Urinary Antigen Testing

Urinary Antigen Testing in Suspected Pneumonia

Clinical Background

Pneumonia is a common illness affecting approximately 450 million people a year and occurring in all parts of the world. It is a major cause of death among all age groups, resulting in 4 million deaths annually – 7% of total world annual deaths.¹

In the UK the annual incidence of pneumonia is approximately 6 cases per 1000 people in the age group 18-39 years
This rises to 75 cases per 1000 people in the age group over 75 years
Roughly 20-40% of individuals contracting pneumonia will require hospital admission
5-10% of these are admitted to a critical care unit
The case fatality rate in the UK is 5-10%.²

Pneumonia is due to infection caused primarily by bacteria or viruses. Although more than 100 strains of infectious agents have been identified as causative pathogens, only a few are responsible for the majority of cases. Bacteria are the most common cause of community-acquired pneumonia (CAP), with Streptococcus pneumoniae isolated in nearly 50% of cases.

Other commonly isolated bacteria include:

Haemophilus influenzae (20%)
Chlamydophila pneumonia (13%)
Legionella (3.8%)
Mycoplasma pneumoniae (3%).^3,4

Streptococcus pneumoniae

S. pneumoniae is a major cause of invasive infection and the WHO estimate that approximately 1.6 million people die of severe pneumococcal infections every year.^5,6The incubation period for pneumococcal pneumonia is only 1-3 days. Infection can lead to bacteraemia, meningitis, pericarditis, empyema, purpura fulminans, endocarditis and/or arthritis.

A rapid diagnosis is essential as these infections can normally be treated with penicillin which is relatively inexpensive and does not induce antibiotic resistance at the same level as other antibiotics.⁷

Legionella pneumophila

Legionella infections can occur as sporadic or epidemic; and as community-acquired or nosocomial infections. The bacteria are usually present in water and can survive (although not reproduce) for a long time in moist environments. As it can be resistant to a degree, to the effects of disinfectants, it can survive in both natural and treated water.

The genus Legionella contains 40 species and more than 70 serogroups, of which Legionella pneumophila is recognised as the most important pulmonary pathogen and accounts for the majority of cases of Legionnaires disease.

Legionnaires disease is a severe pneumonia and if untreated it can lead to mental disorientation and death. For all the cases reported to the European Legionnaires Disease Surveillance Network the case fatality rate was 11% for the cases where the clinical outcome was known.¹⁰

Penicillin is not effective against Legionella as it is an intracellular bacterium
Treatment with antibiotics like macrolides or quinolones that penetrate the infected cells is required
Diagnostic methods to confirm the presence of Legionella in the initial phase are important for the prognosis of the disease

Diagnostic Methods

The diagnosis of pneumococcal pneumonia is based on the presence of clinical signs of pneumonia, chest radiograph, lung infiltrates and microbiological findings. In the clinical situation it is often difficult to make a definite diagnosis and a positive bacterial culture from blood or pleura is required along with the presence of symptoms of pneumonia.

Culture is still the “gold standard” for the diagnosis of Legionella infection and is generally 100% specific. However it is time consuming and expensive and lacks sensitivity. It also requires technical expertise and efficient handling of samples.

Pneumococcal antigen detection dates back to the work done by Dochez and Avery in 191717,18 where they demonstrated capsular polysaccharides in urine from patients with lobar pneumonia. In 1999 the first S. pneumoniae urinary antigen test was developed and commercialised. It was a lateral flow assay that could give a positive or negative result within 15 minutes.

Serology remains an important tool in the diagnosis of Legionnaires Disease but these days the main method in Europe for diagnosing the disease is the Legionella urinary antigen test.²³ Over the past few years rapid bedside assays have been developed by several commercial companies with variations in sensitivity and specificity.

The current UK guideline from the British Thoracic Society recommends:

Pneumococcal urine antigen tests should be performed for all patients with moderate or high severity CAP
Legionella urine antigen tests should be performed for all patients with high severity CAP
A rapid testing and reporting service for both tests should be available to all hospitals admitting patients with CAP.²⁴

Publication of the new NICE guideline for Pneumonia: diagnosis and management of community- and hospital-acquired pneumonia in adults is expected in December 2014. The draft copy in consultation suggested that the use of rapid diagnostic tests for urinary antigen detection could improve antibiotic stewardship.²⁵

References

Testing

The SD Biosensor Standard F system provides a rapid cost effective means of urinary antigen testing. For further information see below:

F200