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Community Acquired Pneumonia

Pathology and Causes || Signs and Symptoms || Diagnosis || Treatment || Overview || Related Articles || References and Resources ||
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Community acquired pneumonia (CAP) refers to pneumonia picked up, you guessed it, in the "community". This differs from pneumonia picked up in the hospital, also known as "nosocomial" pneumonia. It is important to distinguish between the two forms because they differ in the types of bugs that cause them, and what drugs are necessary to treat them. CAP is further subdivided into "typical" and "atypical" forms.

Currently, typical community acquired pneumonia is most commonly caused by the aptly named bacteria, streptococcus pneumoniae. Other common bacteria that cause typical CAP include haemophilus influenzae (not to be confused with viral influenzae), and moraxella catarrhalis. Interestingly, these three bacteria are also the most common cause of ear infections (ie: acute otitis media) in children.

Other less common causes of typical CAP include staphylococcus aureus, klebsiella pneumoniae, pseudomonas aeruginosa, and legionella pneumophila. Most of these bacteria infect people with predisposing chronic medical illnesses like diabetes.

Typical community acquired pneumonia causes inflammation of the lung tissue. As the body attempts to fight off the infection pus and fibrin accumulate in the air spaces of the lung (ie: alveoli). When this occurs in a patchy framework throughout the lung tissue it is called "bronchopneumonia"; when it affects an entire lobe of the lung it is called "lobar pneumonia". Bronchopneumonias can become confluent and turn into lobar pneumonias. When an airspace has junk of any type in it, it is considered "consolidated".

The "atypical" community acquired pneumonias include those caused by viruses, and other bacterial species. The most common cause of atypical CAP is a bacteria without a cell wall known as mycoplasma pneumoniae. Another known causative bacteria is chlamydia pneumoniae. Viruses may in fact be the most common cause of atypical community acquired pneumonia. The common viruses that cause CAP include influenza, adenovirus, and rhinoviruses; however, there are many others that have been implicated as well.

Atypical pneumonia usually causes inflammation that stays within the lining of the lung tissue. In other words, the inflammatory process doesn't normally "spill" out into the airspaces of the lung. On chest x-rays this results in an "interstitial" pattern. However, in severe cases the inflammation may resemble that of typical pneumonias.


Signs and Symptoms

Typical community acquired pneumonias make you feel horrible! They result in high fevers with chills and shaking. In addition, most patients cough up lots of crud (ie: sputum). The sputum is the inflammatory junk that is present in the airspaces. Physical exam can reveal dullness to percussion in any areas of consolidation. Listening with a stethoscope may reveal "crackles" and "rhonchi".

Atypical pneumonia is often times much less severe than its typical counterpart. This type of pneumonia is sometimes referred to as "walking pneumonia" because most patients are able to go about their daily business, although they may feel under the weather. Many patients have a cough that typically doesn't produce much sputum. Patients usually still have fever, myalgias (ie: muscle aches), and headaches.



Diagnosis of community acquired pneumonia is based on clinical symptoms with characteristic findings on chest x-ray. In patient's who are severely ill or fail to respond to empiric antibiotic treatment a sputum culture may be used to try to figure out what bug is causing the pneumonia. In very sick patients with CAP a blood culture is also performed since the bacteria may extravasate into the blood stream causing bacteremia (and possibly septic shock!). A complete blood count (CBC) is also obtained, which usually shows an elevated white blood cell count, a marker of infection.

If legionella pneumophila is suspected a urine test can detect proteins produced by the bacteria. There is a similar urine antigen test for streptococci pneumoniae. If influenza is suspected rapid antigen testing can also be performed.



Treatment for community acquired pneumonia is based on whether or not the patient is sick enough to require hospitalization. There are various scoring guidelines for determining whether or not someone should be hospitalized, some of which are better than others. Regardless, if a patient requires hospitalization the next step is to determine if they are sick enough to require ICU care.

In otherwise healthy patients who do not require hospitalization a macrolide antibiotic is the best initial choice, although tetracycline antibiotics, namely doxycycline can also be used. The treatment is usually given for 5 to 10 days. If the patient is not healthy (ie: diabetes, heart, liver, kidney problems, or previous antibiotic use in the past three months) a respiratory fluoroquinolone or a β-lactam plus a macrolide should be used.

In patients requiring hospitalization, but not in an ICU setting, a respiratory fluoroquinolone is a common starting point. Another regimen includes a β-lactam antibiotic in combination with a macrolide.

If the patient requires ICU care they should be started on a β-lactam in combination with either a fluoroquinolone or macrolide. Aztreonam can be used in patients with allergies to β-lactams (ie: "penicillin" allergies).

If other specific bugs are clinically suspected different antibiotics may be used. For example, if methicillin resistant staphylococcus is suspected vancomycin might be added. If pseudomonas species are a concern, a β-lactam such as piperacillin would have to be added.

Overall, treatment is based on how sick the patient is, as well as what pathogens are suspected clinically based on the patient's history.



Community acquired pneumonia is either typical or atypical depending on the type of "bug" and severity of symptoms. Cough, fever, myalgias, and malaise are common symptoms. Diagnosis is based on clinical history with an abnormal chest x-ray. Other specific tests may be done depending on clinical severity and suspected pathogen. Treatment depends on whether or not the patient requires hospitalization, and if so, whether or not they require an ICU level of care.


Related Articles

- Influenza

- Enterobius vermicularis

- Giardia lamblia

- Borrelia burgdorferi


References and Resources

(1) Brown SM, Dean NC. Defining and predicting severe community-acquired pneumonia. Curr Opin Infect Dis. 2010 Apr;23(2):158-64.

(2) Boersma WG. Assessment of severity of community-acquired pneumonia. Semin Respir Infect. 1999 Jun;14(2):103-14.

(3) Anevlavis S, Bouros D. Community acquired bacterial pneumonia. Expert Opin Pharmacother. 2010 Feb;11(3):361-74.

(4) Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. Seventh Edition. Philadelphia: Elsevier Saunders, 2004.

(5) Bickley LS, Szilagyi PG. Bates' Guide to Physical Examination and History Taking. Ninth Edition. New York: Lippincott Williams and Wilkins, 2007.


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β-lactams are penicillin derivatives. They include amoxicillin (with or without clavulanic acid), ampicillin (the IV formulation of amoxicillin, with or without sulbactam), cefotaxime, ceftriaxone, and ertapenem.
Gemifloxacin, levofloxacin, and moxifloxacin are the fluorquinolones with the best lung penetration.
Macrolides include erythromycin, azithromycin, and clarithromycin.
Inflammation refers to white blood cells flooding an area of the body. Inflammation is a common result of infection and/or injury to a particular body part. The white blood cells are attempting to fight off any "intruding" organisms.