Assessing the Usefulness of Anticardiolipin Antibody Assays
Assessing the Usefulness of Anticardiolipin Antibody Assays
An anticardiolipin antibody (ACA) assay has become a laboratory standard for the detection of antiphospholipid antibodies. We evaluated data from a quality assurance program to assess ACA assay usefulness. Cross-laboratory (n = 56) testing of 12 serum samples yielded interlaboratory coefficients of variation (CVs) for IgG ACA and IgM ACA that were higher than 50% in 17 (71%) of 24 cases. The situation for testing consensus was of equal concern. Total consensus occurred in 6 (25%) of 24 cases. General consensus (interlaboratory agreement, 90% or more) was obtained in 10 (42%) of 24 cases. In the majority of test cases, laboratories could not agree on whether a serum sample tested was ACA-positive or ACA-negative. Differing method-related issues also were evident; some methods tended toward higher or lower ACA values. Method-based majority consensus differed from participant majority consensus on many test occasions. Exceedingly high interlaboratory result variation, combined with a general lack of test result grading consensus and method-based variation, indicate that a cautious clinical approach toward laboratory findings is prudent. Laboratory tests should be repeated at least once before making a clinical diagnosis of any anti-phospholipid syndrome-like disorder.
Largely because of the assortment of disorders with which they are reportedly associated, antibodies that bind phospholipids (antiphospholipid antibodies [APAs]) are a focus of major interest to clinical and laboratory personnel across a variety of disciplines, including hematology, immunology, rheumatology, and obstetrics. It was the initial association of lupus anticoagulants (LAs; evaluated by a clotting-based test and believed to be a laboratory manifestation of APAs) with thrombosis and recurrent fetal loss that first drew the attention of hematology and obstetrics workers (reviewed by Harris and Field et al). Immunologists and rheumatologists also became interested when it was shown that many LA-positive patients had systemic lupus erythematosus. The introduction of a solid-phase assay using cardiolipin as the test phospholipid soon saw the anticardiolipin antibody (ACA) assay become a laboratory standard for the detection of APAs. One reason for wide adoption of the ACA assay was the ability to use it to screen large numbers of serum samples from patients with a variety of clinical disorders. The initial obstetric observation of recurrent fetal loss associated with LAs was followed by later studies showing fetal death and growth retardation in women with ACAs (either with or without systemic lupus erythematosus). More recently, reported association of ACA with stroke and epilepsy has attracted the attention of neurologists and the reported association of skin ulceration, the interest of dermatologists.
In affected patients, ACAs (or APAs) comprise a heterogeneous group of autoantibodies that may include IgG, IgM, and IgA immunoglobulin classes. A number of studies have attempted to evaluate the relative association, or clinical relevance in affected patients, of the presence or absence of these different ACA immunoglobulins and their titers, if present, but results have conflicted. For example, the suggestion that the titer and isotype do not predict the risk of thrombosis is disputed by another group of workers who propose that thrombosis is more likely in the presence of IgG (compared with IgM or IgA) ACA and/or higher antibody titers. It is likely that study discrepancies are due in part to the relatively high incidence of false-positive ACA results in otherwise healthy people and to the difficulties associated with assay standardization. It is generally accepted, however, that the singular presence of ACA activity (in the absence of any other clinical or laboratory marker) is insufficient to diagnose an antiphospholipid syndrome (APS) or an APS-like or APS-related syndrome.
Although originally a radioimmunoassay, the solid-phase ACA assay is now performed most commonly as an enzyme-linked immunosorbent assay. Within Australasia, a variety of reported methods are in use, including 8 different commercial kits and various in-house methods. Irrespective of method, however, it is common practice to attempt some form of standardization in reported ACA activities, with the use of ACA-positive and ACA-negative standards of "known titer" and the terminology originated by Harris et al. In an earlier report from our laboratory, data from 2 external quality assurance programs (QAPs) were used to assess the usefulness of the ACA assay. In the present report, we expand on these findings using more recent and extensive data from 1 large QAP. Our findings indicate that despite attempts at assay standardization, an exceedingly high interlaboratory variation, a general lack of test result consensus, and method-based variations all contribute to generate an assay of limited clinical usefulness.
