7. What laboratory studies should you order to help make the diagnosis, and how should you interpret the results?
No blood test is diagnostic of COPD or useful in excluding it. Similarly, no blood or sputum test is helpful in tailoring or adjusting existing therapy. Studies that look for possible candidates as biomarkers of disease progression are listed in the bibliography. Currently, clinical judgment supersedes existing available laboratory studies in establishing the diagnosis.
If obstructive lung disease is present in the clinical setting of bronchiectasis, immunoglobulin levels, aspergillus serology, A1AT levels, and possibly sweat chloride or rheumatoid factor testing may be indicated as ruling out entities other than cigarette smoking as the predominant cause of obstruction and bronchiectasis. Sputum for non-tuberculous mycobacteria (NTM) may be useful in patients with bronchiectasis and a productive cough; if clinical suspicion of NTM is high, direct sampling with bronchoscopy may be necessary to exclude the diagnosis.
Symptoms of right heart failure in mild or moderate COPD might suggest concomitant obstructive sleep apnea (overlap syndrome). Right heart failure occurring in more severe COPD may represent pulmonary hypertension. A resting room air arterial blood gas can be performed, and if an elevated PaCO2 is present, symptom-guided additional studies are warranted. Given the high prevalence of cardiovascular disease in patients with COPD, symptoms of right heart failure may also be due to left heart failure. A normal PaCO2 in this setting may suggest need for further cardiac work-up. Resting hypoxemia and hypercarbia are independent prognostic factors suggestive of shorter survival in COPD.
The presence of hypoxemia in the setting of severe obstructive lung disease is unpredictable, and the need for supplemental oxygen in severe obstructive lung disease should be considered at baseline as well as when any significant change in clinical status occurs.
What imaging studies will be helpful in making or excluding the diagnosis of COPD?
Many infectious causes of symptoms similar to those in COPD can be evaluated with a simple chest radiograph (e.g., to rule-out active tuberculosis). However, so-called "classic" radiographic findings of COPD, including increased anterior-posterior diameter and flattened diaphragms, are not specific to the disorder. Many of these changes are seen with normal aging.
Although a CT scan may be useful in ruling out the presence of other lung diseases, the clinical relevance of a radiologic description of mild emphysema on CT is unclear since age-appropriate normals are not available. A chest CT scan reading of emphysema should trigger pulmonary function testing rather than establishment of a diagnosis.
If interstitial lung disease or bronchiectasis is clinically suspected, a thin-section, high-resolution chest CT scan is indicated. Mild bronchiectasis may be seen in both A1AT deficiency and smoking-related COPD. However, moderate or severe bronchiectasis warrants additional evaluation for infectious and non-infectious causes other than COPD.
Since most subjects who undergo evaluation for COPD have an increased risk for lung cancer, based on current available data, a "low dose" cancer screening CT scan may be justified. The National Lung Screening Trial (NLST) demonstrated a survival benefit of annual lung CT scans in a population of subjects who were forty-five years of age or older, had a thirty-pack-year or greater smoking history, and who were within five years of quitting smoking or were still actively smoking.
Although small inflammatory nodules are commonly noted on CT scans performed in smokers and in those living in areas where histoplasmosis is endemic, the NLST included sufficient numbers of subjects in these categories to justify the additional, potentially invasive procedures that will be generated by false-positive scans.
Reposted with permission from Decision Support in Medicine, LLC.