A PAS test, short for Periodic Acid-Schiff test, is a laboratory staining technique used to detect sugars and sugar-containing molecules in tissue samples. It works by turning these substances a vivid magenta-pink color under a microscope, helping pathologists identify everything from fungal infections to kidney disease to certain cancers. If your doctor ordered a biopsy and the pathology report mentions PAS staining, it was used to get a closer look at what’s happening inside your tissue at a cellular level.
How the Stain Works
The PAS test is a two-step chemical reaction. First, a chemical called periodic acid is applied to a thin slice of tissue on a glass slide. This breaks apart specific bonds in sugar molecules, creating new reactive groups called aldehydes. Then a second reagent, called Schiff reagent, is applied. It reacts with those aldehydes to produce a bright magenta-pink color wherever sugars are present.
The test picks up a wide range of sugar-based substances: glycogen (the stored form of sugar in your cells), glycoproteins, mucus-producing compounds, and components of the structural membranes that hold tissues together. Anything that doesn’t contain these molecules stays unstained or takes on a pale background color, making the magenta areas stand out clearly.
What a PAS Test Is Used For
PAS staining is one of the most versatile tools in a pathology lab. It shows up in diagnostic workups for a surprisingly wide range of conditions.
- Kidney disease: The membranes that filter blood in your kidneys are rich in sugar-containing molecules, so PAS staining highlights them beautifully. Pathologists use it to evaluate kidney biopsies, looking for thickening or damage to these filtering membranes. In conditions like membranous nephropathy, PAS-stained sections reveal irregular, dimpled membranes that differ visibly from healthy tissue.
- Glycogen storage disorders: Diseases like Pompe disease cause cells to accumulate abnormal amounts of glycogen. PAS staining makes this excess glycogen visible, and clinicians have used the technique on blood and tissue samples since the 1950s to study glycogen-related conditions.
- Liver conditions: When someone with poorly controlled type 1 diabetes develops glycogen hepatopathy (a buildup of glycogen in liver cells), PAS staining shows enlarged liver cells packed with glycogen deposits.
- Fungal infections: The cell walls of fungi stain positive with PAS, making the test useful for spotting fungal organisms in skin, lung, or other tissue biopsies. In one study of palm and sole skin biopsies, PAS staining detected fungi in cases where infection wasn’t even clinically suspected.
- Whipple’s disease: This rare bacterial infection of the small intestine has a hallmark finding on PAS staining: foamy immune cells called macrophages packed with bright PAS-positive granules, filling more than 80% of the tissue lining in some patients.
- Cancer classification: PAS staining helps pathologists distinguish between tumor types. In one study comparing lung adenocarcinoma to mesothelioma, PAS-based staining combined with other markers correctly classified 98% of tumors (80 out of 82 cases).
The Diastase Variation (PAS-D)
Sometimes pathologists need to know whether the magenta staining they see is caused by glycogen specifically or by some other sugar-containing substance. To figure this out, they run the test twice on matching tissue sections: once with PAS alone, and once after treating the tissue with diastase, an enzyme that digests glycogen.
If the magenta color disappears after diastase treatment, the staining was caused by glycogen. If it persists, something else is responsible, such as mucus, basement membrane material, or fungal cell walls. This version, called PAS-D (PAS with diastase), is particularly important for liver biopsies, where pathologists need to distinguish glycogen hepatopathy from non-alcoholic fatty liver disease. The diastase-treated glycogen-filled cells appear as pale “ghost cells” under the microscope, confirming that glycogen was present.
How the Test Is Done
You won’t experience the PAS test directly. It’s performed in a lab on tissue that has already been collected through a biopsy or surgical procedure. The tissue is typically preserved in formalin and embedded in paraffin wax, then sliced into extremely thin sections and mounted on glass slides. Frozen tissue sections can also be used.
A technician applies the periodic acid solution, rinses the slide, applies the Schiff reagent, and then counterstains with a light background color so the magenta areas are easy to identify. The entire process is routine in most pathology labs and doesn’t require any special preparation from the patient beyond whatever biopsy or surgery was needed to collect the tissue.
Reading the Results
PAS results are interpreted by a pathologist, not reported as a simple positive or negative to the patient. What the pathologist looks for depends entirely on the clinical question. In a kidney biopsy, they’re evaluating the thickness and regularity of basement membranes. In a liver biopsy, they’re checking for excessive glycogen in liver cells. In a skin biopsy, they’re scanning the outer layer for fungal organisms.
A “PAS-positive” finding means the target structures lit up magenta. But the significance of that finding varies enormously depending on context. Glycogen in liver cells is normal in small amounts but abnormal when it balloons the cells to twice their usual size. Fungal elements in the outer skin layer are always abnormal. PAS-positive macrophages clogging the small intestine lining point strongly toward Whipple’s disease. Your pathologist and ordering physician interpret the pattern together with your symptoms, imaging, and other lab work to reach a diagnosis.
If PAS staining appears on your pathology report, the specific findings section will describe what was seen: which structures stained, how intensely, and in what pattern. That context, not the stain itself, is what drives the diagnosis.