Multidisciplinary Approach to the Diagnosis of Idiopathic Interstitial Pneumonias: Focus on the Pathologist's Key Role.

Idiopathic Interstitial Pneumonias (IIPs) are a heterogeneous group of the broader category of Interstitial Lung Diseases (ILDs), pathologically characterized by the distortion of lung parenchyma by interstitial inflammation and/or fibrosis. The American Thoracic Society (ATS)/European Respiratory Society (ERS) international multidisciplinary consensus classification of the IIPs was published in 2002 and then updated in 2013, with the authors emphasizing the need for a multidisciplinary approach to the diagnosis of IIPs. The histological evaluation of IIPs is challenging, and different types of IIPs are classically associated with specific histopathological patterns. However, morphological overlaps can be observed, and the same histopathological features can be seen in totally different clinical settings. Therefore, the pathologist's aim is to recognize the pathologic-morphologic pattern of disease in this clinical setting, and only after multi-disciplinary evaluation, if there is concordance between clinical and radiological findings, a definitive diagnosis of specific IIP can be established, allowing the optimal clinical-therapeutic management of the patient.

Molecular and immune signatures, and pathological trajectories of fatal COVID-19 lungs defined by in situ spatial single-cell transcriptome analysis.

Despite intensive studies during the last 3 years, the pathology and underlying molecular mechanism of coronavirus disease 2019 (COVID-19) remain poorly defined. In this study, we investigated the spatial single-cell molecular and cellular features of postmortem COVID-19 lung tissues using in situ sequencing (ISS). We detected 10 414 863 transcripts of 221 genes in whole-slide tissues and segmented them into 1 719 459 cells that were mapped to 18 major parenchymal and immune cell types, all of which were infected by SARS-CoV-2. Compared with the non-COVID-19 control, COVID-19 lungs exhibited reduced alveolar cells (ACs) and increased innate and adaptive immune cells. We also identified 19 differentially expressed genes in both infected and uninfected cells across the tissues, which reflected the altered cellular compositions. Spatial analysis of local infection rates revealed regions with high infection rates that were correlated with high cell densities (HIHD). The HIHD regions expressed high levels of SARS-CoV-2 entry-related factors including ACE2, FURIN, TMPRSS2 and NRP1, and co-localized with organizing pneumonia (OP) and lymphocytic and immune infiltration, which exhibited increased ACs and fibroblasts but decreased vascular endothelial cells and epithelial cells, mirroring the tissue damage and wound healing processes. Sparse nonnegative matrix factorization (SNMF) analysis of niche features identified seven signatures that captured structure and immune niches in COVID-19 tissues. Trajectory inference based on immune niche signatures defined two pathological routes. Trajectory A primarily progressed with increased NK cells and granulocytes, likely reflecting the complication of microbial infections. Trajectory B was marked by increased HIHD and OP, possibly accounting for the increased immune infiltration. The OP regions were marked by high numbers of fibroblasts expressing extremely high levels of COL1A1 and COL1A2. Examination of single-cell RNA-seq data (scRNA-seq) from COVID-19 lung tissues and idiopathic pulmonary fibrosis (IPF) identified similar cell populations consisting mainly of myofibroblasts. Immunofluorescence staining revealed the activation of IL6-STAT3 and TGF-ß-SMAD2/3 pathways in these cells, likely mediating the upregulation of COL1A1 and COL1A2 and excessive fibrosis in the lung tissues. Together, this study provides a spatial single-cell atlas of cellular and molecular signatures of fatal COVID-19 lungs, which reveals the complex spatial cellular heterogeneity, organization, and interactions that characterized the COVID-19 lung pathology.

Resolution of an insidious and migratory Mycobacterium tuberculosis-associated secondary organizing pneumonia: a case report and literature review.

