From the Guidelines
A predominance of monocytes on a bronchoalveolar lavage (BAL) is not a typical finding in healthy individuals, as evidenced by the normal values for BAL total and differential cell counts provided in the study by 1, which shows that alveolar macrophages (AM) are the predominant cell type in the BAL fluid of healthy adults. This finding suggests an ongoing inflammatory process where monocytes have been recruited to the lung tissue and are differentiating into macrophages to address the underlying issue. Common conditions associated with an increased proportion of monocytes or macrophages in the BAL include chronic infections (particularly fungal or mycobacterial), drug-induced pneumonitis, organizing pneumonia, hypersensitivity pneumonitis in subacute phases, and certain interstitial lung diseases. This pattern differs from neutrophil predominance (seen in acute infections) or lymphocyte predominance (seen in sarcoidosis or hypersensitivity pneumonitis). When interpreting a monocyte-predominant BAL, clinicians should correlate these findings with the patient's clinical presentation, medication history, environmental exposures, and other diagnostic tests such as chest imaging and pulmonary function tests, as recommended by 1. Additional BAL studies including cultures, cytology, and special stains may be necessary to determine the specific cause of the inflammation. Some key points to consider when evaluating a BAL with a predominance of monocytes include:
- The clinical context in which the BAL was performed, including the patient's symptoms, medical history, and potential exposures to infectious agents or environmental toxins.
- The results of other diagnostic tests, such as chest imaging and pulmonary function tests, which can help to narrow the differential diagnosis.
- The potential need for additional testing, such as cultures or cytology, to further characterize the inflammatory process and identify a specific cause. Treatment will depend on identifying and addressing the underlying cause rather than the monocyte finding itself. It is essential to consider the guidelines provided by 1 for the clinical utility of BAL cellular analysis in interstitial lung disease, which recommend performing a differential cell count on the BAL fluid, including lymphocyte, neutrophil, eosinophil, and mast cell counts, and using the remaining sample for microbiological, virological, and/or malignant cell cytology laboratory testing, if clinically indicated.
From the Research
Definition of Predominantly Monocytes on a BAL
Having predominantly monocytes on a bronchoalveolar lavage (BAL) refers to an increased number of monocyte-like cells in the BAL fluid.
- This can be an indication of chronic inflammatory lung disease, as seen in studies 2, 3.
- The presence of a high percentage of monocyte-like cells in BAL fluid has been associated with reduced lung function parameters 2.
Characteristics of Monocyte-Like Cells
Monocyte-like cells in BAL fluid exhibit an immunophenotype intermediate between blood monocytes and mature alveolar macrophages.
- They express monocyte-associated surface antigens such as CD11b and CD14 2, 3.
- The expression of these antigens can be used to distinguish monocyte-like cells from mature alveolar macrophages.
Clinical Implications
An increased number of monocytes in BAL fluid can be seen in various interstitial lung diseases, including sarcoidosis, idiopathic pulmonary fibrosis, and extrinsic allergic alveolitis.
- The phenotype of monocyte-like cells in BAL fluid can vary depending on the underlying disease 3.
- The presence of a unique monocyte population in BAL fluid has been linked to allergic asthma and airway hyper-responsiveness 4.
Diagnostic Approach
The diagnosis of monocytosis, including the evaluation of monocytes in BAL fluid, involves a stepwise approach that includes laboratory data, morphology, clinical findings, and ancillary studies 5, 6.
- Flow cytometry studies can be used to distinguish between different etiologies of monocytosis, including chronic myelomonocytic leukemia and reactive or neoplastic causes 5.