Part II: Microscopy
introduction

Tuberculosis is a disease of global importance. One-third of the world?s population is estimated to have been infected with Mycobacterium tuberculosis and nine million new cases of tuberculosis arise each year. The tuberculosis crisis is likely to escalate since the human inmunodeficiency (HIV) epidemic has triggered an even greater increase in the number of tuberculosis cases. The majority of tuberculosis patients are 15 to 45 years of age, persons in their most productive years of life. Tuberculosis kills over two million people world-wide each year, more than all other infectious diseases combined, including AIDS and malaria.

Transmission of tuberculosis is virtually entirely by droplet infection, created through coughing by untreated persons suffering from pulmonary tuberculosis (the most common form) in a confined environment. Infected droplets remain airborne for a considerable time, and may be inhaled by susceptible persons.

Pulmonary tuberculosis usually occurs in the apex of the lung. These develop cavities containing large populations of tubercle bacilli which can be detected in a sputum specimen. Pulmonary tuberculosis is suggested by persistent productive cough for three weeks or longer, weight loss, night sweats and chest pains. The diagnosis can only be made reliably by demonstrating the presence of tubercle bacilli in the sputum by means of microscopy and/or culture in the laboratory.

The cornerstone of the diagnosis of tuberculosis is direct microscopic examination of appropriately stained sputum specimens for tubercle bacilli. The technique is simple, inexpensive and detects those cases of tuberculosis who are infectious, ie. those responsible for maintaining the tuberculosis epidemic. Currently no other diagnostic tool is available which could be implemented affordably.

Between 5 000 and 10 000 tubercle bacilli per millilitre of sputum are required for direct microscopy to be positive. Sputum specimens from patients with pulmonary tuberculosis - particularly those with cavitary disease - often contain sufficiently large numbers of acid-fast bacilli to be readily detected by direct microscopy. The sensitivity can further be improved by examination of more than one smear from a patient. Many studies have shown that examination of two smears will on average detect more than 90% of infectious tuberculosis cases. The incremental yield of acid-fast bacilli from serial smear examinations has been shown to be 80-83% from the first, 10-14% from the second and 5-8% from the third specimen. Therefore three sputum specimens are recommended for suspects of pulmonary tuberculosis. A negative smear result does not exclude the diagnosis of tuberculosis as some patients harbour fewer tubercle bacilli than can be detected by microscopy. A poor quality specimen may also produce negative results.

Sputum examination by microscopy is relatively quick, easy and inexpensive and must be performed on all cases suspected of having tuberculosis. Most patients with infectious tuberculosis have respiratory symptoms and the use of smear microscopy in those presenting to health services with suggestive symptoms constitutes the most efficient means of case detection. Tuberculosis microscopy is also performed to assess response to treatment and to establish cure or failure at the end of treatment.

Smear sensitivity is poor in extra pulmonary tuberculosis and in diseases caused by mycobacteria other than tubercle bacilli (MOTT). It is also virtually impossible to distinguish different mycobacterial species by microscopy. Nevertheless, in high-prevalence countries extra-pulmonary tuberculosis and MOTT disease are far less common than pulmonary tuberculosis and are neither rapidly progressive nor highly infectious. From a public health perspective, early diagnosis is therefore less important.