BACKGROUND: Organizing pneumonia (OP) is a rare interstitial lung disease. Secondary organizing pneumonia (SOP) caused by Mycobacterium tuberculosis (MTB) is extremely rare. Migratory MTB-associated SOP is more deceptive and dangerous. When insidious tuberculosis (TB) is not recognized, SOP would be misdiagnosed as cryptogenic organizing pneumonia (COP). Use of steroid hormone alone leads to the progression of TB foci or even death. Clues of distinguishing atypical TB at the background of OP is urgently needed. CASE PRESENTATION: A 56-year-old female patient was hospitalized into the local hospital because of cough and expectoration for more than half a month. Her medical history and family history showed no relation to TB or other lung diseases. Community-acquired pneumonia was diagnosed and anti-infection therapy was initialized but invalid. The patient suffered from continuous weigh loss. More puzzling, the lesions were migratory based on the chest computed tomography (CT) images. The patient was then transferred to our hospital. The immunological indexes of infection in blood and pathogenic tests in sputum and the bronchoalveolar lavage fluid were negative. The percutaneous lung puncture biopsy and pathological observation confirmed OP, but without granulomatous lesions. Additionally, pathogen detection of the punctured lung tissues by metagenomics next generation sequencing test (mNGS) were all negative. COP was highly suspected. Fortunately, the targeted next-generation sequencing (tNGS) detected MTB in the punctured lung tissues and MTB-associated SOP was definitely diagnosed. The combined therapy of anti-TB and prednisone was administrated. After treatment for 10 days, the partial lesions were significantly resorbed and the patient was discharged. In the follow-up of half a year, the patient was healthy. CONCLUSIONS: It is difficult to distinguish SOP from COP in clinical practice. Diagnosis of COP must be very cautious. Transient small nodules and cavities in the early lung image are a clue to consider TB, even though all pathogen tests are negative. tNGS is also a powerful tool to detect pathogen, ensuring prompt diagnosis of TB-related SOP. For clinicians in TB high burden countries, we encourage them to keep TB in mind before making a final diagnosis of COP.

A case of drug-induced organizing pneumonia caused by amikacin liposome inhalation suspension.

Inhaled liposomal antimicrobials are known to cause hypersensitivity pneumonitis. Amikacin liposome inhalation suspension (ALIS) is a promising novel antimicrobial agent against refractory Mycobacterium avium complex infections. The frequency of drug-induced lung injury caused by ALIS is relatively high. To date, no reports of ALIS-induced organizing pneumonia diagnosed by bronchoscopy are available. We report a case of a 74-year-old female patient presenting with non-tuberculous mycobacterial pulmonary disease (NTM-PD). She was treated with ALIS for refractory NTM-PD. Fifty-nine days after starting ALIS, the patient developed a cough, and her chest radiographs indicated deterioration. She was diagnosed with organizing pneumonia based on pathological findings of the lung tissues obtained by bronchoscopy. After switching from ALIS to amikacin infusion, her organizing pneumonia improved. It is difficult to distinguish between organizing pneumonia and an exacerbation of NTM-PD based on chest radiography alone. Therefore, it is essential to perform an active bronchoscopy for diagnosis.

Pulmonary syphilis with a cicatricial variant of organizing pneumonia: a case report.

BACKGROUND: Syphilis is a chronic disease that progresses in the primary, secondary, latent, and tertiary stages. Pulmonary manifestations of syphilis are rare, and their histological features have not been well-described. CASE PRESENTATION: A 78-year-old man was referred to our hospital because of a solitary nodular shadow in the right middle lung field on a chest radiograph. Five years prior, a rash appeared on both legs. He was tested for syphilis at a public health center, and the non-treponemal test result was negative. When he was approximately 35 years old, he had unspecified sexual intercourse. Chest computed tomography showed a 13-mm nodule with a cavity in S6 of the right lower lobe of the lung. Robot-assisted resection of the right lower lobe was performed because of suspected localized right lower lobe lung cancer. A cicatricial variant of organizing pneumonia (CiOP) was observed, and immunohistochemistry identified Treponema pallidum inside the macrophages in the nodule cavity. The rapid plasma regain (RPR) value was negative, and the Treponema pallidum hemagglutination assay was positive. The patient was diagnosed as having secondary syphilis with pulmonary involvement. Insidious progression of secondary syphilis may result in CiOP and a negative RPR test result. CONCLUSIONS: We report the first case of pulmonary syphilis with a histological pattern of CiOP. It may be asymptomatic and difficult to diagnose because the RPR test may be negative for a long period of time. When either non-treponemal or treponemal test results are positive, the possibility of pulmonary syphilis should be considered along with appropriate medical treatment.

