Clinical significance
Chlamydiae are small Gram-negative bacteria that are obligate intracellular parasites; they have DNA, RNA and ribosomes, replicate by fission and are sensitive to antibiotics. Although they share common antigenic features, the genus Chlamydia includes a variety of microorganisms with distinct biological, serological and pathogenic properties. Chlamydia pneumoniae is an airborne pathogen spread by inhaling aerosols. C. pneumoniae infections often cause no symptoms or only mild symptoms (e.g. sore throat, marked hoarseness, bronchitis or cough). The infection can occasionally cause chronic disease with immunopathological syndromes such as erythema nodosum, joint pain, Guillain-Barré syndrome (GBS) or muscle pain. Chronic C. pneumoniae infections have also been linked to the development of asthma, atherosclerosis and cardiovascular disease.
Epidemiological studies have given C. pneumoniae an important role in causing pneumonia: approximately 5-15% of patients with community-acquired pneumonia have tested positive for C. pneumoniae. 5% of patients with upper respiratory tract infections (bronchitis, sinusitis, otitis, pharyngitis, tracheobronchitis) have also tested positive for C. pneumoniae. It is not currently known whether C. pneumoniae infection is enough on its own to cause pneumonia and sustain the disease; alternatively, it may cause initial damage and pave the way for other pathogens, such as Streptococcus pneumoniae, which is in fact commonly found in patients with pneumonia caused by C. pneumoniae.
Clinical indications
Suspected infection
Sample type
The patient must have a blood sample taken.
Preparation
You must fast for at least 8 hours; a small amount of water is allowed.
Notes
IgG absent, IgM absent: no exposure. If the clinical picture is uncertain, patients should be monitored over time.
IgG present, IgM absent: past infection.
IgG absent, IgM present: early-stage infection.
IgG present, IgM present: acute infection
In primary Chlamydia pneumoniae infection the antibody response is delayed. IgG and IgM antibodies are therefore generally not detectable until 6-8 weeks after the start of the illness.
Patients with acute C. pneumoniae infection have a raised ESR (erythrocyte sedimentation rate) and CRP (C-reactive protein), while the white cell count may remain normal.
Cross-reactions with antibodies against Chlamydia trachomatis or Chlamydia psittaci cannot be ruled out, because of the similarity between the surface antigens of C. pneumoniae and other Chlamydia species, so false positive results are possible.