Reply: Further Progress in Understanding Fibrosing Mediastinitis

From the Authors:

We thank Dr. Strock and colleagues for their interest in our research letter (1).

We agree that IgG4-related disease (IgG4-RD) should be considered in the differential diagnosis of fibrosing mediastinitis (FM) (2). It is not surprising to find elevated IgG4 levels in a small subgroup of patients with FM. IgG4-RD likely only represents a small subgroup of FM (20%), and serum IgG4 levels are known to be normal in 20% of patients with IgG4-RD (2, 3). Normal IgG4 levels can be found in the presence of characteristic tissue changes, as pointed out by Dr. Strock and colleagues. In our clinical practice, we have not yet identified a patient presenting with FM and an elevated IgG4 level, despite measuring IgG4 levels regularly. In contrast, we have encountered patients with a clinical and histological diagnosis of IgG4-RD and normal IgG4 levels. All patients in our rituximab-treated series of FM had normal IgG4 levels (1).

In summary, we agree with Dr. Strock and colleagues that routine measurements of serum IgG4 may add little to the diagnostic workup of patients with FM and that more data regarding the relationship between and FM and IgG4-RD is needed, particularly as rituximab has also been shown to be effective in IgG4-RD (4).