Cytokine release syndrome after bronchoalveolar lavage.

BACKGROUND: Immunosuppressed bone marrow transplant patients with pulmonary infiltrates routinely undergo bronchoscopy with bronchoalveolar lavage (BAL) to investigate potential etiologies. Cytokine release syndrome after BAL is unreported in the literature in general and in this patient population. CASE PRESENTATION: We report on an allogeneic bone marrow transplant patient with non-infectious organizing pneumonia of the lungs who developed delayed and rapidly progressive shock and hypoxia post-procedure over the course of 12 h resulting in intensive care unit admission for supportive care. BAL was characterized by a marked lymphocytic, cytotoxic T cell infiltrate on pathology and flow cytometry without clear evidence of infection. The patient's clinical status improved quickly only after the initiation of high dose intravenous steroids and returned to baseline as an outpatient. CONCLUSION: The patient's clinical data and course suggest a cytotoxic T cell response from the lung and BAL as the etiology. With an increasing number of cellular therapies for cancer entering the clinic, the potential for unusual but morbid complications from routine bronchoscopy should be considered.

Lenalidomide induced pneumonitis.

INTRODUCTION: Iatrogenic lung injury is a very rare, albeit serious complication with antineoplastic therapy, including immunomodulatory drugs. Pneumonitis typically presents clinically with symptoms such as cough, dyspnea, fever, and hypoxemia. Radiographic evaluation often demonstrates diffuse, patchy infiltrates and ground-glass opacities. CASE REPORT: We present a case in which therapy from an immunomodulatory drug, lenalidomide, elicited a pneumonitis in the form of a 9 cm lung mass. An exhaustive workup was completed to rule out viral, bacterial, and fungal infections as well as malignant causes. Lenalidomide-induced lung injury was suspected. MANAGEMENT AND OUTCOME: Lenalidomide was discontinued and corticosteroid therapy was initiated. This resulted in a complete clinical and radiographic resolution of symptoms. DISCUSSION: Several case reports of pneumonitis have been associated with immunomodulatory drug therapy, and while most of these exhibit diffuse ground-glass opacities radiographically, our patient presented with a 9 cm lung mass. Our findings stress the importance of a thorough medication review while ruling out other potential causes of lung injury.

Organizing Pneumonia in A Patient Double-Positive for ANCA and Anti-GBM Antibodies: A Case Report.

Both anti-glomerular basement membrane (GBM) disease and the anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) are common causes of pulmonary-renal syndrome. Organizing pneumonia (OP), a special pattern of interstitial lung disease, is extremely rare either in AAV or anti-GBM disease. We report an old woman presented with OP on a background of co-presentation with both ANCA and anti-GBM antibodies.

[Myelodysplastic syndrome with der (1;7)(q10;p10) complicated with eosinophilia and organizing pneumonia].

The unbalanced translocation der (1;7)(q10;p10) is a characteristic cytogenetic abnormality observed in myelodysplastic syndrome (MDS). A 63-year-old man presented to our hospital with fever and lung disease. The chromosomal analysis of bone marrow cells showed 46, XY, +1, der (1;7)(q10;p10) in all four metaphases. The patient was diagnosed with MDS. Bronchoscope examination revealed organizing pneumonia. The patient's eosinophil count rose to 39% after 30 days. His fever and dyspnea worsened, and a skin rash (systemic erythema) appeared simultaneously. Therefore, the patient was commenced on azacitidine and corticosteroids. Although treatment with both drugs could control disease progression transiently, the WT-1 value and the percentage of myeloblasts in the patient's bone marrow increased. Therefore, the patient received hematopoietic stem cell transplantation from his haplo-identical donor daughter. Some reports have demonstrated that patients with MDS with der (1;7)(q10;p10) have better prognosis than those with other abnormalities, such as -7/7q-. However, reported cases with severe complications show very poor prognosis. MDS with der (1;7)(q10;p10) complicated by eosinophilia and organizing pneumonia have not been reported, and its prognosis is expected to be very poor. Our case suggests that such cases might quickly require hematopoietic stem cell transplantation before the disease worsens.

