Brucellosis is one of the major zoonotic diseases worldwide, responsible for a wide range of medical, public health, and economic problems, especially in those countries where this disease is endemic such as in the Mediterranean region and Arabian Peninsula [14].

The clinical diagnosis of brucellosis with its variations (systemic, complicated, localized, or neurobrucellosis) is challenging since clinical symptoms and signs can overlap with a wide range of infectious and noninfectious diseases (see Chap. 2 in this book). The Brucella-specific laboratory tests are crucial for the diagnosis, as will be discussed in this chapter.

The routine hematology and biochemical profiles in patients with brucellosis are usually within normal limits, with some elevation in erythrocyte sedimentation rate and liver function tests. The cerebrospinal fluid (CSF) analysis, in both adults and children, is nonspecific and can overlap with other central nervous system (CNS) diseases such as mycobacterial, viral, syphilitic, or fungal infections or with noninfectious diseases such as mental and psychiatric illnesses, multiple sclerosis, and cancer [17, 18, 21, 25].

To reach an accurate diagnosis, the physician should be aware of this disease and its suggestive clinical signs and symptoms and must resort to specific Brucella tests. So far, the most practical, helpful, and useful laboratory diagnostic tests for the diagnosis of systemic or complicated cases of brucellosis, including the CNS involvement, are the standard culture and serologic tests. In order to reach the accurate diagnosis, the order for serologic investigation should include a combination of tests: the serum agglutination test (SAT) and the Brucella indirect Coombs test, SAT and Brucellacapt, or SAT and the enzyme-linked immunosorbent assay (ELISA), to be simultaneously tested on blood and CSF specimens [2]. Other tests such as the in-house-developed molecular tests have to be standardized before adopting and incorporating them in routine clinical use [1, 2, 22]. More detailed information on these laboratory tests is discussed below.

The in-house-developed conventional polymerase chain reaction (PCR) and real-time (RT) PCR assays are being attempted to directly detect Brucella from clinical specimens, using Brucella-specific gene targets including: BCS P31 (encodes a 31-kDa cell surface protein), BP26 (encodes a 26-kDa periplasmic protein), 16S rRNA, and the insertion sequence rRNA. However, the sensitivities of these assays are quite variable, ranging from 50 to 100 %. Still, molecular assays have promising potential and can constitute a prominent role in the diagnosis but remain in need of further optimization, standardization, and improvement before their routine incorporation in the diagnosis of human brucellosis [20, 26, 28, 31].

Though Brucella spp. can easily grow on a wide range of culture media, the yield is generally variable, being higher among patients with acute brucellosis (40–90 %) than in patients in the chronic, focal, and complicated stages (5–20 %). The yield in CSF Brucella culture is also low (5–30 %) [4, 15, 25]. Its growth in culture is slow (7 days to 6 weeks) when using the conventional biphasic Ruiz-Castañeda bottle. The automated, continuously monitored blood culture systems like BacT/ALERT (bioMérieux, Durham, NC) and BACTEC (BD Diagnostics, Sparks, MD) show higher Brucella yields and earlier recovery (majority within 1 week) than the conventional culture method [2, 9, 29].

Once recovered in culture, the Brucella genus can be easily and rapidly identified using a couple of conventional tests (gram stain, catalase, oxidase, and specific agglutinating antisera). Most recently, the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry which has been used in many laboratories for routine determination of microorganisms can be helpful in identification of the Brucella species [19].

Because of the above noted technical limitations, Brucella serologic assays are usually relied upon in the laboratory diagnosis of systemic, focal, and complicated brucellosis especially neurobrucellosis. Comprehensive information on the different Brucella-specific serologic tests used in the diagnosis of human brucellosis was detailed by Araj [2] and will be summarized below.

In order to have an accurate assessment, serologic results should be interpreted in the context of the evolution of antibody responses after infection with Brucella spp. IgM first appears, followed by the appearance of IgG, IgA, and other immunoglobulins within 10–14 days. Their evolution will depend on several factors, for example, the clinical category, response to treatment, and relapse. It is of interest to note that in this disease and after treatment and cure, Brucella-specific IgG and some IgM can persist for a very prolonged time (months and sometime years) in 15–20 % of asymptomatic patients, and the explanation for this remains elusive [14, 22].