Vertebral osteomyelitis, spondylodiscitis, and epidural abscess are clinical pictures with an incidence of 2.4/100,000/year, which increases with age. Many patients have at least one of the risk factors, such as diabetes, immunosuppression, malignancy, or spinal surgery history. Vertebral infection should be suspected clinically with insidious back pain that increases significantly at night, accompanying fever with a rate of 35–60% and focal neurological symptoms [8]. Bacteria are generally responsible for the etiology, and Staphylococcus aureus (32–67%) is the most frequent finding [9]. Coagulase-negative staphylococci, E. coli, and Pseudomonas species may also be responsible, especially in those with a history of injecting drug use. In countries where tuberculosis and brucellosis are endemic, Mycobacterium tuberculosis and Brucella species should be considered causative agents [10]. Microbiological methods should prove diagnosis supported by imaging methods, and microbiological diagnosis of the samples taken by interventional procedures in patients without surgical indication is essential in the correct diagnosis and management of the patient. Thanks to the serological test results, a connection was established with the vertebral complication in our case. However, the development of paravertebral abscesses and bilateral empyema, which are rare complications, necessitated the differential diagnosis of other diseases, especially tuberculosis. A definitive diagnosis could only be made due to the evaluation of pleural fluid samples with microbiological examinations. The diagnosis can be made when Brucella species are isolated from vertebral, paravertebral, or epidural tissue samples, abscess material, or blood cultures or when RBPT positivity and a Brucella standard tube agglutination titer above 1/160 are detected in addition to clinical findings. Owing to the specificity of laboratory evaluation, only 5% of patients with suspected vertebral brucellosis require a biopsy [11].
RBPT is often used as a rapid screening test to diagnose brucellosis. Although the sensitivity of RBPT has been reported to be very high, its specificity may be low, especially in endemic areas and chronic cases [12]. The test’s positive predictive value is low and positive results must be confirmed by a more specific test. In the presence of a high antibody titer, false negativity may result due to the prozone phenomenon [13]. Therefore, RBPT or STA negativity does not exclude Brucella infection. Negative results can be detected in the early stages of the disease and in the presence of blocking antibodies. In clinically compatible patients, dilution of patient serum at high titers increases the sensitivity of the SAT-Coombs combination [14]. Coombs test (antihuman globulin test), which is necessary for complicated and chronic cases, eliminates the effects of blocking antibodies. Evidence obtained in a limited number of these cases reports that the Coombs test is the best indicator of relapses and seroconversion in parallel with treatment [13, 15].
BrucellaCapt test is also an immuno-agglutination technique. It is a modification of the Coombs test to detect incomplete or blocking IgG and IgA antibodies. It has similar sensitivity and specificity to classical tests in diagnosing brucellosis, and its ease of application is an important advantage [16,17,18].
The gold standard for diagnosis is the growth of bacteria in blood cultures or tissue cultures (for example, joint fluid or bone marrow). However, the isolation of the microorganism is complicated. Bone marrow aspirate culture is invasive and clinically impractical. The sensitivity of blood cultures is reported to be between 17% and 85% [19].
In our case, although STA was positive at a titer of 1/320 and RBPT was negative, Brucella agglutination test with Coombs was found to be > 1/1280 positive. Ordu province is endemic to brucellosis, and the interpretation of serological test results can be difficult. Therefore, it may be necessary to confirm negative results in both positive and suspicious patients.
It should be taken into account that negative RBPT results may be observed, especially in cases with high clinical suspicion, and may need to be supported by other serological diagnostic tests when necessary. We think that using the BrucellaCapt test is valuable in faster diagnosis in endemic areas and effective patient management. Despite the confusing clinical and laboratory results in this case, in which more than one rare complication developed, it was possible to reach the diagnosis by applying adequate and correct microbiological examinations.