A case of acute severe cryptogenic organic pneumonia with secondary hemophilia. / 继发嗜血现象的重症急进型隐源性机化性肺炎1例.

Cryptogenic organic pneumonia (COP) refers to organic pneumonia that has not been identified a clear cause by current medical methods. A small proportion of COP can exhibit severe and progressive characteristics, while severe COP can cause systemic inflammatory storms and can be secondary to hemophilia. This article reported a case of acute severe COP secondary to hemophilia. A 67-year-old male patient was admitted to the hospital due to cough, shortness of breath, and fever. At first, he was misdiagnosed as severe pneumonia, but failed to receive anti infection treatments. Sputum pathogenetic examination and Macrogene testing of alveolar lavage fluid were performed, and no etiology was found to explain the patient's condition. The condition was gradually worsened and hemophilia occurred to explain, suggesting that acute severe COP was relevant. After receiving hormone treatment, the condition gradually relieved and the absorption of lung lesions improved. Hemophilia secondary to COP is rare, and the specific mechanism needs further study.

Prognostic implications of and clinical risk factors for acute lung injury and organizing pneumonia after lung transplantation: Data from a multicenter prospective cohort study.

We determined prognostic implications of acute lung injury (ALI) and organizing pneumonia (OP), including timing relative to transplantation, in a multicenter lung recipient cohort. We sought to understand clinical risks that contribute to development of ALI/OP. We analyzed prospective, histologic diagnoses of ALI and OP in 4786 lung biopsies from 803 adult lung recipients. Univariable Cox regression was used to evaluate the impact of early (≤90 days) or late (>90 days) posttransplant ALI or OP on risk for chronic lung allograft dysfunction (CLAD) or death/retransplantation. These analyses demonstrated late ALI/OP conferred a two- to threefold increase in the hazards of CLAD or death/retransplantation; there was no association between early ALI/OP and these outcomes. To determine risk factors for late ALI/OP, we used univariable Cox models considering donor/recipient characteristics and posttransplant events as candidate risks. Grade 3 primary graft dysfunction, higher degree of donor/recipient human leukocyte antigen mismatch, bacterial or viral respiratory infection, and an early ALI/OP event were significantly associated with increased late ALI/OP risk. These data from a contemporary, multicenter cohort underscore the prognostic implications of ALI/OP on lung recipient outcomes, clarify the importance of the timing of these events, and identify clinical risks to target for ALI/OP prevention.

Secondary organizing pneumonia after recovery of mild COVID-19 infection.

A 36-year-old male with diffuse large B-cell lymphoma on maintenance rituximab therapy presented to the emergency department with high fever and fatigue. A chest X-ray showed a lobar infiltrate, 40 days before admission the patient suffered from a mild coronavirus disease 2019 (COVID-19) infection and fully recovered. PCR nasopharyngeal swab was negative for COVID-19. Comprehensive biochemical, radiological, and pathological evaluation including 18-fluorodeoxyglucose positron emission tomography with computed tomography and transbronchial lung biopsy found no pathogen or lymphoma recurrence. Treatment for pneumonia with antibiotic and antifungal agents was nonbeneficial. A diagnosis of secondary organizing pneumonia (OP) was made after pneumonia migration and a rapid response to corticosteroids. OP secondary to a viral respiratory infection has been well described. Raising awareness for post-COVID-19 OP has therapeutic and prognostic importance because those patients benefit from steroid therapy. We believe the condition described here is underdiagnosed and undertreated by doctors worldwide. Because of the ongoing global pandemic we are now encountering a new kind of patient, patients that have recovered from COVID-19. We hope that this case may contribute to gaining more knowledge about this growing patient population.

