Since mucous traps AFB and protects other organisms

from effective decontamination a combination of 2% NaOH

(decontaminant) and 0.5% N-acetyl-L-cysteine (mucolytic

agent) is preferably employed. Neutralization of strong

decontaminating solutions before using the sample

for AFB stain and culture is usually accompanied with

sequential buffered wash of the concentrated sample

because if the pH of the concentrate remains alkaline

or acidic it can destroy the culture medium and prevent

the growth of mycobacteria and staining efficiency of the

AFB smears. The buffered wash also helps in reducing

the specific gravity of specimen and sediments the

Mycobacterium more effectively.

Another important aspect post-decontamination is

the specimen concentration and relative centrifugal force

applied to the specimen. Improvement in correlation

between specimen showing a positive smear for AFB and a

positive culture has been demonstrated by increasing the

centrifugal force applied to pellet the specimen.

Microbiology and Bacteriology 847

Recommendations for sample collection for mycobacterial isolation and acid fast staining

Specimen type Specimen requirements Special instructions Unacceptable specimen

Abscess contents aspirated fluid As much as possible in syringe

with Luer tip cap

Cleanse skin with alcohol before

aspirating sample. Laboratory

may provide 7H9 broth/Kirchner

medium for transport of small

volumes of aspirates

Dry swab

Blood 10 mL SPS (yellow top) blood

collection tube or 10 mL isolator

tube

Disinfect site as for routine

blood culture. Mix tube contents

immediately after collection.

SPS is preferred anticoagulant

Heparinized blood is also

acceptable

Blood collected in EDTA, which

greatly inhibits mycobacterial

growth even in trace amounts

Coagulated blood

Body fluids (pleural, pericardial,