Given that symptoms and signs of brucellosis are nonspecific, cultures and serology are usually necessary for diagnosis. Some general laboratory findings might suggest the diagnosis (eg, leukopenia, relative lymphocytosis, or pancytopenia [36, 37] [in as many as 20% of cases]). The standard test for diagnosis of brucellosis is the isolation of the organism from blood or tissues (eg, through bone marrow biopsy or liver aspiration).
In the United States, federal regulations applicable to clinicians, pathologists, and laboratory staff govern the possession, use, and transport of specimens or cultures containing B abortus, B suis, and B melitensis. These are the select agent rules of the Department of Health and Human Services (DHHS) and the Department of Agriculture (DOA). Isolations of these species and instances of potential laboratory exposure must be reported either to the Centers for Disease Control and Prevention (CDC) or to the Animal and Plant Health Inspection Service. 
Reporting requirements that are at least equally strenuous are in place in other parts of the world, especially those areas that are certified as Brucella -free. That designation carries economic importance of the first degree in Brucella -free countries such as New Zealand. The consequences of importation of Brucella to New Zealand may be extremely severe.
Complete blood count
A complete blood count (CBC) typically is ordered routinely as part of an evaluation for a patient with potential infectious disease. Leukocytosis is rare in brucellosis, and a significant number of patients are neutropenic. Anemia is reported in 75% of patients (particularly with chronic infection), thrombocytopenia is reported in 40% (secondary to hepatosplenomegaly or from immune thrombocytopenia), and pancytopenia is reported in 6% of patients.
A slight elevation in liver enzyme levels is a very common finding. These elevated levels may reflect the severity of hepatic involvement and correlate clinically with hepatomegaly.
Diagnosis of brucellosis is definitive when Brucella organisms are recovered from blood, bone marrow, or other tissue. Some Brucella species require 5-10% carbon dioxide for primary isolation. Because of the ease of aerosol transmission, any potential Brucella specimens should be handled under a biohazard hood.
The sensitivity of blood cultures with improved techniques such as the Castaneda bottles is further improved by the lysis-centrifugation technique. With these methods, the sensitivity is approximately 60%.
Subcultures are still advised for at least 4 weeks; thus, if brucellosis is suspected, the laboratory should be alerted to keep the cultures for 3-4 weeks, which is not done routinely for most bacterial cultures.
Because the reticuloendothelial system holds a high concentration of brucellae, bone marrow culture is thought to be the criterion standard. Sensitivity is usually 80-90%. 
Any fluid (eg, synovial fluid, pleural fluid, or cerebrospinal fluid [CSF]) can be cultured, but the yield is usually low.
In patients with neurobrucellosis, analysis of CSF reveals a mild-to-modest lymphocytic pleocytosis of 88-98%. Protein levels are elevated in conjunction with normal glucose levels. CSF cultures are positive for brucellosis less than 50% of the time, but antibody testing of the fluid yields a diagnosis. CSF cultures are indicated for suggested meningitis.
Although significant joint effusion is uncommon, arthrocentesis may occasionally be needed to exclude septic arthritis. The joint aspirate demonstrates an exudative fluid with low cell counts and mononuclear predominance. Patients with brucellosis rarely present with acute monoarticular arthritis.
Serologic testing is the most commonly used method of diagnosing brucellosis. Repeated serologic testing is recommended if the initial titer is low.
The tube agglutination test, developed by Bruce, measures antibodies against smooth lipopolysaccharide (LPS); it remains the most popular test tool for the diagnosis of brucellosis. The 2-mercaptoethanol test detects immunoglobulin G (IgG), and titers higher than 1:80 define active infection. A high IgG antibody titer or a titer that is higher after treatment suggests persistent infection or relapse. Other tests, such as tray agglutination (TAT) and modified TAT, are also popular.
Titers higher than 1:160 in conjunction with a compatible clinical presentation are considered highly suggestive of infection. Titers higher than 1:320 are considered to be more specific, especially in endemic areas. Seroconversion and evolution of the titers can also be used for diagnosis.
The shortcomings of agglutination tests test include potential cross-reactivity with IgM of other organisms such as Francisella tularensis, Salmonella urbana, Yersinia enterocolitica serotype O9, Vibrio cholerae, Afipia clevelandensis, and some other bacteria.