Significance of alveolar nitric oxide concentration in the airway of patients with organizing pneumonia after allogeneic hematopoietic stem cell transplantation.

Organizing pneumonia (OP) is a complication of allogeneic hematopoietic stem cell transplantation (allo-HSCT) and a manifestation of peripheral airway/alveolar inflammation. Recently, alveolar nitric oxide concentration (Calv) has been revealed as a noninvasive marker of peripheral airway inflammation; however, whether Calv levels are associated with OP and peripheral airway in patients after allo-HSCT remains unclear. Herein, we evaluated whether Calv levels could reflect the presence of OP and structural airway changes in patients after allo-HSCT. We measured the eNO levels of 38 patients (6 with OP and 32 without OP) who underwent allo-HSCT. Three-dimensional computed tomography (CT) analysis of the airway was performed in 19 patients. We found that in patients with OP, Calv levels were significantly higher than in those without OP (10.6 vs. 5.5 ppb, p < 0.01). Receiver-operating characteristic analyses revealed a Calv cut-off value for OP detection of 10.2 ppb. No significant differences in the patient characteristics, except for the presence of OP (p < 0.01), were noted between the two groups stratified by the Calv cut-off value. Three-dimensional CT images of the airway revealed gradually increasing positive correlations between Calv levels and airway wall area of the third-, fourth-, and fifth-generation bronchi (r = 0.20, 0.31, 0.38; p = 0.42, 0.19, 0.038, respectively), indicating that Calv levels are strongly correlated with the wall thickness of the distal bronchi. Our results suggest that the Calv level may be a useful noninvasive detectable marker for OP after an allo-HSCT.

Organizing pneumonia as a possible pulmonary manifestation of systemic lupus erythematosus: Three cases and a review of literature.

Background: Organizing pneumonia (OP) is a rare manifestation of systemic lupus erythematosus (SLE). It has been described in very few case reports.Purpose and Methods: We encountered OP in three patients with SLE in 1 year; two manifested OP as an initial presentation of SLE, while the third manifested OP when SLE relapsed. To elucidate the clinical features and outcomes of OP in patients with SLE, we screened the PubMed database for cases diagnosed with OP either at or after the diagnosis of SLE; the search was restricted to articles that were published after 1990, when OP became widely recognized.Results: We identified 15 cases of OP in patients with SLE. Thus, we analyzed a total of 18 cases (including our three). OP developed at the initial diagnosis of SLE in 15 cases and at relapse of SLE in three cases. In most cases, the disease activity of SLE was moderate or high. In all cases, OP was accompanied by other extrapulmonary symptoms of SLE, namely, fever (77.8%), cutaneous manifestations (61.1%), arthralgia/arthritis (50%), and lupus nephritis (33.3%). Steroid monotherapy or increasing the dose of the steroids was effective in seven cases (38.8%); however, steroid monotherapy was ineffective and led to death due to respiratory failure in two cases (11.1%). Combination therapies of steroids with hydroxychloroquine, azathioprine, tacrolimus, mycophenolate mofetil, intravenous pulse cyclophosphamide therapy, and belimumab were effective in seven cases (38.8%).Conclusions: Based on the review of previously reported and our cases, we concluded that OP is an important pulmonary manifestation of SLE. Clinicians should be aware of it as it may require intensive immunosuppressive therapy either at or after the diagnosis of SLE.

Scoping review: The state of research on cryptogenic organizing pneumonia therapeutics.