An anticardiolipin antibody (ACA) assay has become a laboratory standard for the detection of antiphospholipid antibodies. We evaluated data from a quality assurance program to assess ACA assay usefulness. Cross-laboratory (n = 56) testing of 12 serum samples yielded interlaboratory coefficients of variation (CVs) for IgG ACA and IgM ACA that were higher than 50% in 17 (71%) of 24 cases. The situation for testing consensus was of equal concern. Total consensus occurred in 6 (25%) of 24 cases. General consensus (interlaboratory agreement, 90% or more) was obtained in 10 (42%) of 24 cases. In the majority of test cases, laboratories could not agree on whether a serum sample tested was ACA-positive or ACA-negative. Differing method-related issues also were evident; some methods tended toward higher or lower ACA values. Method-based majority consensus differed from participant majority consensus on many test occasions. Exceedingly high interlaboratory result variation, combined with a general lack of test result grading consensus and method-based variation, indicate that a cautious clinical approach toward laboratory findings is prudent. Laboratory tests should be repeated at least once before making a clinical diagnosis of any anti-phospholipid syndrome-like disorder.
Largely because of the assortment of disorders with which they are reportedly associated, antibodies that bind phospholipids (antiphospholipid antibodies [APAs]) are a focus of major interest to clinical and laboratory personnel across a variety of disciplines, including hematology, immunology, rheumatology, and obstetrics. It was the initial association of lupus anticoagulants (LAs; evaluated by a clotting-based test and believed to be a laboratory manifestation of APAs) with thrombosis and recurrent fetal loss that first drew the attention of hematology and obstetrics workers (reviewed by Harris and Field et al). Immunologists and rheumatologists also became interested when it was shown that many LA-positive patients had systemic lupus erythematosus. The introduction of a solid-phase assay using cardiolipin as the test phospholipid soon saw the anticardiolipin antibody (ACA) assay become a laboratory standard for the detection of APAs. One reason for wide adoption of the ACA assay was the ability to use it to screen large numbers of serum samples from patients with a variety of clinical disorders. The initial obstetric observation of recurrent fetal loss associated with LAs was followed by later studies showing fetal death and growth retardation in women with ACAs (either with or without systemic lupus erythematosus). More recently, reported association of ACA with stroke and epilepsy has attracted the attention of neurologists and the reported association of skin ulceration, the interest of dermatologists.
In affected patients, ACAs (or APAs) comprise a heterogeneous group of autoantibodies that may include IgG, IgM, and IgA immunoglobulin classes. A number of studies have attempted to evaluate the relative association, or clinical relevance in affected patients, of the presence or absence of these different ACA immunoglobulins and their titers, if present, but results have conflicted. For example, the suggestion that the titer and isotype do not predict the risk of thrombosis is disputed by another group of workers who propose that thrombosis is more likely in the presence of IgG (compared with IgM or IgA) ACA and/or higher antibody titers. It is likely that study discrepancies are due in part to the relatively high incidence of false-positive ACA results in otherwise healthy people and to the difficulties associated with assay standardization. It is generally accepted, however, that the singular presence of ACA activity (in the absence of any other clinical or laboratory marker) is insufficient to diagnose an antiphospholipid syndrome (APS) or an APS-like or APS-related syndrome.
Although originally a radioimmunoassay, the solid-phase ACA assay is now performed most commonly as an enzyme-linked immunosorbent assay. Within Australasia, a variety of reported methods are in use, including 8 different commercial kits and various in-house methods. Irrespective of method, however, it is common practice to attempt some form of standardization in reported ACA activities, with the use of ACA-positive and ACA-negative standards of "known titer" and the terminology originated by Harris et al. In an earlier report from our laboratory, data from 2 external quality assurance programs (QAPs) were used to assess the usefulness of the ACA assay. In the present report, we expand on these findings using more recent and extensive data from 1 large QAP. Our findings indicate that despite attempts at assay standardization, an exceedingly high interlaboratory variation, a general lack of test result consensus, and method-based variations all contribute to generate an assay of limited clinical usefulness.
