Investigations

It is also useful in the identification of alternative diagnoses such as pulmonary oedema and exclusion of others e.g. pneumothorax.

The BTS guidelines recommend that chest radiographs are performed on all patients being admitted from the ED with CAP in order to confirm or refute the diagnosis [1].

Chest radiographs are not necessary on patients discharged from the ED with a diagnosis of CAP unless the diagnosis is in doubt or the patient is considered at risk of underlying pathology such as lung cancer.

Changes consistent with pneumonia

The lung parenchymal inflammation and fluid-filled alveoli are seen as areas of consolidation on the chest radiograph. The areas of infection are usually limited by pleural surfaces to a specific lobe of one lung but occasionally it may involve more than one lobe or both lungs.

Small pleural effusions are common and usually reactive. Occasionally, chest radiographs may appear normal in the early stages of pneumonia.

Anteroposterior (AP) films performed on very unwell patients in the resuscitation room are often of poor quality and distinguishing between pulmonary oedema and pneumonia can be difficult. The following signs may help to confirm a diagnosis of pneumonia:

• Silhouette sign:  The silhouette sign refers to the loss of the normal border between structures for example:
  • Right middle lobe pneumonia, where the right heart margin is classically lost
  • Right lower lobe pneumonia, where the border of the diaphragm on the right side is obscured, while the right heart margin remains clear
• Air Bronchogram: As the bronchi branch a point is reached where the cartilaginous bronchial walls are too thin to visualise (unless calcified) and it is not possible to distinguish air in the bronchi from air in the surrounding alveoli. However, if the surrounding alveoli fill with fluid or pus then branching radiolucent air passages (air bronchograms) may be seen

Left lower lobe pneumonia may be missed if the area behind the cardiac shadow is not routinely inspected.

Blood tests

The BTS guidelines recommend that the following tests be performed on all patients admitted with CAP:

• Full blood count: A white cell count >15 x 109/l is suggestive of a bacterial pneumonia
• U&Es: Urea forms part of the CURB-65 scoring system. Occasionally, patients will be identified with unsuspected renal failure or hyponatraemia
• C-Reactive Protein (CRP): CRP levels on admission have not proven to be clinically useful either diagnostically or prognostically but forms a baseline for later comparison. Failure of the CRP level to fall 72 hours after admission suggests treatment failure
• Liver function tests (LFTs): The value of routine measurement of LFT’s in patients that are not jaundiced, with non-severe pneumonia is unclear

Arterial Blood Gases/Lactate

The BTS guidelines recommend that ABGs only be performed in patients with oxygen saturations below 94% [6].

However, the measuring of lactate levels (which is now routinely measured on most blood gas analysers) is essential for identifying tissue hypo-perfusion in patients who are not yet hypotensive but who are at risk for septic shock.

A raised lactate (particularly levels >4 mmol/l) is a poor prognostic indicator independent of the CURB-65 score.

Microbiology – Blood cultures

Overall, blood cultures have a low sensitivity in CAP (<10% cases). The yield is particularly low for patients with non-severe CAP and no co-morbid disease, and for those who have received antibiotic therapy prior to admission [7,8].

The BTS guidelines recommend that blood cultures are taken from all patients with moderate or severe CAP and most other patients admitted with CAP, preferably before antibiotic therapy is commenced.

However, blood cultures may be omitted if the following criteria are met:

• A diagnosis of CAP has been definitely confirmed

and

• The patient has low severity pneumonia

and

• The patient has no significant co-morbid disease

Blood cultures are not required for patients with confirmed low severity pneumonia in the absence of co-morbid conditions.

Microbiology – sputum culture

Purulent sputum samples should be sent for culture and sensitivity from patients with the following characteristics [1]:

• Severe pneumonia
• Moderate pneumonia that have not received prior antibiotics
• Failure to improve with treatment
• Patients who test positive for urine legionella antigen

Since the aim is to deliver antibiotics within 4 hours of arrival at the ED, purulent sputum samples that are produced by patients being admitted with CAP should in general be sent for culture and sensitivity. Samples obtained later on the ward will have a lower yield due to antibiotic administration in the ED.

Serology

A number of serological tests are available for identification of specific organisms but are of little relevance to management of the patient in the ED. The BTS guidelines recommend the following:

• Pneumococcal urine antigen: all patients with moderate or severe CAP should be tested
• Legionella urine antigen: patients with severe pneumonia, specific risk factors and all patients during epidemics should be tested. NICE recommends tested patients with moderate or severe pneumonia
• PCR: PCR testing is available for mycoplasma pneumonia, Chlamydophlia and respiratory viruses but should be selectively requested by respiratory physicians

ECG

Pneumonia is not associated with specific ECG changes though sinus tachycardias are common. In the elderly it is not uncommon for pneumonia to trigger atrial fibrillation or rate related ischaemia.

The following investigations should be omitted:

CT Chest: has no role in the routine investigation of patients with CAP.