Cryptogenic organizing pneumonia is a diffuse interstitial lung disease that starts in the alveolar wall and subsequently expands to the alveolar ducts and respiratory bronchioles. Randomized controlled trials are lacking to guide the treatment of cryptogenic organizing pneumonia, so treatment decisions and practice guidelines are often based upon observations from case series or expert clinical opinions. The backbone of treatment involves immunosuppression via corticosteroids. In refractory cases, cytotoxic therapy is considered. The evidence that supports the use of these regimens are limited. The goal of this scoping review is to conduct a systematic search of the literature to determine what regimens have been utilized to treat steroid refractory organizing pneumonia and to characterize the evidence supporting their use.

Clinical features and long-term prognosis of acute fibrinous and organizing pneumonia histologically confirmed by surgical lung biopsy.

BACKGROUND: Acute fibrinous and organizing pneumonia (AFOP) is a rare interstitial pneumonia characterized by intra-alveolar fibrin deposition and organizing pneumonia. The clinical manifestations and long-term prognosis of AFOP are unclear. Our objective was to investigate the clinical features and prognosis of AFOP. METHODS: We identified patients diagnosed with AFOP by surgical lung biopsy between January 2011 and May 2018 at Seoul National University Bundang Hospital. We retrospectively reviewed clinical and radiologic findings, treatment, and outcomes of AFOP. RESULTS: Fifteen patients with histologically confirmed lung biopsies were included. The median follow-up duration was 2.4 (range, 0.1-82) months. The median age was 55 (range, 33-75) years, and four patients were immunocompromised. Fever was the most common clinical presentation (86.7%). Patchy ground-glass opacities and/or consolidations were the most predominant findings on chest computed tomography images. Nine patients (60%) received mechanical ventilator care, and eight patients (53.3%) died. The non-survivors tended to have slightly higher body mass index (BMI) and a long interval between symptom onset and diagnosis than the survivors, but these findings were not statistically significant. Among seven survivors, five patients were discharged without dyspnea and oxygen supplement. CONCLUSIONS: The clinical course of AFOP was variable. Although AFOP was fatal, most of the patients who recovered from AFOP maintained normal life without supplemental oxygen therapy and respiratory symptoms.

Acute fibrinous and organizing pneumonia in a patient with Sjögren's syndrome and Legionella pneumonia: a case report and literature review.

BACKGROUND: Acute fibrinous and organizing pneumonia (AFOP) is a rare clinicopathological condition. Studies in the literature have reported that AFOP may be associated with respiratory infections, such as respiratory syncytial virus, influenza virus, Pneumocystis jirovecii, Penicillium citrinum, and Chlamydia infections. However, AFOP associated with Legionella infection has not been reported previously. Here, we report a case of a patient with AFOP secondary to Sjögren's syndrome and Legionella infection. CASE PRESENTATION: A 47-year-old man was admitted to the hospital because of fever, expectoration, and shortness of breath. Lung imaging showed irregular patchy consolidation. A diagnosis of Legionella pneumonia was initially considered on the basis of the patient's history of exposure to soil before disease onset, signs of extrapulmonary involvement, and a positive Legionella urine antigen test result. However, the patient's symptoms and lung imaging did not improve after treatment with levofloxacin, moxifloxacin, and tigecycline for Legionella infection. In addition, Sjögren's syndrome was diagnosed on the basis of clinical manifestations and immunological indicators. Pathological changes associated with AFOP were confirmed from the results of ultrasound-guided percutaneous lung biopsy. The patient's clinical symptoms improved rapidly after a short course of low-dose corticosteroid therapy, and lung imaging showed significant improvement. CONCLUSIONS: The possibility of secondary AFOP should be considered when Legionella pneumonia does not improve after standard antibiotic therapy. Lung biopsy and histopathological examination are important for the adjustment of treatment strategy. Our case also highlights the importance of screening for autoimmune diseases in patients with AFOP.

[Organizing pneumonia as a pulmonary manifestation of post-COVID syndrome: features of diagnosis and treatment].

Treatment of patients with long-term persistent symptoms after COVID-19 is an urgent problem for clinicians around the world. One of the most significant manifestations of post-COVID-19 syndrome is organizing pneumonia that is usually treat with corticosteroids. The paper presents a clinical case of typical course of post-COVID-19 organizing pneumonia in a patient without previous lung disease. Risk factors, diagnostic methods and treatment options in this group of patients are also discuss.

