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editorial
. 1999 Dec;37(12):4201. doi: 10.1128/jcm.37.12.4201-4201.1999

Field Evaluation of Rapid Tests for Tuberculosis Diagnosis

Voahangy Rasolofo 1,*, Suzanne Chanteau 1
PMCID: PMC85929  PMID: 10636726

We read with great interest the letter previously published by M. P. Grobusch et al. (1). Evaluation of diagnostic tests is indeed a topical problem, especially in developing countries. We agree with the authors' remark that the rapid test (AMRAD/ICT) could be considered only as an adjunct test to standard techniques of tuberculosis (TB) diagnosis. However, we would like to comment on the last conclusion drawn by the authors. The reported study was realized in a European hospital, and the specificity observed was very good. This would not be always the case in developing countries, particularly in tropical areas where people are permanently in contact with various pathogens and develop cross-reacting antibodies responsible for poor specificity. Therefore, it is important that every new test and, particularly, every new serodiagnostic assay be validated with the controls of the country where the test will be applied. Furthermore the low sensitivity of the AMRAD/ICT assay (50%) will result in false-negative results for many patients. Thus, although the specificity is excellent, the negative predictive value of the test will be unacceptably low even in countries with a high prevalence of TB. Therefore, we do not believe that such a test would be suitable as a first-line method of diagnosis. Sputum smear microscopy has been proven to be very useful for diagnosis of contagious TB cases in developing countries where diagnostic tools such as radiology are missing. Still, a serological test for the diagnosis of paucibacillary and extrapulmonary TB would be of interest if it were more sensitive and faster and could be used on large series of samples. Also, keep in mind that serodiagnostic tests necessitate blood sample collection and disposable materials that are not readily available in most countries with a high prevalence of human immunodeficiency virus or hepatitis. In resource-poor areas, all these factors have to be taken into account when evaluating the efficacy and cost of a rapid test.

REFERENCE

  • 1.Grobusch M P, Schürmann D, Schwenke S, Teichmann D, Klein E. Rapid immunochromatographic assay for diagnosis of tuberculosis. J Clin Microbiol. 1998;36:3443. doi: 10.1128/jcm.36.11.3443-3443.1998. [DOI] [PMC free article] [PubMed] [Google Scholar]
J Clin Microbiol. 1999 Dec;37(12):4201.

AUTHOR'S REPLY

Martin P Grobusch 1

We thank Drs. Rasolofo and Chanteau for their thoughtful comments on our previously published work (1-4). It is obvious from the data published so far that we are aware of (1-1, 1-4, 1-6) that the sensitivity and specificity of the test vary with the region where it is used. We understand that when one ponders the possible use of this rapid immunochromatographic antigen detection assay for TB diagnosis in a certain world region, its significance in terms of high positive and negative predictive values has to be judged on the basis of sensitivity and specificity data obtained in pilot trials in that particular setting in comparison to locally used standard methods.

As stated in our letter, we conclude that the currently available assay as presently applied cannot be considered an ultimate breakthough in the serodiagnosis of TB, particularly in countries where reliable diagnostic tools are available regardless of costs. However, we do not share Rasolofo's and Chanteau's belief that the use of this assay in certain settings as a first-line diagnostic tool should be dismissed on the grounds of these data.

At present, TB diagnosis in developing countries relies largely on clinical features, X ray, and microscopy of acid-fast sputum stains. Cultivation as the diagnostic “gold standard” is a costly (and therefore not ubiquitously available) and complex technique, requiring up to several weeks before a definite diagnosis is established (1-1). By using serodiagnostic methods, the time required for reaching a clinical decision to treat a suspected case of tuberculosis can be drastically reduced, as in some cases the actual proof of an organism's identity does not depend on its isolation or identification in a specific culture (1-3). At present, serodiagnosis is a rapid technique that is technically facile, can be automated, and, if in routine use, is inexpensive. The technique is also usable for diagnosing nonpulmonary TB and is particularly attractive for the identification of TB manifestations in which specimens are not easily accessible, e.g., skeletal TB.

Serodiagnosis in its current form carries a potential which is not entirely sounded so far, particularly if, as hypothesized earlier, the assay detecting human antibody response to the 38-kDa antigen indicates mycobacterial disease requiring treatment in the immunocompetent patient rather than tuberculosis sensu strictu (1-5). We would therefore like to underline again that this simple technique and its attractive price appear to make it particularly interesting for use in developing countries (1-2).

REFERENCES

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