PURPOSE:We had noted large numbers of PFTs with isolated reductions in diffusing capacity (DLCO). Thus, we studied this incidence while evaluating the associated spectrum of diseases.
METHODS:We prospectively evaluated PFTs with normal spirometry (FEV1 and FVC >80% predicted, FEV1/FVC >70%) over 3-months from 2 pulmonary function labs (County and University Hospitals). Reference DLCOs were compared using CRAPO, a database from natives at a mile high altitude currently used at our hospitals, and Miller, a database from residents across the state of Michigan. Low DLCO was set at <70% predicted for both databases. Imaging, clinical and smoking history, and race (12% lower DLCO for African Americans) of all subjects were considered.
RESULTS:Of the 302 PFTs, CRAPO yielded 139 subjects (46.03%) with isolated reductions in DLCO, 62 non-smokers; while Miller yielded 81 patients (26.82%), 39 non-smokers. The 58 PFTs with normal diffusing capacity using Miller and low DLCO using CRAPO were distributed as follows; 7 with history of sarcoid (2 had abnormal imaging), 5 with evidence of pulmonary emphysema on imaging, 27 with miscellaneous diseases (histoplasmosis, extrapulmonary sarcoid, connective tissue disease or cancer) while 19 had no obvious clinical or radiological disease. After accounting for all variables, there were 11 non-smokers using Crapo and 5 non-smokers using Miller with unexplained low DLCO. No patient who was abnormal by Crapo, but normal by Miller had a reason for an abnormal DLC0 by chart review.
CONCLUSION:A significant percentage of PFTs have isolated reductions in DLCO; 46.03% vs. 26.82% depending on data base. Considering the large number of false positives using CRAPO and the fact that none of the diffusing capacities that were low for CRAPO yet normal for Miller had an identifiable pulmonary pathology, we recommend using Miller reference values in Indianapolis. Race, medical and smoking histories are all helpful in determining the significance of an isolated reduction in DLCO.
CLINICAL IMPLICATIONS:The significant prevalence of isolated reductions in DLCO warrants more reliable reference databases and further evaluation of associated diseases.
DISCLOSURE:Salah Najm, None.