Prior studies have demonstrated increased IgA in the bronchial secretions of patients with lung cancer. Most of the studies report on immunoglobulin concentrations of bronchial washings. These samples are usually obtained by irrigation followed by rapid aspiration of 10 to 20 ml of saline solution through a major bronchus. One study also found increased IgA in bronchoalveolar lavage specimens obtained from tumor-containing lungs.
In studies to date, elevated slgA levels have been found in patients with bronchoscopically visible disease. Although this observation has enhanced our knowledge of the local humoral immune response to lung cancer, further studies need to be done to determine clinical applicability. Of most importance would be studies aimed at establishing the positive predictive value of measuring BAL slgA in patients whose diagnosis may not be established by standard bronchoscope techniques and in whom the prevalence of cancer is neither extremely high nor low.
One clinical setting in which slgA measurements might be of value, therefore, is in patients with indeterminate pulmonary nodules who do not have endoscopically visible tumors. The likelihood of making a definitive diagnosis using transbronchoscopic or transthoracic biopsy techniques is dependent on nodule size. In nodules 2 cm or smaller, transbronchoscopic biopsy specimens will often fail to provide a diagnosis and transthoracic needle biopsies expose the patient to an increased risk of pneumothorax. The prevalence of malignant neoplasm among patients with solitary pulmonary nodules is dependent on nodule size, patient age, and smoking status. We intentionally chose entry criteria that would recruit a population of patients with a cancer prevalence of approximately 50 percent so as to optimize the predictive value of BAL slgA measurements. If increased slgA concentrations could predict the presence of a malignant neoplasm, thoracotomies might be avoided in patients with benign processes.
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The results of this pilot study suggest that we cannot rely on BAL slgA as an indirect indicator of a malignant neoplasm. The 95 percent confidence interval for the standardized BAL slgA difference between lungs in a cancer patient shows that this difference is not compatible with as much as a doubling of slgA associated with cancer. This conflicts with the findings reported by others and mitigates against clinical application of this measurement.