Smears and cultures for diagnosis of pulmonary tuberculosis in an asymptomatic immigrant population

Roberto Assael, Joaquin Cervantes, Gerardo Barrera

Clinica Medica Internacional, Ciudad Juarez, Mexico

Background: The World Health Organization estimated in 2010 that 8.8 million new tuberculosis (TB) cases. About one-third of the world's population is infected and 10% will develop active TB disease. While cultures remain the international gold standard for diagnosing TB disease, in many other low-income countries, sputum smears remain the only and most accessible tool with which to diagnose active TB disease. As a consequence, in patients with TB who have negative smears, their TB remains undetected.
Aim: The objective of the study reported here was to demonstrate the proportion of smear-positive/culture-positive cases compared with smear-negative/culture-positive TB cases in Mexican immigrants bound for the USA.
Methods: A retrospective study was undertaken of the medical records of 122 active TB cases diagnosed at a clinic in Ciudad Juarez, Mexico, from 2009 to 2012. All cases were confirmed by culture, regardless of the sputum smear results.
Results: Of the cases, 80% (97 active TB cases) had negative sputum smears, while only 25 cases (20%) had at least one positive smear. All of the cultures were confirmed as positive for Mycobacterium tuberculosis complex.
Conclusion: The fact that 80% of the TB cases were smear negative and 20% smear positive shows that there is a clear gap between the actual state of active TB disease within patients under screening conditions, meaning that eight out of ten actual cases are being missed when sputum smear is the only diagnostic tool in asymptomatic patients with abnormal chest X-rays. Based on these results, it is highly recommended that countries that have not standardized culturing as the gold standard for the diagnosis of active TB do so, so that TB cases – which may endanger global public health – are not missed. It is also recommended that further studies be undertaken to determine the clinical background of the patients diagnosed by smear and culture to identify a direct relationship between clinical signs and symptoms and the smear result.

Keywords: TB, screening, Mexico, mycobacteria growth indicator tube, Löwenstein–Jensen agar