Bronchiolitis obliterans organizing pneumonia as the pulmonary manifestation of lupus: A review of three cases.

OBJECTIVE: Bronchiolitis obliterans organizing pneumonia (BOOP) is a clinico-patho-radiological diagnosis which rarely presents as a pulmonary manifestation of lupus. In this concise report, organizing pneumonia was found as the sole pulmonary manifestation of SLE in different age groups. METHOD: All three patients diagnosed with SLE according to SLICC 2012 classification criteria, were admitted in rheumatology ward of NIMS hospital, Hyderabad, India from May to November, 2018. Their diagnosis of BOOP was either biopsy proven or imaging guided. Review of literature was done with MeSH terms (SLE, BOOP) in PubMed and approximately 10 articles were reviewed including latest of 2019 published in Scientific Reports. RESULT: There were three patients - one juvenile lupus and two adults. Two patients were male and one female. All three patients had SLE with high disease activity. They all had organising pneumonia as pulmonary manifestation with other organ involvement. Juvenile patient had a fatal outcome while the others had a good recovery with steroid and immunosuppressive. CONCLUSION: BOOP is a rare pulmonary manifestation in lupus. It can be diagnosed early with more precision using computerised tomography of lung without waiting for biopsy report. This will result in a better prognosis by rapid initiation of corticosteroid and immunosuppressive treatment.

Comparison of Chest CT Findings of COVID-19, Influenza, and Organizing Pneumonia: A Multireader Study.

BACKGROUND. Previous studies compared CT findings of COVID-19 pneumonia with those of other infections; however, to our knowledge, no studies to date have included noninfectious organizing pneumonia (OP) for comparison. OBJECTIVE. The objectives of this study were to compare chest CT features of COVID-19, influenza, and OP using a multireader design and to assess the performance of radiologists in distinguishing between these conditions. METHODS. This retrospective study included 150 chest CT examinations in 150 patients (mean [± SD] age, 58 ± 16 years) with a diagnosis of COVID-19, influenza, or non-infectious OP (50 randomly selected abnormal CT examinations per diagnosis). Six thoracic radiologists independently assessed CT examinations for 14 individual CT findings and for Radiological Society of North America (RSNA) COVID-19 category and recorded a favored diagnosis. The CT characteristics of the three diagnoses were compared using random-effects models; the diagnostic performance of the readers was assessed. RESULTS. COVID-19 pneumonia was significantly different (p < .05) from influenza pneumonia for seven of 14 chest CT findings, although it was different (p < .05) from OP for four of 14 findings (central or diffuse distribution was seen in 10% and 7% of COVID-19 cases, respectively, vs 20% and 21% of OP cases, respectively; unilateral distribution was seen in 1% of COVID-19 cases vs 7% of OP cases; non-tree-in-bud nodules was seen in 32% of COVID-19 cases vs 53% of OP cases; tree-in-bud nodules were seen in 6% of COVID-19 cases vs 14% of OP cases). A total of 70% of cases of COVID-19, 33% of influenza cases, and 47% of OP cases had typical findings according to RSNA COVID-19 category assessment (p < .001). The mean percentage of correct favored diagnoses compared with actual diagnoses was 44% for COVID-19, 29% for influenza, and 39% for OP. The mean diagnostic accuracy of favored diagnoses was 70% for COVID-19 pneumonia and 68% for both influenza and OP. CONCLUSION. CT findings of COVID-19 substantially overlap with those of influenza and, to a greater extent, those of OP. The diagnostic accuracy of the radiologists was low in a study sample that contained equal proportions of these three types of pneumonia. CLINICAL IMPACT. Recognized challenges in diagnosing COVID-19 by CT are furthered by the strong overlap observed between the appearances of COVID-19 and OP on CT. This challenge may be particularly evident in clinical settings in which there are substantial proportions of patients with potential causes of OP such as ongoing cancer therapy or autoimmune conditions.