The Modern Diagnostic Approach to Community-Acquired Pneumonia in Adults

James D. Chalmers, MBChB, PhD, FRCPE


Semin Respir Crit Care Med. 2016;37(6):876-885. 

In This Article

Abstract and Introduction


Respiratory tract infections, the majority of which are community acquired, are among the leading causes of death worldwide and a leading indication for hospital admission. The burden of disease demonstrates a "U"-shaped distribution, primarily affecting young children as the immune system matures, and older adults as the process of immunosenescence and accumulation of comorbidities leads to increased susceptibility to infection. Diagnosis of community-acquired pneumonia (CAP) is traditionally based on demonstration of a new infiltrate on a chest radiograph in a patient presenting with an acute respiratory illness or sepsis. Advances in diagnosis have been slow, and although there are increasing data on the value of computed tomography or lung ultrasound as more sensitive diagnostic methodologies, they are not widely used as initial diagnostic tests. There are a wide range of differential diagnoses and pneumonia "mimics" which should be considered in patients presenting with CAP. Once the diagnosis of CAP has been made, identifying the causative microorganism is the next stage in the diagnostic process. Traditional culture-based approaches are relatively insensitive and achieve a positive diagnosis in only 30 to 70% of cases, even when rigorously applied. Urinary antigen tests, polymerase chain reaction assays, and even next-generation sequencing technologies have become available and are increasing the rates of positive diagnosis. In an era of increasing antimicrobial resistance, the accurate diagnosis of CAP and determining the causative pathogen are ever more important. Getting these both right is key in reducing both morbidity and mortality from CAP, and appropriate antimicrobial stewardship which is now an international healthcare priority.


Acute respiratory tract infections are the leading cause of death in developing countries, while remaining a leading cause of death in developed nations.[1,2] The Global Burden of Disease study revealed that the number of deaths from acute respiratory tract infections has fallen over the past two decades. Nevertheless, respiratory infections will continue to have a profound impact worldwide.[1,2] The exact prevalence of acute respiratory tract infections worldwide is nearly impossible to calculate, but there are an estimated 4 million deaths per year, with up to three-fourths of those deaths being in children younger than 5 years.

Determining the precise impact of community-acquired pneumonia (CAP) worldwide is made more difficult by the use of a definition that relies on chest radiographic evidence of consolidation.[3–5] It is estimated that up to 80% of episodes of CAP in developed countries are treated without the patient ever receiving a chest X-ray. In developing nations, this proportion will be higher still.[5–7]

Whether in primary care or in the hospital setting, CAP must be quickly recognized and treated, as severe CAP can be life threatening.[8] In the emergency setting, prompt resuscitation and administration of intravenous antibiotics are essential steps in reducing the morbidity and mortality from CAP which stands at 5 to 15% for hospitalized patients and often >25% for patients requiring admission to an intensive care unit.[8–10] In this context, CAP must be differentiated from other conditions causing acute respiratory failure and septic shock. In the less acute setting, in outpatients or in primary care, the challenge is more to differentiate patients with CAP who may benefit from antibiotic therapy from that much larger group of patients with acute cough who will not benefit from antibiotic therapy.[5,7,11,12]

Once the diagnosis of CAP has been made, antibiotic treatment is commenced according to the severity of disease and to cover the most likely causative pathogens.[13,14] Testing to determine the possible causative pathogen serves several important functions which include (1) to deescalate initial broad-spectrum antibiotic treatment if a pathogen is identified that can be treated with narrower spectrum therapy,[15] (2) to identify unusual or drug-resistant pathogens that will not be covered by the initial empirical regime,[16] and (3) to provide local data on the frequency of different causative pathogens and antibiotic resistance rates that can be used to guide future empirical treatment guidelines.[17]

The goal of this review is to discuss the diagnostic approach in CAP, with a focus on how to make the initial diagnosis and the emerging methods for determining the underlying pathogen.