en تخصصي Special پژوهشي orginal article <strong><em>Background & Aims</em></strong>: &nbsp;To determine attenuation threshold for detection and quantification of air trapping in obstructive airway disease. Quantify airway dysfunction in patients of obstructive airway disease & its correlation with pulmonary function tests.<br> <strong><em>Materials & Methods:</em></strong> Paired HRCT scans of 48 patients were done and correlated with Pulmonary Function Tests taken within 2 weeks of the study. Threshold attenuation value on expiratory scan that signifies air trapping was obtained by correlating relative volumes with PFT parameters (PEF 25-75% & RV/TLC). The lung volumes at this threshold were then correlated with PFT values signifying airway dysfunction (FEV1, FEV1/FVC and PEF 25-75%) and airway dysfunction was then quantified based on these volumes.<br> <strong><em>Results</em></strong><strong>:</strong> Maximum correlation of PFT parameters signifying air trapping is with relative volume of limited lung at -850HU (l850) (p<0.005) which was taken as the threshold for air trapping. Using this threshold (-850HU), we calculated the relative volume change of limited and whole lung (l850 & t850) and expiratory relative volume of limited and whole lung (ERV l850 & ERV t850). Significant correlation was seen between l850 and PFT parameters signifying airway dysfunction (p<0.005). A severity classification of obstructive airway disease was formulated based on l850 and classified patients into mild (l850<-30%), moderate (l850= -20 - -30%), severe (l850 =-10 - -20%), and very severe (l850 > -10%).<br> <strong><em>Conclusion</em></strong><strong>:</strong> l850 can be used as a CT parameter to quantify airway dysfunction irrespective of presence or absence of emphysema. Severity classification of obstructive airway disease was formulated based on l850. FEV1 Forced Expiratory Volume in 1 sec, FVC Forced Vital Capacity, HRCT High-Resolution Computed Tomography, PEF25-75% Peak Expiratory Flow (25 – 75 %), PFT Pulmonary Function Test, RV Residual Volume 241 251 http://ijrabms.umsu.ac.ir/browse.php?a_code=A-10-135-1&slc_lang=en&sid=1 Omair Shah shahomair133@gmail.com 1003194753284600988 1003194753284600988 Yes sheri kashmir institute of medical sciences Naseer Choh naseerchoh@rediffmail.com 1003194753284600989 1003194753284600989 No sheri kashmir institute of medical sciences Faiz Shera faizthebro@gmail.com 1003194753284600990 1003194753284600990 No sheri kashmir institute of medical sciences Tariq Gojwari tariqgojwari@gmail.com 1003194753284600991 1003194753284600991 No sheri kashmir institute of medical sciences Jan Suhail jansuhailshah1@gmail.com 1003194753284600992 1003194753284600992 No sheri kashmir institute of medical sciences Fahad Shafi shah_omair@yahoo.com 1003194753284600993 1003194753284600993 No sheri kashmir institute of medical sciences Rafi Jan rafiahmedjan@gmail.com 1003194753284600994 1003194753284600994 No sheri kashmir institute of medical sciences