A 28-year-old woman with systemic lupus erythematosus has serum tested for anti-dsDNA antibodies by Crithidia luciliae immunofluorescence. The kinetoplast of this organism fluoresces brilliantly. What is the significance of this finding compared with a standard ELISA for anti-dsDNA?
- A Crithidia assay detects only low-avidity anti-dsDNA antibodies which are more specific for lupus nephritis
- B Crithidia assay detects high-avidity anti-dsDNA antibodies; these correlate better with nephritis and disease activity than ELISA ✓
- C ELISA is more specific than Crithidia assay because it uses purified circular dsDNA antigen
- D Crithidia assay can also detect anti-ssDNA antibodies, reducing its specificity for SLE
Correct answer: B. Crithidia assay detects high-avidity anti-dsDNA antibodies; these correlate better with nephritis and disease activity than ELISA
Explanation
The kinetoplast of Crithidia luciliae contains pure circular dsDNA with no histones or other nuclear antigens, so fluorescence specifically detects high-avidity anti-dsDNA antibodies. High-avidity anti-dsDNA correlates strongly with lupus nephritis flares and overall disease activity. ELISA detects both low- and high-avidity antibodies, reducing its specificity for nephritis. The Crithidia assay does not detect anti-ssDNA because ssDNA is absent from the kinetoplast.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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