A 43-year-old woman complains of gradually worsening dyspnea over the past year. She smokes 1 pack of cigarettes a day. She is trying to “cut back,” because her father, also a smoker, died at age 52 of emphysema. She works as an equestrian riding instructor, often with exposure to animals and hay, but has not noticed exacerbation of symptoms while at work. She has 3 healthy children, one of whom has childhood asthma. On examination, she is comfortable at rest. Her O2 saturation is 93%. She has no basilar crackles or wheezing, but her breath sounds are distant. Chest x-ray shows hyperexpansion especially prominent in the lung bases. Spirometry reveals FEV1 of 46% of predicted but near normal forced vital capacity (FVC). The ratio of FEV1 to FVC is 52%. In addition to advice about smoking cessation, what study would be most important to obtain?
A. Sweat chlorideThis woman has COPD (chronic symptoms, obstructive defect on spirometry) at age younger than 45. Early-onset symptoms, even in a smoker, coupled with a positive family history, should raise the possibility of alpha-1 antitrypsin (AAT) deficiency, and a serum AAT level should be ordered. If it is low, a phenotype assay will confirm the abnormal gene product. AAT deficiency tends to cause more prominent alveolar destruction in the lower lung zones, as opposed to usual smoker’s emphysema, which has an upper lobe predominance. Diagnosing AAT deficiency would be important for her family members. In addition, infusion of pooled human AAT, although quite expensive, can raise AAT levels and probably slows progression of the disease. Cystic fibrosis, which is diagnosed by the sweat chloride level, can present with lung disease in adulthood. However, this woman’s lack of cough and sputum production, as well as her normal fertility, makes this a less likely diagnosis than AAT deficiency. Diffusing capacity will be low in any cause of emphysema, and CT scanning will confirm bullous changes, but neither is recommended in the routine management of COPD. High-resolution CT scanning is used in the diagnosis of interstitial, not obstructive, lung disease. This woman has a history of exposure to organic compounds known to cause hypersensitivity pneumonitis, but her lack of symptoms during or soon after exposure, as well as the absence of patchy infiltrates on CXR, makes this diagnosis less likely. Many agricultural workers have immunoprecipitins to thermophilic actinomycetes. In the absence of convincing history, these results are nonspecific.