The most likely diagnosis is idiopathic pulmonary fibrosis (IPF). This patient's presentation of progressively worsening cough and dyspnea is typical for this rare disease. He has multiple risk factors, including smoking history, organic dust exposure from his previous work as a carpenter, and current age; the prevalence of IPF increases significantly with each decade of life, and the disease essentially only affects individuals who are older than 50 years. This is important because an individual younger than 40 years who presents with similar symptoms has a significantly higher likelihood of an alternative cause, such as connective tissue disease–related pulmonary disease. Therefore, in younger patients with a similar presentation, more intensive screening for autoimmune disorders may be appropriate. In addition, this patient's high-resolution CT (HRCT) findings are classic for usual interstitial pneumonia (UIP), which is the typical pathologic correlate of IPF. A definite UIP pattern on CT imaging in a patient whose clinical history does not suggest an underlying alternative disorder is consistent with a diagnosis of IPF.
Although this patient is at risk for COPD because of his smoking history, his pulmonary function tests show a normal FEV1/FVC ratio. Clubbing is also noted on physical examination. Although the etiology of clubbing remains unclear, this is more commonly reported in patients with diffuse parenchymal lung disease rather than COPD. Most importantly, CT imaging is not consistent with the findings usually seen with COPD.
Hypersensitivity pneumonitis presents in acute and chronic forms. This patient's presentation is not consistent with acute disease given the lack of flulike symptoms. The chronic form of the disease can present similarly to IPF, but this patient lacks a history of a clear ongoing exposure to a likely offending agent associated with hypersensitivity pneumonitis (such as organic antigens from domestic birds), and his CT findings are basal predominant. Hypersensitivity pneumonitis typically affects the upper and mid-lung zones, and micronodules are often present.
Respiratory bronchiolitis–associated interstitial lung disease is strongly associated with smoking exposure. However, the CT imaging demonstrates mid to upper lung–predominant centrilobular micronodules. Although advanced cases can be associated with septal line thickening and scarring, the distribution on the CT scan is not consistent with this.