Persistent pulmonary infiltrate results when a substance denser than air (e.g., pus, oedema, blood, surfactant, protein, or cells) lingers within the lung parenchyma. Non-resolving and slowly resolving pneumonias are the most common broad categories of persistent pulmonary infiltrate.    Persistence is attributed to defects in host immune defence mechanisms, presence of unusual or resistant organisms, or diseases that mimic pneumonia.   
The classification of these disorders may become quite complex, as some clinicians focus primarily on the radiological abnormalities, while others emphasise the accompanying clinical features. Non-resolving or slowly resolving pneumonia is loosely defined as a pneumonia that does not improve clinically, or even worsens, despite a minimum of 10 days of adequate antibiotic therapy, or as radiographic infiltrate that does not resolve within 12 weeks.   Slowly resolving pneumonias are usually defined as the persistence of radiographic infiltrate in a clinically improved patient for longer than 4 weeks (<50% resolution in 1 month).    
A waiting period of 12 to 14 weeks is suggested for slowly resolving pneumonia to be considered non-resolving (or chronic) in older patients with non-tuberculous bacterial pneumonia.  Non-responding pneumonia is an inadequate clinical response despite antibiotic treatment. It is an independent risk factor for death and delayed resolution of pulmonary infiltrate.   Non-infectious causes are responsible for about 20% of cases of non-resolving pneumonia. 
A good clinical response to pulmonary infiltrate is defined as 50% clearing of chest radiographic findings at 4 weeks of therapy.  Clinical improvement and resolution of leukocytosis supports the conclusion that the patient has responded to antibiotic therapy, even when chest radiographic abnormalities persist.   In the case of pneumonia, most patients have a normal temperature and decreased cough within 3 to 5 days after beginning treatment. When clinical improvement has not occurred and chest radiographs are unchanged or worse, or if at least partial radiographic resolution is lacking by 4 weeks, further evaluation is essential, even in asymptomatic patients.   
Resolution of non-resolving pneumonias varies and depends on the causal agent, the severity of disease, and host factors.   Several risk factors may hinder the rate of radiographic clearing of the condition:
Comorbid conditions (COPD, cardiac failure, diabetes, renal failure, immunodeficiency, alcohol intake, smoking, occupational exposure, cancer, cancer treatment, systemic illness): patients with haematological malignancies, immunosuppressive disorders, or exposure to silica, aluminium, or titanium dust are prone to persistent pulmonary infiltrate
Initial severity of the infection
Delay in initiation of therapy.
AP declares that she has no competing interests.
Dr Athanasia Pataka would like to gratefully acknowledge Dr Paraskevi Argyropoulou-Pataka, a previous contributor to this topic. PAP declares that she has no competing interests.
CR declares that she has no competing interests.
Clinical Lead in Respiratory Medicine
Whipps Cross University Hospital
MD declares that she has no competing interests.
Pulmonary Medicine and Infectious Diseases
Pulmonary Medicine Clinic
IPK declares that he has no competing interests.
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