Differences between total IgG and sum of the IgG subclasses in clinical samples

Summary Aims IgG subclasses measurement is used in the investigation of patients with immunodeficiency and autoimmune diseases. In some patients a significant discrepancy between the sum of IgG subclasses (IgGsum) and total IgG may be seen. This study aimed to assess frequency and degree of such discrepancies in routine samples. Methods Data were collected retrospectively from 571 consecutive IgG subclass samples performed by an nephelometric/turbidimetric assay. Total IgG measurement was performed by nephelometry or turbidimetry. Fifty prospective samples with a difference between the IgGsum and total IgG >15% were re-run at dilution. Results IgGsum was a mean of 3.7% higher than the total IgG. Sixty-two samples (10.9%) had a difference between IgGsum and total IgG of >15%. Difference between IgGsum and total IgG correlated with the proportion but not level of IgG1. Repeat testing at dilution of samples with differences >15% did not significantly reduce the difference between results. Conclusions Differences of >15% between IgGsum and total IgG are common. Using an adjusted range based on our data would reduce the number of samples requiring additional testing. Samples falling outside this range should be reviewed.