic acid✔
b) Skipping the hematoxylin step
c) Decreasing Schiff’s reagent time
d) Omitting the oxidation step - If nuclei stain pink instead of blue, what was likely skipped?
a) Periodic acid step
b) Schiff’s reagent
c) Hematoxylin counterstain✔
d) Diastase digestion - PAS-negative fungi may indicate:
a) Over-fixation
b) Use of Carnoy’s fixative
c) Insufficient periodic acid time
d) All of the above✔ - Why might glycogen be absent in PAS-stained liver tissue?
a) Over-oxidation
b) Prolonged formalin fixation
c) Inadequate diastase digestion
d) All of the above✔
- Which oxidant is preferred for fungal staining to reduce background?
a) Periodic acid
b) Chromic acid✔
c) Hydrogen peroxide
d) Potassium permanganate - Microwave PAS staining is used for:
a) Enhancing fungal visibility
b) Rapid results (e.g., intraoperative)✔
c) Reducing reagent costs
d) Eliminating counterstaining - PAS-D (diastase) staining helps differentiate:
a) Glycogen vs. mucin✔
b) Fungi vs. bacteria
c) Collagen vs. reticulin
d) Amyloid vs. hyaline - Which stain is MORE sensitive for fungi than PAS?
a) H&E
b) Gram stain
c) Grocott’s methenamine silver (GMS)✔
d) Ziehl-Neelsen - PAS is inferior to mucicarmine for detecting:
a) Glycogen
b) Fungal walls
c) Epithelial mucins✔
d) Basement membranes
- Schiff’s reagent should be stored:
a) At room temperature
b) In a freezer
c) Refrigerated (2–8°C)✔
d) In direct sunlight - Periodic acid is hazardous because it is:
a) Radioactive
b) Corrosive✔
c) Carcinogenic
d) Flammable - Expired Schiff’s reagent may cause:
a) Faint staining
b) Blue-purple discoloration
c) Both a and b✔
d) Enhanced nuclear staining - Which fixative interferes with diastase digestion?
a) Formalin
b) Bouin’s (picric acid)✔
c) Ethanol
d) Glutaraldehyde - PAS staining is contraindicated for:
a) All bacterial infections
b) Tissues fixed in osmium tetroxide
c) Frozen sections
d) None of the above✔
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