The diagnosis of nocardiosis is established with culture of the causative organism from the infection site(s). Because nocardiae grow slower than common bacteria, the microbiology laboratory should always be notified when nocardiosis is clinically suspected. This is particularly true when sputum is the submitted specimen. Respiratory secretions, skin biopsy samples, and aspirates from abscesses are the most common specimens from which Nocardia species are identified. Direct smears or histopathologic stains of these specimens can be highly suspicious, as noted above. Nocardia species can usually be isolated in 3-5 days.
Blood cultures are positive for Nocardia organisms in a minority of patients, but they always should be obtained when pulmonary or disseminated nocardiosis is suspected.
Immunodominant antigens of Nocardia species have been identified and used in serological assays. However, no serologic technique or molecular technique is yet available for routine clinical use. Similarly, nucleic acid amplification assays have been described but are not available for routine clinical use.
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High-power microscopic appearance of Nocardia. Image courtesy of CDC.
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Photomicrograph of tissue biopsy stained with Gomori methenamine silver demonstrating acute inflammatory response and organisms compatible with Nocardia.
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Plain chest radiograph in a patient with nocardiosis. Image courtesy of Applied Radiology, Anderson Publishing, LTD.
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Chest CT scan in a patient with pleuropulmonary nocardiosis. Image courtesy of Applied Radiology, Anderson Publishing, LTD.
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Brain CT scan in a patient with nocardial brain abscess. Image courtesy of Applied Radiology, Anderson Publishing, LTD.