Prozone phenomenon may occur secondary to hyperantigenemia, possibly leading to false-negative results. Accordingly, routine dilution of sera (typically beyond 1:320) is necessary to avoid this problem.
Enzyme-linked immunosorbent assay (ELISA) typically uses the cytoplasmic proteins as antigens and measures IgM, IgG, and IgA, allowing better interpretation, especially in cases of brucellosis relapse.  This is because antibodies against LPS, which are used in agglutination tests, might persist for longer periods and are believed to yield higher sensitivity and specificity. ELISA of CSF also helps diagnose neurobrucellosis. Because levels should decrease with effective treatment, ELISA is also helpful in follow-up.
Rapid point-of-care assays
Point-of-care assays are available that offer fast and accessible diagnostic capabilities, especially in areas were special laboratory resources are lacking.
Polymerase chain reaction
Polymerase chain reaction (PCR) tests have been developed for the detection and rapid diagnosis of Brucella species in human blood specimens.  Two major genetic targets are the Brucella gene BCSP31 and the 16S-23S rRNA operon. The 16S-23S rRNA operon has been shown in studies to be more reliable in terms of sensitivity but is not yet widely used in clinical practice and needs more standardization. Possible applications would include evaluating cases of relapse and monitoring response to therapy.
Urinalysis and urine culture
Urinalysis, urine culture, sensitivity testing, or a combination thereof may be indicated in the presence of symptoms of urinary tract infection (UTI). The most likely finding is a sterile pyuria, similar to that seen with tuberculosis. Urine cultures may be helpful; the organism grows from the urine if the genitourinary tract is infected.
A chest radiograph should be obtained if respiratory symptoms are present or if a source of infection is not apparent. Radiographic findings are typically absent in brucellosis, even in patients with prominent respiratory symptoms. Findings that may be observed in patients with active pulmonary involvement include hilar and paratracheal lymphadenopathy, pulmonary nodules, pleural thickening, and pleural effusion.
Spinal radiographic findings in patients with osteoarticular disease occur later in the course of illness, usually 2-3 weeks after the onset of symptoms. In patients with sacroiliitis, the most commonly observed abnormalities include blurring of articular margins and widening of the sacroiliac spaces.  Spondylitis-related abnormalities include anterosuperior vertebral angle epiphysitis, spinal straightening, narrowing of the intervertebral disc spaces, end-plate sclerosis, and osteophytes. 
Other Imaging Studies
Echocardiography is used to evaluate for possible endocarditis. The primary site of vegetation is the aortic valve, with the sinus of Valsalva most commonly affected, followed by the mitral valve. Mycotic aneurysms of the aorta or carotids may be observed on duplex arteriography.
Use of ultrasonography to diagnose testicular abscess from brucellosis has been reported; low-resistance flow appears to be characteristic for these tumors. 
Radionuclide scintigraphy  is more sensitive for revealing skeletal abnormalities, especially early in the disease, when standard radiographic findings are usually normal. This modality may be especially helpful in distinguishing hip involvement from sacroiliitis. To facilitate prompt diagnosis, radionuclide scintigraphy also may have a role in screening for new-onset brucellosis and musculoskeletal symptoms.
In patients with altered mental status or focal neurologic deficits, cranial computed tomography (CT) is warranted. Although the CT scan is often normal, it may reveal evidence of acute or chronic Brucella leptomeningitis, subarachnoid hemorrhage, or cerebral abscess.
Bone marrow aspiration and biopsy may be required to establish a diagnosis in certain patients. Bone marrow examination may reveal erythrophagocytosis. Microangiopathic hemolytic anemia, thrombocytopenic purpura,  and Coombs-positive hemolytic anemia have been reported in brucellosis.
Percutaneous liver biopsy may be needed in the patient with liver granulomas to obtain a specimen for diagnosis. Analysis of liver biopsy specimens may reveal granulomatous hepatitis and hepatic microabscesses.
Histologic findings in brucellosis usually include mixed inflammatory infiltrates with lymphocytic predominance and granulomas (in up to 55% of cases) with necrosis (see the images below). 
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