Congo Red Staining for Amyloidosis in Ascites Fluid
Congo red staining is the gold standard for detecting amyloidosis in ascites fluid, with positive samples showing characteristic apple-green birefringence under polarized light. 1
Diagnostic Approach for Amyloidosis in Ascites Fluid
- Congo red staining is the primary stain used to detect amyloid deposits in ascites fluid and other tissue samples 1
- Positive Congo red staining must demonstrate apple-green birefringence when viewed under polarized light to confirm the presence of amyloid 2, 3
- After confirming amyloid presence with Congo red, further typing is essential to determine the specific type of amyloidosis (AL, ATTR, etc.) 1, 2
Optimizing Congo Red Staining Technique
- For optimal results, tissue sections should be 8μm thick, as this improves visualization of small amyloid deposits 4
- The staining solution should be fully saturated with sodium chloride (NaCl) to increase specificity for amyloid and reduce false positives 4
- Examination under both conventional light microscopy and polarized light is necessary for accurate diagnosis 5
Enhanced Detection Methods
- Fluorescence microscopy using a Texas red filter significantly improves the diagnostic yield when examining Congo red-stained samples 6
- Using a metallurgical polarized microscope with strain-free optics and matching polarizers increases sensitivity for identifying amyloid material 5
- Congo red/immunohistochemical double staining on a single slide can improve diagnostic accuracy by allowing focused evaluation of Congo red-positive areas 7
After Congo Red Staining: Confirming Amyloid Type
- Mass spectrometry-based analysis (LC-MS/MS) is the gold standard for amyloid typing with 88% sensitivity and 96% specificity 2
- Immunohistochemistry can be used to identify the precursor protein type if mass spectrometry is not available 2, 3
- For suspected AL amyloidosis, additional testing should include serum free light chain assay, serum and urine immunofixation electrophoresis, and bone marrow biopsy 1
Avoiding Pitfalls in Amyloid Detection
- False positives can occur with heat artifacts due to cautery, which may show Congo red positivity and apple-green birefringence 8
- Plastic coverslips can interfere with polarized light examination and should be avoided 5
- Congo red staining with only 50% saturation of NaCl can cause collagen to show green birefringence, making differentiation from amyloid difficult 4
- Negative Congo red staining does not completely exclude amyloidosis, especially with small deposits that may be missed by conventional microscopy 6