infectious diseases
Extrapulmonary Tuberculosis (TB)
Diagnosis of Extrapulmonary TB
Acid-fast staining, microscopic analysis, and mycobacterial culture of fluid and tissue samples, and, when available, nucleic acid amplification testing (NAAT) Chest x-ray Tuberculin skin test (TST) or interferon-gamma release assay (IGRA) Testing is similar to that for pulmonary TB (see Diagnosis ), including chest x-ray, TST or IGRA, and microscopic analysis (with appropriate staining) and mycobacterial cultures of affected body fluids (cerebrospinal fluid, urine, or pleural, pericardial, or joint fluid) and tissue for mycobacteria. Blood culture results are positive in about 50% of patients with disseminated TB; such patients are often immunocompromised, often by HIV infection. However, cultures and smears of body fluids and tissues are often negative because few organisms are present; in such cases, NAATs may be helpful. NAATs can be done on fresh fluid or biopsy samples and on fixed tissue (eg, if TB was not suspected during a surgical procedure and cultures were not done). NAATs typically are not approved for extrapulmonary TB diagnosis but are commonly used in hopes of an early diagnosis for medical care and public health reasons, pending culture. Although a positive NAAT result almost always supports a TB diagnosis, a negative result does not rule out TB in most cases because the negative predictive value is generally unknown and may depend on specimen processing and other factors that are not standardized. Typically, lymphocytosis is present in body fluids. A very suggestive finding in the cerebrospinal fluid is a glucose level < 50% of that in serum and an elevated protein level. If all tests are negative and miliary TB is still a concern, biopsies of the bone marrow and the liver are done. If TB is highly suspected based on other features (eg, granuloma seen on biopsy, positive TST or IGRA result plus unexplained lymphocytosis in pleural fluid or cerebrospinal fluid), treatment should usually proceed despite inability to demonstrate TB organisms. Chest x-ray and other imaging can also provide helpful diagnostic information. Chest x-ray may show signs of primary or active TB; in miliary TB, it shows thousands of 2- to 3-mm interstitial nodules evenly distributed through both lungs. Other imaging tests are done based on clinical findings. Abdominal or genitourinary involvement usually requires CT or ultrasonography; renal lesions are often visible. Bone and joint involvement requires CT or MRI; MRI is preferable for spinal disease. TST and IGRA may initially be negative, but a repeat test in a few weeks is likely to be positive. If it is not, the diagnosis of TB should be questioned or causes of anergy (no reaction to any skin test) should be sought. 