CT scan findings of probable usual interstitial pneumonitis have a high predictive value for histologic usual interstitial pneumonitis.

BACKGROUND: The current usual interstitial pneumonitis (UIP)/idiopathic pulmonary fibrosis CT scan classification system excludes probable UIP as a diagnostic category. We sought to determine the predictive effect of probable UIP on CT scan on histology and the effect of the promoter polymorphism in MUC5B (rs35705950) on histologic and CT scan UIP diagnosis. METHODS: The cohort included 201 subjects with pulmonary fibrosis who had lung tissue samples obtained within 1 year of chest CT scan. UIP diagnosis on CT scan was categorized as inconsistent with, indeterminate, probable, or definite UIP by two to three pulmonary radiologists. Tissue slides were scored by two expert pulmonary pathologists. All subjects with available DNA (N = 200) were genotyped for rs35705950. RESULTS: The proportion of CT scan diagnoses were as follows: inconsistent with (69 of 201, 34.3%), indeterminate (72 of 201, 35.8%), probable (34 of 201, 16.9%), and definite (26 of 201, 12.9%) UIP. Subjects with probable UIP on CT scan were more likely to have histologic probable/definite UIP than subjects with indeterminate UIP on CT scan (82.4% [28 of 34] vs 54.2% [39 of 72]; P = .01). CT scan and microscopic honeycombing were not associated with each other (P = .76). The minor (T) allele of the MUC5B polymorphism was associated with concordant CT scan and histologic UIP diagnosis (P = .03). CONCLUSIONS: Probable UIP on CT scan is associated with a higher rate of histologic UIP than indeterminate UIP on CT scan suggesting that they are distinct groups and should not be combined into a single CT scan category as currently recommended by guidelines. CT scan and microscopic honeycombing may be dissimilar entities. The T allele at rs35705950 predicts a UIP diagnosis by both chest CT scan and histology.