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Mycoplasmas are the smallest free-living organisms. Their genome size is only 10-15% of that of usual bacteria, and they lack the genes necessary for cell wall synthesis. Mycoplasma pneumoniae is estimated to be the cause of up to 20% of atypical pneumonia cases. Disease onset is insidious. Fever, headache, and malaise occur 2 to 4 days before the onset of respiratory symptoms. Another common clinical syndrome caused by M. pneumoniae is acute or subacute tracheobronchitis. Some studies have shown a peak incidence of infection in the fall in temperate climates although sporadic cases may occur without seasonality. Complications of infection may include severe pneumonia, dermatitis including erythema nodosum and Stevens-Johnson syndrome, cardiac arrhythmias, pericarditis, myocarditis, coagulopathies, hemolytic anemia and central nervous system involvement. Vascular complications include Raynaud's phenomenon, which may be associated with the production of cold agglutinins. There is accumulating evidence over the last few years that links M. pneumoniae with chronic asthma in some patients due to persistent infection of the lower respiratory tract. Evidence for an extended spectrum of disease associated with M. pneumoniae has come about in part because of improved diagnostic tests. The classic serologic method of diagnosis is measurement of cold agglutinins. A cold agglutinin titer of ³ 1:32 in the appropriate setting is considered indicative of infection. However, the sensitivity of the cold agglutinin test is low, approximately 50%. The specificity is also low since cold agglutinins are an acute phase reactant and may be seen in a variety of diseases, such as autoimmune disorders, dysproteinemic states, other atypical pneumonias including legionellosis, mononucleosis caused by cytomegalovirus or Epstein-Barr virus, and lymphoma. Although isolation of M. pneumoniae from respiratory specimens is possible, growth usually requires 1-2 weeks, which limits the clinical utility of culture. Specific serological tests for IgM and IgG antibodies to M. pneumoniae have become the preferred laboratory method of diagnosis. Both immunofluorescence assays (IFA) and enzyme-linked immunosorbent assays (ELISA) are available and have sensitivities and specificities >90%. The presence of IgM antibodies, or a fourfold or greater rise in IgG antibody between paired sera indicates recent infection. Results are reported as negative or positive. The reference value is negative. Specimen requirement is one plain red top tube of blood. |
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