TB diagnostics and laboratory strengthening - WHO policy
Definition of a new sputum smear-positive TB case, 2007
The revised definition of a new sputum smear-positive pulmonary TB case is based on the presence of at least one acid fast bacilli (AFB+) in at least one sputum sample in countries with a well functioning external quality assurance (EQA) system.
Detailed background information
Proposal for a revision of the case definition of "Sputum Smear-Positive Tuberculosis"
As highlighted in the Stop TB Strategy, quality-assured bacteriological examination is an essential element for diagnosis and management of TB patients harbouring susceptible or resistant bacilli. During the last two years, an increasing number of countries are scaling up external quality assurance programmes for smear microscopy by means of blinded re-checking of slides. As a result, the quality of smear microscopy examination reached a satisfactory level in some countries. Evidence suggests that countries with a functional EQA system have very low frequency of false positive cases.
Key issues for WHO action
A number of key meetings and workshops were held where the TB case definition was discussed. These meetings included the Stop TB Partnership Laboratory Strengthening Subgroup (SLCS), an expert group meeting organized by the UNION held in Belgium and a technical expert workshop held in the Netherlands. Recent scientific evidence [Ref. 1,2] was reviewed and it was concluded that where a functional EQA for smear microscopy is in place, the finding of a single AFB in at least one single sputum smear examination in a TB suspect would satisfy the criterion to report a patient as having "sputum smear-positive tuberculosis" and to subsequently start treatment.
It should be noted that the definition of bacteriological failures has not been reviewed; hence, no change in definition of failure cases is proposed at this stage.
Given this policy revision, WHO will:
- guide and support countries in making country-specific plans of action for modifying all normative, training, and recording and reporting tools;
- provide technical assistance to countries to upgrade and fully expand functional external quality assurance (EQA) systems for TB laboratory services;
- provide guidance on study design, and sampling methodologies, in order to evaluate new diagnostic technologies (in collaboration with the Special Programme for Research and Training in Tropical Diseases (TDR));
- monitor and evaluate the impact of the change of policy on case detection at country level.
1. Mase S, Ramsay A, Ng N, Henry M, Hopewell PC, Cunningham J, Urbanczik R, Perkins M, Aziz MA, Pai M. Yield of serial sputum specimen examinations in the diagnosis of pulmonary tuberculosis: a systematic review. Int J Tuberc Lung Dis 2007;11(5):485-95)
2. Bonnet M, Ramsay A, Gagnidze L, Githui W, Guerin PJ, Varaine F. Reducing the number of sputa examined, and thresholds for positivity: An opportunity to optimize smear microscopy. Accepted for publication, Int J Tuberc Lung Dis