Brucellacapt® uses the Brucella abortus strain although, serologically, there is no difference between Brucella abortus, Brucella melitensis and Brucella suis. According to the Manual of Diagnostic Tests and Vaccines for Terrestrial Animals from the OIE: "It should be emphasised that antigen made with one of these B. abortus strains is also used to test for B. melitensis or B. suis infection" (http://www.fao.org/fileadmin/templates/rap/files/meetings/2014/140318-reference.pdf)
According to the Manual of Clinical Microbiology, 8th ed from the ASM: "The standard SAT method uses killed whole-cell B. abortus antigen and detects antibodies against B. abortus, B. suis and B. melitensis but not B. canis."
According to WHO, the only way to distinguish between B. abortus and B. melitens is through bacteriological culture (isolation of the bacteria). Brucellacapt®, like the other current serological methods (ELISA, Rose Bengal, SAT), will not discriminate between Brucella abortus, Brucella melitensis or any of the strains that contain LPS, which is common to all species of Brucella.
Scientific literature supports that the treatment for both infections is the same (both are sensitive to the same antibiotics) with the exception of some rare strains such as B. melitensis strain REV-1.Therefore, differentiation is only relevant from the epidemiological point of view only. The use of Brucellacapt® offers an additional advantage (compared ELISA, SAT, Rose Bengal etc…) since it allows patient follow-up during their treatment, and an early detection of reinfections and chronic phases of the illness. In any case, please refer to the recommendations published by the WHO for diagnosis and treatment of brucellosis.
As with Rose Bengal, the acidic medium used in the reaction avoids the prozone phenomenon. Prozone in Brucellacapt® is extremely rare. Sometimes we have found it in highly positive sera, but only in the first dilution at 1/40, and never at titers higher than that. With the dilution proposed for screening there should never be prozone, if the technique is performed correctly. In the case of titration, the first dilution will be negative and the rest positive. Again, this is rare, and more commonly found in other techniques such as common agglutination.
Brucellacapt® can be used in two ways: in titration (24 tests), or in screening (96 tests). Both procedures are explained in the product package insert. The kit includes enough diluent to perform the titration. To use it in screening mode, additional serum diluent will be required (Ref. B0002). This can be acquired separately through your local distributor. In both cases, it is highly recommended to use of a humid chamber during incubation time, to avoid desiccation of the sample.
Brucellacapt® can be used for detection of acute brucellosis. Its sensitivity and specificity in both acute and chronic cases is reflected in the scientific literature. However, in prevalent areas where many patients are tested every year, there are faster methods for easy detection of acute cases (Rose Bengal, ELISA). The application of Brucellacapt® in endemic areas becomes relevant for the detection of chronic cases, since none of the mentioned methods is sufficiently sensitive for an adequate diagnosis of the focalized forms of brucellosis.
The three methods (Rose Bengal, ELISA and Brucellacapt®) would be equally accurate for the detection of acute cases. Rose Bengal would show false negatives in samples of chronic and past infections, (approximately 75% of chronic cases are not detected). ELISA and Brucellacapt® would detect all samples from chronic and past infections. ELISA, however, cannot distinguish between them. Using Brucellacapt®, titres above 1/320 are indicative of brucellosis, but other clinical tests and the seroprevalence of the disease in the area should always be evaluated in parallel before a diagnosis is made. In addition, Brucellacapt® allows monitoring of the treatment of chronic disease.
8. The results of the Brucellacapt® test are given in terms of positive and negative results only. How to identify IgA and IgG?
This test is not intended to identify the IgA and IgG parameters. Brucellacapt® is a quantitative test that allows the detection of agglutinating and non-agglutinating antibodies and their quantification through an appropriate titration. It allows a better correlation with the diagnosis of the chronic stages of brucellosis and unlike the IgG ELISA kits, Brucellacapt® differentiates the chronic cases from the past infections already cured (those related to IgG assays). If the customer needs to detect IgG or IgM, it will be necessary to perform an ELISA test with specific anti-IgG or IgM conjugates (or any other alternative technique such as IFA). The agglutination techniques do not allow the detection and specific differentiation of IgG and/or IgM.
This will depend on the prevalence of the disease among the population in a particular country or area. In a low prevalence area, detection of acute brucellosis can be done by either IgG or IgM, since the presence of any of them is a significant marker of infection. Besides the prevalence, other factors such as the clinical history of the patient and the stage of the disease when the sample was taken should always be considered.
The SAT test refers to Wright’s Serum Agglutination Test. Rose Bengal is a rapid agglutination test that contains acidic medium which makes it more sensitive and specific than the SAT.
12. Why does Brucellacapt® establish a titer of 1/320 - 1/640 as a limit to distinguish between healthy and sick?
Although widely used in all countries nowadays, Brucellacapt® was designed in its origin for detection of chronic cases in areas of high prevalence. The cut-off dilution for Brucellacapt® is based on clinical evidence and seroprevalence data of the disease in endemic areas, and it was established during the course of several published studies, which are available on our website. Laboratories in endemic countries screen samples at 1/320 and 1/640, as in those areas lower titers are seldom considered significant.
The correct way to establish the limit titer is to compare samples from patients with samples from healthy population, and carry out a ROC analysis. In an area of low prevalence of the disease, a 1/320 cut off for the screening assay seems too high, and in such circumstances, a complete titration would be advisable. The screening procedure detailed in the IFU was designed for specific circumstances, but there would be no technical inconvenience in carrying out the screening procedures at different titers, whenever the calculations to achieve the final titers used are carried out carefully. For instance, in a country like Germany, a screening dilution of 1/80 1/160 would be adequate.