Am. J. Respir. Crit. Care Med.,
Volume 161, Number 5, May 2000, 1559-1562
A Minimum 5.0 ml of Sputum Improves the Sensitivity
of Acid-fast Smear for Mycobacterium tuberculosis
JOHN R.
WARREN,
MONDIRA
BHATTACHARYA,
KLEPER N. F.
DE ALMEIDA,
KATHY
TRAKAS,
and
LANCE R.
PETERSON
Departments of Pathology and Medicine, Northwestern University Medical School, and the Clinical Microbiology Laboratory, Northwestern
Memorial Hospital, Chicago, Illinois
Detection of acid-fast bacilli (AFB) by sputum smear supports
treatment decisions with pulmonary tuberculosis (TB), but smear sensitivity for Mycobacterium tuberculosis is only approximately 45 to 75%. In an effort to increase sensitivity, smears were prepared using a minimum sputum volume of 5.0 ml. Sensitivity of smears during a 39-mo period (n = 1,849) using
5.0 ml of sputum was 92.0%, significantly greater (p < 0.001) than a sensitivity of 72.5%
in a previous 24-mo period (n = 3,486) when all specimens were
processed regardless of volume. All new cases of TB (n = 18) were
smear-positive with
5.0 ml of sputum before treatment, and all
were receiving antituberculosis drugs at hospital discharge. In
contrast, significantly fewer new cases of TB (14 of 26, p = 0.002)
were positive before treatment when smears were prepared using
sputum of any volume, and significantly fewer of these new TB
cases (18 of 26, p = 0.03) were receiving treatment at hospital discharge. The eight cases without treatment were smear-negative. These results indicate that acid-fast smear using
5.0 ml of sputum increases sensitivity for M. tuberculosis and accelerates treatment of TB.