dsDNA Ab in serum

Introduction

Anti-dsDNA

Indications

• positive with systemic lupus erythematosus (60-70%)

Reference interval

• normal: negative for antibodies to dsDNA [< 100 IU/mL]

Principle

Calf thymus DNA is highly purified under conditions which maintain the antigen in its native state. This antigen is then bound to polystyrene microwells & stabilized for extended shelf life. Diluted patient sera are placed in the microwells & incubated. If anti-dsDNA antibodies are present, they will bind to the antigen in the microwell. The microwells are then washed to remove residual sample & a second incubation with anti-human IgG & IgM conjugated to alkaline phosphatase is carried out. The conjugate will bind immunologically to the anti-dsDNA IgG and/or IgM portion of the bound antigen-antibody complex, forming a 'sandwich' consisting of:

Conjugate (Enzyme-labeled Anti-human IgG & IgM)

Human anti-dsDNA (IgG and/or IgM)

Well Coated with dsDNA

Unbound conjugate is removed in the subsequent washing step. Enzyme substrate is then added to the microwell & if bound conjugate is present, the colorless substrate (p-nitrophenyl phosphate), will be hydrolyzed to form a yellow end product, (p-nitrophenol). The reaction is then stopped & the color fixed. The intensity of the color is measured photometrically at 405 nm & is proportional to the concentration of anti-dsDNA present in the patient sample.

Clinical significance

• 60% sensitivity, >95% specificity for SLE*
• found in patients with more severe disease, especially renal disease
• antibody levels follow disease activity, thus useful for monitoring disease severity

* Crithidia IFA or Farr assays more specific than ELISA

This test is intended for the evaluation of sera for the presence of IgG & IgM antibodies to double-stranded (ds) DNA. In addition this test may be used to follow the level of antibodies to dsDNA in individual patients during the treatment & remission of SLE. Antibodies to dsDNA correlated with renal disease in SLE.

Antibodies to dsDna occur in at least 60-70% of SLE patients & there is considerable evidence to implicate immune complexes containing anti-DNA & DNA in the pathogenesis of SLE. The presence or absence of anti-dsDNA antibodies should not be used as the sole criterion for the diagnosis of SLE.

Low levels of anti-dsDNA antibodies may occur in other connective tissue diseases (i.e. 0-5% of patients with rheumatoid arthritis) & may occur at a very low frequency (2-3%) in individuals without any symptoms of connective tissue disease. It has also been reported that the appearance of anti-dsDNA in patients with connective tissue disease can occur prior to the development of the complete clinical pattern.

Specimen

Serum is separated from the clot & refrigerated, 2-8 degrees C for short term storage or stored frozen, -20 degrees C, for long term storage. Avoid freeze-thaw cycles.CAUTION: Serum samples should not be heat inactivated, as this may cause false positive results.

No special patient preparation required.

Interpretation

• < 100 IU/mL: negative for antibodies to dsDNA.
• 100-300 IU/mL:
  • borderline for antibodies to dsDNA.
  • Possible SLE or other connective tissue disease (CTD).
  • Other testing is required for differential diagnosis.
  • See limitations & clinical indications.
• 300-800 IU/mL:
  • Moderately positive for antibodies to dsDNA.
  • Probable SLE, however other CTD'S cannot be ruled out by this result alone.
  • Other testing is required for differential diagnosis.
  • See limitations & clinical indications.
• > 800 IU/mL: Highly positive for antibodies to dsDNA. Virtually diagnostic for SLE.

More general terms

• dsDNA Ab in body fluid

Component of

• connective tissue disease panel; collagen disease panel; SLE panel