P:Patients with suspected TB

I: AFB stain for diagnosis


O: Measured sensitivity of AFB stain (adverse events from releasing smear negative pts who turn out to be culture positive)

Clinical Question:

Should we discharge patients with suspected TB with smear negative results without confirmed culture results?

Evidenced-Based Sources:

1. Chest. 1989 Jun;95(6):1193-7.

A reevaluation of sputum microscopy and culture in the diagnosis of pulmonary tuberculosis.

Levy H, Feldman C, Sacho H, van der Meulen H, Kallenbach J, Koornhof H.
Department of Medicine, Hillbrow Hospital, Johannesburg, South Africa.

This prospective study was undertaken to determine the interpretation of "scanty-positive" acid-fast bacilli on microscopy and to reevaluate simultaneous microscopy and culture of sputum for the accurate diagnosis of pulmonary tuberculosis (PTB). A total of 2,560 specimens were processed from 727 patients. There were 435 positive specimens (17.0 percent), originating from 139 patients, 10 by microscopy only, 176 by culture only, and 249 on both microscopy and culture. Review of the hospital records showed that 107 patients had PTB, 1 had Mycobacterium kansasii colonization, and 31 were thought not to have PTB. Sensitivity and specificity were 53.1 and 99.8 percent for microscopy, 81.5 and 98.4 percent for culture, and 77.6 and 100 percent for microscopy and culture, respectively. Seventy-five microscopy specimens (46 patients) were reported as scanty-positive, of which five (four patients) were deemed false positives, yielding a positive predictive value of 93.3 percent. In those patients with positive sputum microscopy, acid-fast bacilli were detected in one of the first four specimens. Seven isolates (three patients) were mycobacteria other than tubercle (0.27 percent of specimens and 1.6 percent of mycobacteria cultured). Despite the ready availability of laboratory evidence of disease, only 73 percent of cases were diagnosed by ward staff and 36 percent notified by the primary physician. Eleven patients (10.3 percent) died, six of whom had not received diagnoses of PTB before death. Sputum microscopy and culture remains reliable despite Bayesian predictions when applied to a population with a decreasing incidence of tuberculosis.

PMID: 2656111 [PubMed - indexed for MEDLINE]

2. Jpn J Infect Dis. 2007 May;60(2-3):73-5.

Is it valuable to examine more than one sputum smear per patient for the diagnosis of pulmonary tuberculosis?

Ozkutuk AOzkutuk A, Terek GTerek G, Coban HCoban H, Esen NEsen N.

Department of Microbiology and Clinical Microbiology, Dokuz Eylul University Faculty of Medicine, Izmir, Turkey.

The simplest, cheapest, and fastest diagnostic method for tuberculosis (TB) is the detection of acid-fast bacilli (AFB) by microscopy. The algorithm advised for the diagnosis of TB recommends examination of three consecutive sputum specimens from TB suspects for the presence of AFB. In the present study, we evaluated the contribution of each specimen to the final detection of TB suspect patients with culture-proven disease. The collection and analysis of retrospective data on patients with culture-proven pulmonary TB, from June 2002 to August 2006 at Dokuz Eylul University Hospital, Turkey, have enabled us to assess the value of examining two sputum specimens in diagnosing this disease. AFB were detected from one or more sputum specimens with direct microscopy in 42% of the cases. An analysis of results of smear examination showed that 97% of AFB were detected from the first specimen and only 3% were obtained from the second smear. The third specimen did not have any additional diagnostic value for the detection of AFB by microscopy. As a conclusion the present study shows that examining two sputum smears is sufficient for the early detection of AFB in our laboratory.
PMID: 17515635 [PubMed - indexed for MEDLINE]

3. Centers for Disease Control and Prevention


4. Lancet. 1999 Feb 6;353(9151):444-9.

Transmission of Mycobacterium tuberculosis from patients smear-negative for acid-fast bacilli. Behr MA, Warren SA, Salamon H, Hopewell PC, Ponce de Leon A,Daley CL, Small PM

Department of Medicine, McGill University Health Centre, Montreal, Quebec, Canada.

BACKGROUND: The microscopic examination of sputum for acid-fast bacilli, is a simple and rapid test that is used to provide a presumptive diagnosis of infectious tuberculosis. While patients with tuberculosis with sputum smears negative for acid-fast bacilli are less infectious than those with positive smears, both theoretical and empirical evidence suggest that they can still transmit Mycobacterium tuberculosis. We aimed to estimate the risk of transmission from smear-negative individuals. METHODS: As part of an ongoing study of the molecular epidemiology of tuberculosis in San Francisco, patients with tuberculosis with mycobacterial isolates with the same DNA fingerprint were assigned to clusters that were assumed to have involved recent transmission. Secondary cases with tuberculosis, whose mycobacterial isolates had the same DNA, were linked to their presumed source case to estimate transmission from smear-negative patients. Sensitivity analyses were done to assess potential bias due to misclassification of source cases, unidentified source cases, and HIV-1 co-infection. FINDINGS: 1574 patients with culture-positive tuberculosis were reported and DNA fingerprints were available for 1359 (86%) of these patients. Of the 71 clusters of patients infected with strains that had matching fingerprints, 28 (39% [95% CI 28-52]) had a smear-negative putative source. There were 183 secondary cases in these 71 clusters, of whom a minimum of 32 were attributed to infection by smear-negative patients (17% [12-24]). The relative transmission rate of smear-negative compared with smear-positive patients was calculated as 0.22 (95% CI 0.16-0.32). Sensitivity analyses and stratification for HIV-1 status had no impact on these estimates. INTERPRETATION: In San Francisco, the acid-fast-bacilli smear identifies the most infectious patients, but patients with smear-negative culture-positive tuberculosis appear responsible for about 17% of tuberculosis transmission.PMID: 9989714 [PubMed - indexed for MEDLINE]

Key Points:

1. The sensitivity of Acid-fast bacilli on soutum smear is 53.1% but the sensitivity of culture is 81.5%

2. In a series of AFB direct microscopy 68 of 70 known+ TB pts (97%) were detected from the 1st smear, the remaining 3% were detected on the 2nd smear. In this group a 3rd smear was not needed.

3. The CDC recommendation is to "test three expectorated sputa for mycobacterial culture."

4. "The acid fast bacilli smear identifies the most infectious patients, but patients with smear-negative culture-positive tuberculosis appear responsible for about 17% of tuberculosis transmission."