SERPINA1 and hyperinsulinemic hypoglycemia, familial, 4: Several previous studies have linked BTC expression to COPD, and BTC has been found to be higher in ex-smokers with COPD than those without COPD, has been associated with emphysema in alpha-1 antitrypsin deficiency,35 and has been found to be elevated in acute exacerbations in COPD.36