What are the key diagnostic tests and differences between Type 1 and Type 2 celiac disease, including biopsy assessments and additional investigative tests after esophagogastroduodenoscopy (EGD)?

Diagnostic Tests and Differences Between Type 1 and Type 2 Refractory Celiac Disease

Flow cytometry and immunohistochemistry are the two essential tests that should be ordered on biopsies to differentiate between Type 1 and Type 2 refractory celiac disease, with T-cell receptor rearrangement studies as a critical additional investigative test after EGD.

Biopsy Tests for Differentiating Type 1 and Type 2 Refractory Celiac Disease

Two Key Tests for Biopsies:

1. Flow Cytometry

   • Requires fresh, unfixed specimens placed in RPMI medium or normal saline 1
   • Most accurate method to identify aberrant intraepithelial lymphocytes (IELs) 1
   • Can quantify the percentage of aberrant IELs (critical for Type 2 diagnosis)

2. Immunohistochemistry (IHC)

   • Can be performed on formalin-fixed or fresh unfixed tissue 1
   • Evaluates CD3, CD8, and other surface markers on IELs
   • Helps identify the immunophenotype of the IELs

Key Differences Between Type 1 and Type 2 Refractory Celiac Disease

Feature	Type 1 (RCD1)	Type 2 (RCD2)
IEL Population	Normal IEL phenotype	Aberrant, clonal IEL population
Flow Cytometry	sCD3+, cytCD3+, CD8+, sTCR+	sCD3-, cytCD3+, CD8-, sTCR-
IEL Percentage	Normal distribution	Aberrant IELs ≥20% of total IELs [1]
IHC Findings	CD8+	Typically CD8- (occasionally CD8+)
TCR Gene	Polyclonal TCR rearrangements	Clonal TCR rearrangements
Prognosis	Better prognosis	Poorer prognosis, risk for EATL [1]

Type 1 RCD shows villous atrophy with a normal IEL population similar to conventional celiac disease, while Type 2 RCD is characterized by aberrant clonal T-cell expansion that can progress to enteropathy-associated T-cell lymphoma (EATL) 1.

Additional Investigative Test After EGD

T-cell receptor (TCR) gene rearrangement studies should be ordered as the additional investigative test after EGD 1.

• Performed using polymerase chain reaction (PCR) on biopsy samples
• Detects clonal or monoclonal TCR β or γ gene rearrangements
• Supports the diagnosis of Type 2 RCD when positive
• Important caveat: TCR clonality alone is insufficient for RCD2 diagnosis as clones can be detected in newly diagnosed celiac disease, celiac disease on GFD, and RCD1 1

Clinical Approach to Suspected Refractory Celiac Disease

1. Confirm original celiac disease diagnosis by reviewing prior diagnostic testing 1
2. Exclude ongoing gluten ingestion through serologic testing, dietitian review, and detection of immunogenic peptides in stool or urine 1
3. Perform EGD with multiple biopsies:
   • 4-6 biopsies from distal duodenum
   • 2-3 biopsies specifically for flow cytometry (in RPMI/saline)
   • Additional biopsies for IHC and TCR studies 1
4. Interpret results with hematopathology expertise to accurately classify RCD type 1

Important Considerations

• RCD is rare, occurring in only approximately 1% of celiac disease patients 1
• Consultation with expert hematopathologists is necessary for proper interpretation of these specialized studies 1
• Type 2 RCD has a significantly worse prognosis and requires more aggressive management due to risk of EATL development 1
• False positives can occur with TCR clonality testing alone, so results must be interpreted in conjunction with flow cytometry and IHC findings 1

Remember that these specialized tests should be performed at centers with expertise in diagnosing and managing refractory celiac disease for optimal patient outcomes.