*Purpose: Acute cellular rejection (ACR) in lung transplantation is defined by perivascular and interstitial mononuclear infiltrates on lung allograft pathology. Procuring a good quality transbronchial biopsy (TBB) sample is essential for assessment and generally requires at least five well-expanded alveolated lung parenchymal fragments from at least three levels of the paraffin block based on ISHLT guidelines. Insufficient TBB samples are graded ‘AX.’ We hypothesized that AX might be due to an anatomic or physiologic underlying pulmonary process, such as airway narrowing, lung fibrosis or hypoxemia requiring early procedure termination, that reduces the yield of biopsies and may, in turn, be associated with a higher risk for CLAD or death.

*Methods: This single-center cohort was drawn from all consecutive adult, first, bilateral lung transplants performed between 1999 and 2015. We reviewed surveillance and for-cause biopsies obtained according to a standard protocol within the first-year post-transplant. All patients who had at least one TBB and who were alive and CLAD-free at 1-year post-transplant were included. Outcomes of patients with at least one biopsy event graded as AX were compared with patients with no AX events. For AX subjects, we computed the percentage of AX biopsies (number of AX biopsies divided by total number of biopsies). Association of any AX or percent AX with time to CLAD or death was assessed using Cox PH models. Multivariable models included age, sex, native lung disease, CMV mismatch and transplant ‘era’ (1999-2008 vs. 2009-2015). Sensitivity analyses evaluated the above associations after excluding patients with less than three biopsies.

*Results: The study cohort consisted of 809 patients with a median number of 6 (IQR 5-6) biopsy events within the first-year post-transplant. 439 patients (54.3%) had at least one AX biopsy and a median percent AX of 20% (IQR 17-33). Median post-1-year time to CLAD and death was 761 (IQR 320-1,587) and 1,200 (IQR 662-2,308) days, respectively. In the multivariable analysis, there was no significant difference in risk for CLAD (HR=1.05, 95% CI 0.87-1.28, P=0.60), or death (HR=1.14, 95% CI 0.92-1.42, P=0.24) between patients with at least one AX biopsy versus none. Among the subjects with any AX, having more than 50% AX biopsies was not associated with either CLAD (HR=0.82, 95% CI 0.54-1.23, P=0.34) or death (HR=1.22, 95% CI 0.82-1.82, P=0.33). Excluding patients with less than three biopsies yielded similar results.

*Conclusions: We demonstrate that AX biopsies are not associated with an increased risk of CLAD or death after lung transplant. This finding suggests that insufficient biopsy tissue is not necessarily related to an underlying anatomical or physiological lung process.

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