What is Crithidia (Crithidia) dsDNA (double-stranded deoxyribonucleic acid) and how is it interpreted in a clinical diagnosis?

What is Crithidia dsDNA and How to Interpret It

The Crithidia luciliae immunofluorescence test (CLIFT) is a highly specific method for detecting anti-double-stranded DNA (anti-dsDNA) antibodies using the kinetoplast of the hemoflagellate protozoan Crithidia luciliae as substrate, and should be used as a confirmatory test following positive solid-phase assays in the diagnostic workup of systemic lupus erythematosus (SLE). 1

What is the Crithidia Test?

The CLIFT uses Crithidia luciliae, a protozoan parasite, as the substrate for detecting anti-dsDNA antibodies. 2 The key feature is the kinetoplast—a specialized organelle containing a large amount of pure double-stranded DNA in a circular configuration. 2, 3

How it works:

• Patient serum is applied to slides containing fixed Crithidia luciliae organisms 2
• If anti-dsDNA antibodies are present, they bind to the kinetoplast DNA 2
• Fluorescent-labeled anti-human immunoglobulin reveals binding as a characteristic annular (ring-like) fluorescent pattern around the kinetoplast 2, 4

Diagnostic Performance and Clinical Use

Specificity and Sensitivity:

• The CLIFT offers the highest specificity for SLE among anti-dsDNA detection methods, approaching 98-100% in healthy controls and patients without autoimmune disease 1, 3
• Sensitivity is lower than solid-phase assays (SPAs), detecting approximately 42-76% of SLE patients depending on disease activity 2, 3
• Sensitivity is higher in patients with active SLE compared to those in remission 2

Recommended Testing Strategy:

• Anti-dsDNA testing should follow a double-screening strategy: first-line testing with a last-generation solid-phase assay (SPA), followed by CLIFT as the confirmatory test 1
• If the SPA is negative, CLIFT should only be performed when other clinical signs of SLE are present 1
• Without clinical suspicion, a negative SPA result should be reported directly as negative anti-dsDNA 1

Interpretation Algorithm

When both tests are positive (SPA+ and CLIFT+):

• SLE is very likely 1, 5
• This combination provides the highest diagnostic confidence 1

When SPA is positive but CLIFT is negative (SPA+ and CLIFT-):

• Evaluate in the context of clinical characteristics 1
• SLE is possible but less certain 1
• Clinical follow-up is recommended, with repeat testing in 6 months if diagnosis remains unclear 1

When both are negative:

• SLE diagnosis cannot be established based on anti-dsDNA criteria at this time 1
• Does not rule out SLE, as some patients may be seronegative 5

Role in Disease Monitoring

For monitoring established SLE:

• A quantitative assay (such as ELISA/FEIA) should be used to monitor disease activity, preferably using the same method and laboratory used at diagnosis 1
• CLIFT provides only semiquantitative information and should not be the sole monitoring method 1
• CLIFT reinforces clinical decision-making but lacks the precision needed for serial quantitative monitoring 1
• Anti-dsDNA and complement levels should be measured in follow-up even if previously negative 1

Important Caveats and Pitfalls

Technical considerations:

• The CLIFT can be irreproducible if slides are allowed to dry after washing with PBS, causing local increases in salt concentration that affect antibody binding 6
• Careful control of assay conditions is essential for reliable results 6
• Different laboratories may use different methods and cutoffs, affecting interpretation 1

Atypical fluorescence patterns:

• Besides the typical annular kinetoplast staining, atypical images may occur showing fluorescence of the membrane, flagellum, or basal corpuscle 4
• These atypical patterns can result from cross-reactions with antibodies to Trypanosoma cruzi (Chagas disease) or Leishmania species 4
• When atypical images are observed, consider serological testing for Chagas disease and leishmaniasis, particularly in endemic areas 4

Antibody heterogeneity:

• Anti-dsDNA antibodies are heterogeneous and can target various DNA structures including single-stranded DNA, Z-DNA, B-DNA, RNA, and DNA-histone complexes 1, 5
• The kinetoplast may contain antigens other than pure dsDNA, including DNA-histone complexes 7
• Some sera show positive CLIFT due to antihistone antibodies rather than true anti-dsDNA 7

Avidity differences:

• The CLIFT detects both high and low avidity anti-dsDNA antibodies, while the Farr assay (radioimmunoassay) primarily detects high avidity antibodies 6
• This explains why some sera are CLIFT-positive but negative by other methods 6

Clinical context is essential:

• Anti-dsDNA positivity can occur in healthy individuals, other autoimmune diseases, infections (bacterial, viral, parasitic), and malignancies 1, 5
• Results must always be interpreted in conjunction with clinical findings and other laboratory tests 1
• The laboratory should receive relevant clinical information to validate results and provide appropriate recommendations 1