Comparison of Direct and Concentrated Acid-Fast Smears To Identify Specimens Culture Positive for Mycobacterium spp

Abstract

Microscopic examination of respiratory specimens for acid-fast bacilli (AFB) plays a key role in the initial diagnosis of tuberculosis, monitoring of treatment, and determination of eligibility for release from isolation. The objective of this study was to compare the sensitivity obtained with smears for detection of AFB (AFB smears) made directly from respiratory specimens (direct AFB smears) to that obtained with parallel smears made from concentrates of the specimens (concentrated AFB smears). A total of 2,693 specimens were evaluated; 1,806 were from the University of California Irvine Medical Center Medical Microbiology Laboratory (UCIMC), which serves a tertiary-care hospital with outpatient clinics, and 887 were from the Microbial Disease Laboratory at the California Department of Public Health (MDL), which receives specimens from outpatient facilities and clinics on Pacific islands. Of the 353 AFB culture-positive specimens at UCIMC, there was a statistically significant difference in the sensitivity of the direct AFB smear (34%) and that of the smear made from the concentrated specimen (58%) (P < 0.05). This was also true for the 208 specimens positive for Mycobacterium tuberculosis, for which the sensitivity of the direct smear was 42% (87 of 208) and that for the smear made from the concentrated specimen was 74% (154 of 208). At MDL, where all but 1 of the 45 culture-positive specimens grew M. tuberculosis, the sensitivity of the smear made from the concentrated specimen was 93% (42 of 45) and was not significantly higher than the sensitivity of the direct smear, which was 82% (37 of 45). By combining the results from both laboratories, 42 patients from whom at least three specimens were received were culture positive for M. tuberculosis. The cumulative results for the initial three specimens from these patients showed that the direct smear detected M. tuberculosis in 81% of these patients, whereas the smear made from the concentrate detected M. tuberculosis in 91% of these patients. In summary, when all culture-positive specimens are considered, the sensitivity of the direct smear compared to that of a smear made from the concentrated specimen was significantly different overall in the two different laboratory settings. However, this difference was reduced only if the cumulative results for the initial three specimens received from patients who were culture positive for M. tuberculosiswere evaluated.

Evaluation of two pretreatment methods for the detection of Mycobacterium tuberculosis in suspected pulmonary tuberculosis

“…The most widely used method to diagnose pulmonary TB is the detection of AFB in sputum by ZN staining technique in TB control programs. However, microscopy of direct sputum smear has a low sensitivity (analytical sensitivity 10,000 bacilli per milliliter), often not more than 20 to 65%, in comparison with culture . This statement was virtually supported in the current study as well, wherein the sensitivity and specificity of direct AFB smear was found to be 67.03% and 100%, respectively.…”

“…The detection of acid fast bacilli (AFB) directly from sputum using Ziehl‐Neelsen (ZN) staining continues to be the rapid technique for the diagnosis of pulmonary TB in global TB control programs . However, the direct AFB smear has discouragingly low sensitivity . In the preceding years, various methods have been attempted to improve smear microscopy for AFB.…”

Revisiting Acid-Fast Bacilli Microscopy of Concentrated Sputum Smears as an Efficient Tool for the Diagnosis of Tuberculosis: A Study from a Tertiary Care Centre in Southern India

“…One of these is a study conducted in Bangladesh (TB endemic country) by Uddin et al [16] which concurs with this opinion and reports a sensitivity of 83% by the concentrated smear technique as opposed to 71% by the direct smear. Another one of these is a study by Peterson et al conducted [17] in two differing laboratory set-ups (a tertiary care hospital laboratory and multiple other outpatient clinics) found that in the tertiary care hospital arm of the study, the direct smear was considerably less sensitive than the concentrated sputum smear (28% and 51%, respectively).…”

A Study for Detection of Missed Tuberculosis Cases at a Tertiary Care Centre in Northern Bihar, India

“…The reaction was stopped by addition of phosphate buffer followed by centrifugation at 2500 x g for 15 mins (modified Petroff 's method). 4 Smears were prepared from the deposit. ZN staining was performed on these smears and microscopy was done to search for the presence of Mycobacterium tuberculosis if any.…”

“…1 Direct sputum microscopy by Ziehl-Neelsen technique still remains the most important tool for detecting Mycobacterium tuberculosis after smear preparation from the yellow and most purulent part of the sputum. 3 The importance of centrifugation and concentration methods (like modified Petroff's method) have been proved to increase the sensitivity of sputum smear examination for Mycobacterium tuberculosis by many studies, 4 but still is not being followed at DOTS and Designated Microscopy Centres (DMCs) under RNTCP.…”