title: Differential assay reactivity of immunglobulin A anti-ß2glycoprotein I antibodies: implications for the clinical interpretation of antiphospholipid antibody testing.
abstract: OBJECTIVE
  The routine measurement of IgA anticardiolipin (aCL) and IgA anti-β2glycoprotein I (anti-β2GPI) antibodies remain controversial despite several studies demonstrating an association with thromboembolic disease in patients with systemic lupus erythematosus (SLE) and antiphospholipid syndrome (APS). This controversy may be a contributing factor for the current under use of IgA antiphospholipid antibodies. We aimed to investigate the nature of discrepant IgA anti-β2GPI reactivity to help define the diagnostic value of IgA antiphospholipid antibodies.
  
  
  METHODS
  Four sera selected from SLE/APS patients and positive for antiphospholipid antibodies but having discrepant IgA anti-β2GPI reactivity on two commercial assays were studied. IgA antibodies were affinity purified to investigate anti-β2GPI reactivity. Column wash through and eluent fractions were tested on both IgA anti-β2GPI assays. Results were normalized to total protein. Assay conjugates and standards from the discrepant assays were interchanged.
  
  
  RESULTS
  The diseased samples were strongly positive in one assay [144-388 IgA antiphospholipid (APL) units] and negative or weakly positive in another assay (9.9-53 APL units). IgA eluents from IgA anti-β2GPI positive samples reacted 10 times stronger on the reactive assay. When normalized to protein content, the eluents showed no cross-reactivity for IgG or IgM anti-β2GPI antibodies, confirming IgA isotype specificity. Conjugate interchange confirmed that both assays bound IgA anti-β2GPI antibodies, but the anti-IgA conjugate from the reactive assay was 4 times stronger, suggesting that its ability to detect IgA anti-β2GPI antibodies was partially dependent on the anti-IgA conjugate and calibration.
  
  
  CONCLUSIONS
  These results confirm not only the presence of IgA anti-β2GPI antibodies in the selected patient samples but also highlight an IgA conjugate issue for the unreactive assay, causing an underestimation of IgA anti-β2GPI. This finding may assist in the ongoing standardization efforts of APS antibody testing. In addition, conclusions from published clinical studies may need to be revised as some assays may understate IgA significance.
authors.affiliation: Corgenix Medical Corporation, Broomfield, Colorado, USA.
authors.forename: David B
authors.initials: DB
authors.lastname: Hood
authors.affiliation: Corgenix Medical Corporation, Broomfield, Colorado, USA.
authors.forename: Karin R
authors.initials: KR
authors.lastname: Snyder
authors.affiliation: Corgenix Medical Corporation, Broomfield, Colorado, USA.
authors.forename: Tammy R
authors.initials: TR
authors.lastname: Buckner
authors.affiliation: Corgenix Medical Corporation, Broomfield, Colorado, USA.
authors.forename: Beth L
authors.initials: BL
authors.lastname: Hurley
authors.affiliation: Corgenix Medical Corporation, Broomfield, Colorado, USA.
authors.forename: Kelly R
authors.initials: KR
authors.lastname: Pitts
authors.affiliation: Corgenix Medical Corporation, Broomfield, Colorado, USA.
authors.forename: Luis R
authors.initials: LR
authors.lastname: Lopez
keywords: IgA anti-β2 glycoprotein I; Systemic lupus erythematosus; antiphospholipid antibodies; antiphospholipid syndrome