Bulk acid-fast staining of sputum smears: time to end a taboo.
METHODS
A high-throughput laboratory routinely performing fluorescence microscopy for acid-fast bacilli (AFB) smear with automated bulk staining.


OBJECTIVE
To determine the risk of false-positive AFB sputum smears from bulk staining showing as smear-positive, culture-negative specimens, or a decrease in smear- and culture-positives.


METHODS
Direct AFB smear and Löwenstein-Jensen culture were performed for a total of 39,350 routine sputum specimens. Of these, 6633 were randomly selected for individual AFB staining, while the remaining 32,717 were processed by bulk machine staining. Positives for smear and culture were compared.


RESULTS
Overall, 111 specimens yielded a positive individually stained smear; of these, 100 (90.1%, 95%CI 83.0-95.0) were also culture-positive compared to 504/543 smear-positives after bulk staining (92.8%, 95%CI 90.6-95.0). The proportions of smear-positive, culture-negative and smear- and culture-positive specimens were respectively 1.8% vs. 2.2% and 90.1% vs. 92.8%, for individual and bulk staining (non-significant).


CONCLUSIONS
The risk of transferring AFB from positive to negative smears during bulk AFB staining is negligible, if it occurs at all. Bulk staining should not be discouraged, as even in low-income countries this method will save significant resources, particularly manpower, and improve staining results in laboratories with a high workload.
Kam
K M
KM
TB Reference Laboratory, Public Health Laboratory Services Branch, Centre for Health Protection, Department of Health, Hong Kong, China.
Yip
C W
CW
Tang
H S
HS
Van Deun
A
A
D057205:Automation, Laboratory; D001431:Bacteriological Techniques; D005189:False Positive Reactions; D006801:Humans; D008856:Microscopy, Fluorescence; D009169:Mycobacterium tuberculosis / Q000302:isolation & purification*; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D013048:Specimen Handling; D013183:Sputum / Q000382:microbiology*; D013194:Staining and Labeling; D013609:Taboo; D014376:Tuberculosis / Q000175:diagnosis* / Q000382:microbiology