Raised levels of anti-glucose-6-phosphate isomerase IgG in serum and synovial fluid from patients with inflammatory arthritis

M Schaller; W Stohl; S M Tan; V M Benoit; D M Hilbert; H J Ditzel

doi:10.1136/ard.2004.025502

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Raised levels of anti-glucose-6-phosphate isomerase IgG in serum and synovial fluid from patients with inflammatory arthritis

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Re: Raised levels of anti-glucose-6-phosphate isomerase IgG in serum and synovial fluid from patient
Mart Mannik
Published on: 13 April 2016

Published on: 13 April 2016
Re: Raised levels of anti-glucose-6-phosphate isomerase IgG in serum and synovial fluid from patient
- Mart Mannik, Professor Emeritus of Medicine
Dear Editor
Methodological problems in the detection of IgG antibodies to glucose -6-phosphate isomerase in serum and synovial fluid from patients with inflammatory arthritis.
Examination of the methods used by M Schaller et al in their article on the detection of IgG antibodies to glucose-6-phosphate isomerase (GPI) in patients with inflammatory arthritis (1) revealed serious deficiencies.
The a...
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Dear Editor
Methodological problems in the detection of IgG antibodies to glucose -6-phosphate isomerase in serum and synovial fluid from patients with inflammatory arthritis.
Examination of the methods used by M Schaller et al in their article on the detection of IgG antibodies to glucose-6-phosphate isomerase (GPI) in patients with inflammatory arthritis (1) revealed serious deficiencies.
The authors expressed the results in micrograms/ml of IgG antibodies to GPI in the studied serums and synovial fluids. To calculate the micrograms/ml of antibodies to GPI from the measured optical densities, the authors used the linear range of a standard curve on the same ELISA plate. The standard curve was constructed by coating the wells with serial dilutions of a human serum with known concentrations of total IgG, ranging from 0.02 to 5 micrograms/ml in the dilutions. The optical densities measured for the serial dilutions of the standard serum were plotted against the amount of IgG in the dilution for each data point. Unfortunately, the authors used the whole serum with many other proteins which competed for binding to the wells of the ELISA plate. Thus, the amount of IgG that actually bound to the wells was not known. Using the calculated amount of IgG in the used dilutions of serum for the construction of the standard curve is wrong and renders the reported data useless. As an example, if only a tenth of the IgG present in the diluted serum bound to the wells, the calculated amount of the antibodies to GPI would be one tenth of the reported values. The same problem renders the calculations of IgG subclass antibodies to GPI useless.
Another concern is that using serum and synovial fluid specimens the authors did not use a blank for each specimen. The use of a blank is essential for testing the presence of any antibodies in the serum or synovial fluid of patients with RA or with other autoimmune diseases owing to the possible nonspecific binding of IgG or of immune complexes containing IgG to the wells or to the blocking agent used. This point is well illustrated by the authors in Figure 1A. The binding of IgG from one patient with RA to rabbit or human GPI is only a little higher than IgG binding to bovine serum albumin, which was used as a blocking agent in all ELISA wells.
In conclusion, the reported quantities of IgG antibodies to GPI in the tested serum and synovial fluid specimens are wrong.
Reference
1. Schaller M, Stohl W, Tan S M, Benoit V M, Hilbert D M, Ditzel H J. Raised levels of anti-glucose-6-phosphate isomerase IgG in serum and synovial fluid from patients with inflammatory arthritis. Ann Rheum Dis; 64:743-749.
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