What is the recommended method for diagnosing active Cytomegalovirus (CMV) infection?

Diagnosis of Active CMV Infection

Immunohistochemistry (IHC) on tissue biopsies, possibly combined with tissue PCR, is the essential standard test for diagnosing active CMV infection, particularly in patients with suspected CMV colitis or end-organ disease. 1

Tissue-Based Testing: The Gold Standard

For suspected CMV colitis or end-organ disease, obtain endoscopic biopsies and request IHC with monoclonal antibodies against CMV antigens on all tissue samples. 1, 2, 3

Why IHC is Superior

• IHC has specificity of 92-100% and sensitivity of 78-93%, significantly outperforming conventional H&E staining which has sensitivity of only 10-87% 1, 3
• IHC detects infected cells that appear morphologically normal without the classic "owl's eye" inclusion bodies, capturing cases that H&E staining would miss 1, 2, 3
• IHC is the only method that confirms active tissue invasion versus mere viral presence, which is critical for treatment decisions 1

Tissue PCR as Adjunct

• Tissue PCR (qualitative or quantitative) can be performed alongside IHC to increase diagnostic yield, though the clinical significance of positive PCR without histologic evidence of infection remains unclear 1
• No standardized cut-off values exist for tissue PCR, and quantitative thresholds have not been established for distinguishing infection from disease 1

Blood-Based Testing: Limited Role

Blood-based tests (serum PCR, pp65 antigenemia) have poor sensitivity for diagnosing CMV colitis and should NOT be used as primary diagnostic tools for end-organ disease. 1

Performance Characteristics of Blood Tests

• Blood PCR and pp65 antigenemia have pooled sensitivity of only 50.8% for detecting CMV colitis (39.7% for pp65, 60% for blood PCR), though specificity is high at 99.9% 1
• Blood tests may be completely negative in patients with active CMV colitis, as viremia does not reliably correlate with tissue disease 1
• Qualitative PCR has 100% sensitivity but only 72% specificity, leading to false positives that represent viral shedding rather than disease 4

When Blood Tests May Be Useful

• Consider blood-based testing as an adjunct when contemplating cessation of immunosuppressive therapy or to assess for systemic dissemination 1
• In transplant recipients, quantitative blood PCR with established cut-offs (e.g., >1330 copies/mL) can predict progression to disease with sensitivity 87% and specificity 98% 5, 6

Serology: Not for Active Infection

CMV serology (IgM/IgG) should NOT be used to diagnose active infection in adults. 1

• High seroprevalence in adults (>90%) means IgG positivity only indicates past exposure, not active disease 1
• CMV IgM has limited value and may represent false positives, cross-reactivity with EBV, or persistent antibodies from remote infection 7
• Serology is only useful for identifying at-risk patients (IgG-positive) who could develop reactivation under immunosuppression 1

Clinical Context for Testing

When to Test for CMV

Test all patients with steroid-refractory or severe acute colitis, particularly those on immunosuppressive therapy. 1

• CMV colitis prevalence ranges from 10-30% in steroid-refractory acute colitis and is associated with worse outcomes including toxic megacolon and colectomy 1
• Risk factors include: refractory disease (OR 4.24), immunosuppressive agents like azathioprine (OR 1.95), anti-TNF therapy (OR 11.13), and age >30 years 1

Biopsy Technique Matters

Obtain multiple biopsies from both ulcer bases (higher yield for CMV) and ulcer edges (higher yield for HSV) during endoscopy 3

Diagnostic Algorithm

1. For suspected CMV colitis in immunocompromised patients with refractory disease:

   • Obtain endoscopic biopsies from affected tissue 1, 2
   • Request IHC with CMV monoclonal antibodies on all samples 1, 2, 3
   • Consider tissue PCR as adjunct if IHC is negative but clinical suspicion remains high 1

2. Report semi-quantitatively:

   • Number of CMV-positive cells 2
   • Number of biopsy fragments with CMV 2

3. Correlate with blood tests only after tissue diagnosis:

   • Blood PCR or antigenemia can help distinguish localized from systemic disease 2, 3
   • Resolution of viremia may guide treatment duration 1

Critical Pitfalls to Avoid

• Do not rely on blood PCR alone for diagnosing CMV colitis – it will miss >40% of cases 1
• Do not confuse viral detection (PCR positive) with invasive disease – only tissue confirmation with IHC establishes true CMV disease requiring treatment 3, 7
• Do not use IgM serology to diagnose active infection – it lacks specificity and may be falsely positive 7
• Do not assume negative H&E staining excludes CMV – IHC must be performed as it detects infected cells without classic inclusion bodies 1, 2, 3