Details of the search strategy and selection criteria can be found in the Methods section.
ReviewSputum processing methods to improve the sensitivity of smear microscopy for tuberculosis: a systematic review
Introduction
The burden of disease and death caused by tuberculosis is immense, with 8·8 million cases and nearly 2 million deaths estimated to have occurred in 2003 alone.1 The HIV epidemic has had a huge impact, driving up incidence rates dramatically in sub-Saharan Africa.1, 2, 3, 4 In addition, tuberculosis is a major cause of death among people who are HIV infected, currently accounting for at least 11% of AIDS deaths worldwide.5 An important barrier to global tuberculosis control is the low rate of case detection. Although the proportion of smear-positive cases identified is increasing, the proportion that were identified globally under directly observed therapy (short-course) programmes (the internationally recognised tuberculosis control strategy) was only 45% in 2003.1 The World Health Assembly set a global target to detect 70% of new smear-positive cases (70% case detection rate) by 2005. This target was not met.6 To prevent transmission of Mycobacterium tuberculosis and to provide appropriate care for patients, prompt and accurate diagnosis of tuberculosis is a matter of great urgency.7, 8
Sputum microscopy is the most important test for the diagnosis of pulmonary tuberculosis in low-income and middle-income countries, where 95% of tuberculosis cases and 98% of deaths occur.9 In these countries, most laboratories use smears of unconcentrated sputum (direct smears) with Ziehl-Neelsen staining. Microscopy is fast, simple, inexpensive, widely applicable, and highly specific for M tuberculosis in tuberculosis-endemic countries. In addition, microscopy identifies the most infectious patients.10, 11, 12 Although microscopy has been reported to have greater than 80% sensitivity for identifying cases of pulmonary tuberculosis in some settings,13, 14 the sensitivity of the test has been low and variable in other reports (range 20–60%).15 Smear-negative tuberculosis is disproportionately higher in HIV-positive than in HIV-negative individuals,16, 17 and has been linked to poor treatment outcomes, including death, especially in areas devastated by the HIV epidemic.18, 19 Microscopy contributes little to the diagnosis of paediatric pulmonary tuberculosis,20 and does not, by definition, identify smear-negative pulmonary tuberculosis. Clearly, improvement in the sensitivity of microscopy would be of great potential value.
Reports describing newer sputum processing methods as well as calls for re-examination of existing methods have prompted interest in the assessment of chemical processing and sputum concentration to improve the sensitivity of microscopy.11, 21, 22, 23 Chemicals, such as sodium hydroxide (NaOH) and a solution of N-acetyl L-cysteine and sodium hydroxide (NaLC-NaOH) to liquefy sputum, together with centrifugation, are widely used in modern laboratories.24, 25 A recent review of studies using sodium hypochlorite (NaOCl; bleach) to treat sputum followed by centrifugation found a significant increase in sensitivity compared with the direct smear method.21 This review, however, did not address gravity sedimentation and other physical or chemical methods that have been investigated.
We did a systematic review to assess the ability of various sputum processing methods to improve the accuracy of microscopy, compared with the direct (unconcentrated) method. We specifically addressed two questions: (1) Do sputum processing methods increase the sensitivity of microscopy in persons with and without HIV infection? (2) What is the influence on overall test accuracy of specific chemical treatments of sputum, of physical concentration methods, and of combinations of these methods?
Section snippets
Search strategy and selection criteria
Our initial searches were done in 2004 and updated in 2005. We searched the following databases for primary studies: PubMed (1950 to May, 2005), BIOSIS (1969 to November, 2004), Embase (1974 to 2004), and Web of Science (1945 to 2004). The search terms used included the following: “tuberculosis”, “Mycobacterium tuberculosis”, “acid-fast bacilli”, “sputum microscopy”, “bacteriology”, “sensitivity and specificity”, “sputum concentration”, and “direct microscopy”. We hand searched two journals
Description of included studies
Of the 3538 citations identified after literature searches, 46 articles (44 published,23, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74 [reference 73 as an abstract] and two unpublished [L Cuevas, Liverpool School of Tropical Medicine, Liverpool, UK, personal communication) consisting of 83 studies, met our eligibility criteria (figure 1). We considered most studies to be
Discussion
Our systematic review of 83 studies, most using Ziehl-Neelsen staining, that compared microscopy using processed sputum with the direct smear method, indicates the following: (1) sputum treated with bleach or NaOH and concentrated by centrifugation is, on average, more sensitive; (2) sputum subjected to overnight sedimentation preceded by treatment with ammonium sulphate or bleach, is, on average, more sensitive, based on a small number of studies; (3) the specificity for processed smears is
Search strategy and selection criteria
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